PHYSICS-PPY

Power supply

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For the Budgie album, see Power Supply (album).
Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely to others.
Contents
[hide]
1 Electrical power supplies
2 Power supply types
2.1 Battery power supply [1]
2.2 Linear power supply
2.2.1 AC/ DC supply
2.3 Switched-mode power supply
2.4 Programmable power supply
2.5 Uninterruptible power supply
2.6 High-voltage power supply
2.7 Voltage multipliers
3 Power supply applications
3.1 Computer power supply
3.2 Welding power supply
3.3 AC adapter
3.3.1 Polarity
4 Overload Protection[11]
4.1 Fuses
4.2 Circuit Breakers
5 Power conversion
6 Mechanical power supplies
7 Terminology
8 See also
9 References
10 External links
//
[edit] Electrical power supplies
This term covers the power distribution system together with any other primary or secondary sources of energy such as:
Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company (see electricity generation) to a well-regulated lower voltage DC for electronic devices. Low voltage, low power DC power supply units are commonly integrated with the devices they supply, such as computers and household electronics. For other examples, see switched-mode power supply, linear regulator, rectifier and inverter (electrical).
Batteries
Chemical fuel cells and other forms of energy storage systems
Solar power
Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has torque to spare, or in semi-portable units containing an internal combustion engine and a generator) (For large-scale power supplies, see electricity generation.)
Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.
The regulation of power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input. This kind of regulation is commonly categorized as a Stabilized power supply.
[edit] Power supply types
Power supplies for electronic devices can be broadly divided into linear and switching power supplies. The linear supply is a relatively simple design that becomes increasingly bulky and heavy for high current devices; voltage regulation in a linear supply can result in low efficiency. A switched-mode supply of the same rating as a linear supply will be smaller, is usually more efficient, but will be more complex.
[edit] Battery power supply [1]
A battery is a type of linear power supply that offers benefits that traditional line-operated power supplies lack: mobility, portability, and reliability. A battery consists of multiple electrochemical cells connected to provide the voltage desired.
The most commonly used dry-cell battery is the carbon-zinc dry cell battery.[2] Dry-cell batteries are made by stacking a carbon plate, a layer of electrolyte paste, and a zinc plate alternately until the desired total voltage is achieved. The most common dry-cell batteries have one of the following voltages: 1.5, 3, 6, 9, 22.5, 45, and 90. During the discharge of a carbon-zinc battery, the zinc metal is converted to a zinc salt in the electrolyte, and magnesium dioxide is reduced at the carbon electrode. These actions establish a voltage of approximately 1.5 V.
The lead-acid storage battery may be used. This battery is rechargeable; it consists of lead and lead/dioxide electrodes which are immersed in sulfuric acid. When fully charged, this type of battery has a 2.06-2.14 V potential. During discharge, the lead is converted to lead sulfate and the sulfuric acid is converted to water. When the battery is charging, the lead sulfate is converted back to lead and lead dioxide.
A nickel-cadmium battery has become more popular in recent years. [3]This battery cell is completely sealed and rechargeable. The electrolyte is not involved in the electrode reaction, making the voltage constant over the span of the batteries long service life. During the charging process, nickel oxide is oxidized to its higher oxidation state and cadmium oxide is reduced. The nickel-cadmium batteries have many benefits. They can be stored both charged and uncharged. They have a long service life, high current availabilities, constant voltage, and the ability to be recharged.
[edit] Linear power supply


A home-made linear power supply (used here to power amateur radio equipment)
An AC powered linear power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produce DC, a rectifier is used. A capacitor is used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known as ripple. These pulsations occur at a frequency related to the AC power frequency (for example, a multiple of 50 or 60 Hz).
The voltage produced by an unregulated power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications a linear regulator will be used to stabilize and adjust the voltage. This regulator will also greatly reduce the ripple and noise in the output direct current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from overcurrent.
Adjustable linear power supplies are common laboratory and service shop test equipment, allowing the output voltage to be set over a wide range. For example, a bench power supply used by circuit designers may be adjustable up to 30 volts and up to 5 amperes output. Some can be driven by an external signal, for example, for applications requiring a pulsed output.
The simplest DC power supply circuit consists of a single diode and resistor in series with the AC supply. This circuit is common in rechargeable flashlights.
[edit] AC/ DC supply
Main article: AC/DC (electricity)
In the past, mains electricity was supplied as DC in some regions, AC in others. A simple, cheap linear power supply would run directly from either AC or DC mains, often without using a transformer. The power supply consisted of a rectifier and a capacitor filter. The rectifier was essentially a conductor, having no sudden effect when operating from DC.
[edit] Switched-mode power supply
Main article: Switched-mode power supply


A computer's switched mode power supply unit.
A switched-mode power supply (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase.
The input power slicing occurs at a very high speed (typically 10 kHz — 1 MHz). High frequency and high voltages in this first stage permit much smaller step down transformers than are in a linear power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current draw by the load.
Modern switched-mode power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[4] In the event that an abnormal high current power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. For decades PC computer power supplies have also provided a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.
Switched mode power supplies have an absolute limit on their minimum current output. [5] They are only able to output above a certain power level and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolation transformer to act as a tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-power dummy load such as a ceramic power resistor or 10 watt light bulb can be attached to the supply to allow it to run with no primary load attached.
Power factor has become a recent issue of concern for computer manufacturers. Switched mode power supplies have traditionally been a source of power line harmonics and have a very poor power factor. Many computer power supplies built in the last few years now include power factor correction built right into the switched-mode supply, and may advertise the fact that they offer 1.0 power factor.
By slicing up the sinusoidal AC wave into very small discrete pieces, the portion of the alternating current not used stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the actual power used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.
[edit] Programmable power supply
Programmable power supplies are those in which the output voltage can be varied remotely. One possible option is digital control by a computer interface. Variable properties include voltage, current, and frequency. This type of supply is composed of a processor, voltage/current programming circuits, current shunt, and voltage/current read-back circuits.
Programmable power supplies can furnish DC, AC, or both types of output. The AC output can be either single-phase or three-phase. Single-phase is generally used for low-voltage, while three-phase is more common for high-voltage power supplies.
When choosing a programmable power supply, several specifications should be considered. For AC supplies, output voltage, voltage accuracy, output frequency, and output current are important attributes. For DC supplies, output voltage, voltage accuracy, current, and power are important characteristics. Many special features are also available, including computer interface, overcurrent protection, overvoltage protection, short circuit protection, and temperature compensation. Programmable power supplies also come in a variety of forms. Some of those are modular, board-mounted, wall-mounted, floor-mounted or bench top.
Programmable power supplies are now used in many applications. Some examples include automated equipment testing, crystal growth monitoring, and differential thermal analysis [6].
[edit] Uninterruptible power supply
Main article: Uninterruptible power supply
An Uninterruptible Power Supply (UPS) takes its power from two or more sources simultaneously. It is usually powered directly from the AC mains, while simultaneously charging a storage battery. Should there be a dropout or failure of the mains, the battery instantly takes over so that the load never experiences an interruption. Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data. Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains. The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption. Such a scheme might be found in hospitals or telephone central offices.
[edit] High-voltage power supply
High voltage refers to an output on the order of hundreds or thousands of volts. High-voltage power supplies use a linear setup to produce an output voltage in this range.
When choosing a high-voltage power supply, there are several options to consider. Some of these are maximum current, maximum power, maximum voltage, output polarity, user interface, and style. The first four of these characteristics of course depend upon the supply's intended application. There are many available types of user interfaces. For example, the interface may be local in the form of a digital meter, or analog meter. Also, the interface can be remote, as in a computer connection. Numerous styles of high-voltage power supplies are also manufactured. Available models come in printed circuit board mount, open frame (as designed to be incorporated into an instrument), and rack mount. Models with multiple outputs can also be found [7].
[edit] Voltage multipliers
Voltage multipliers, as the name implies, are circuits designed to multiply the input voltage. The input voltage may be doubled (voltage doubler), tripled (voltage tripler), quadrupled (voltage quadrupler), etc. Voltage multipliers are also power converters. An AC input is converted to a higher DC output. These circuits allow high voltages to be obtained using a much lower voltage AC source.
Typically, voltage multipliers are composed of half-wave rectifiers, capacitors, and diodes. For example, a voltage tripler consists of three half-wave rectifiers, three capacitors, and three diodes. Full-wave rectifiers may be used in a different configuration to achieve even higher voltages. Also, both parallel and series configurations are available. For parallel multipliers, a higher voltage rating is required at each consecutive multiplication stage, but less capacitance is required. The voltage capability of the capacitor limits the maximum output voltage.
Voltage multipliers have many applications. For example, voltage multipliers can be found in everyday items like televisions and photocopiers. Even more applications can be found in the laboratory, such as cathode ray tubes, oscilloscopes, and photomultiplier tubes.[8][9]
[edit] Power supply applications
[edit] Computer power supply
Main article: Computer power supply
A modern computer power supply is a switched-mode supply designed to convert 110-240 V AC power from the mains supply, to several output both positive (and historically negative) DC voltages in the range 12V to 3.3V. The first computer power supplies were linear devices, but as cost became a driving factor, and weight became important, switched mode supplies are almost universal.
The diverse collection of output voltages also have widely varying current draw requirements, which are difficult to all be supplied from the same switched-mode source. Consequently most modern computer power supplies actually consist of several different switched mode supplies, each producing just one voltage component and each able to vary its output based on component power requirements, and all are linked together to shut down as a group in the event of a fault condition.
The most common modern computer power supplies are built to conform to the ATX form factor. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer.[10]A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. The more reputable makers advertise "True Wattage Rated" to give consumers the idea that they can trust the power advertised.
[edit] Welding power supply
Main article: Welding power supply
Arc welding uses electricity to melt the surfaces of the metals in order to join them together through coalescence. The electricity is provided by a welding power supply, and can either be AC or DC. Arc welding typically requires high currents typically between 100 and 350 amps. Some types of welding can use as few as 10 amps, while some applications of spot welding employ currents as high as 60000 amps for an extremely short time. Older welding power supplies consisted of transformers or engines driving generators, while modern ones implement semiconductors and even microprocessors, greatly reducing their size and weight.
[edit] AC adapter


Switched mode mobile phone charger
See also: Wall wart
A linear or switched-mode power supply (or in some cases just a transformer) that is built into the top of a plug is known as a "wall wart", "power brick", "plug pack", "plug-in adapter", "adapter block", "domestic mains adapter" or just "power adapter". They are even more diverse than their names; often with either the same kind of DC plug offering different voltage or polarity, or a different plug offering the same voltage. "Universal" adapters attempt to replace missing or damaged ones, using multiple plugs and selectors for different voltages and polarities. Replacement power supplies must match the voltage of, and supply at least as much current as, the original power supply.
The least expensive AC units consist solely of a small transformer, while DC adapters include a few additional diodes. Whether or not a load is connected to the power adapter, the transformer has a magnetic field continuously present and normally cannot be completely turned off unless unplugged.
Because they consume standby power, they are sometimes known as "electricity vampires" and may be plugged into a power strip to allow turning them off. Expensive switched-mode power supplies can cut off leaky electrolyte-capacitors, use powerless MOSFETs, and reduce their working frequency to get a gulp of energy once in a while to power, for example, a clock, which would otherwise need a battery.
This type of power supply is popular among manufacturers of low cost electrical items because:
Devices sold in the global marketplace don't need to be individually configured for 120 volt or 230 volt operation, just sold with the appropriate AC adapter.
The device itself doesn't need to be tested for compliance with electrical safety regulations. Only the adapter needs to be tested.
Product development becomes faster and cheaper, because the heat produced by the power supply is outside of the product.
The device itself can be smaller and lighter, since it does not contain the power supply.
[edit] Polarity


Diagram showing positive tip polarity on the left and negative tip polarity on the right. To read diagram: The center positive drawing on the left indicates that the center (tip) of the output plug is positive (+) and the barrel of the output plug is negative (-). Some multi-adaptors use "tip" to indicate the center, but include no explanation that tip is used to mean center.
AC-to-DC adapters have polarity; even if the plug fits into a device, the positive and negative connections may be oriented the wrong way. It is necessary to use an adapter with the correct polarity to avoid damage. Standardized polarity symbols are usually used to label polarity.
[edit] Overload Protection[11]
Power supplies should have some type of overload protection. Overload protection is important to protect the electronic equipment hooked up to the power supply and to also prevent overheating, which could potentially lead to an electrical fire. Fuses and circuit breakers are two of the more frequent mechanisms used for overload protection.
[edit] Fuses
A piece of wire is connected between two metal ends. The two metal ends of the fuse are connected by either a tube of glass or plastic which surrounds the wire. If too much current flows, the wire overheats and melts. This interrupts the power supply, and the equipment stops working until the problem that caused the overload is identified and the fuse is replaced.
There are two types of fuses, slow-blow and fast-blow. In a fast-blow fuse, the wire inside the fuse will melt if the current exceeds the rated current, even if it is just for a fraction of second. This concise process is important in electronic equipment where even a small spike in the current could damage the equipment. A slow-blow fuse is designed to only melt when there is a continuous overload. Slow-blow fuses are ideal for motor systems.
[edit] Circuit Breakers
One benefit of using a circuit breaker as opposed to a fuse is that it can simply be reset instead of having to constantly replace the blown fuse. A circuit breaker works once an overloaded current causes some element to heat and trigger a spring which shuts the circuit down. Once the element cools, and the problem is identified the breaker can be reset and the power restored.
[edit] Power conversion
The term "power supply" is sometimes restricted to those devices that convert some other form of energy into electricity (such as solar power and fuel cells and generators). A more accurate term for devices that convert one form of electric power into another form (such as transformers and linear regulators) is power converter. The most common conversion is AC-DC. This is a conversion from the household current AC, to the direct current that is used in your car, and most electronics.
[edit] Mechanical power supplies
Cavitation
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Cavitating propeller model in a water tunnel experiment.

High speed jet of fluid impact on a fixed surface.


Cavitation damages on a valve plate for an axial piston hydraulic pump.
Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Noninertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc.
Since the shock waves formed by cavitation are strong enough to significantly damage moving parts, cavitation is usually an undesirable phenomenon. It is specifically avoided in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics.
Contents
[hide]
1 Inertial cavitation
2 Noninertial cavitation
3 Cavitation damage
4 Hydrodynamic Cavitation
5 Chemical engineering applications
6 Biomedical application
7 Cleaning application
8 Pumps and propellers
8.1 Suction cavitation
8.2 Discharge cavitation
9 Cavitation in engines
10 Vascular plants
11 Marine life
12 Coastal erosion
13 List of cavitation tunnels
13.1 Canada
13.2 France
13.3 Germany
13.4 India
13.5 Iran
13.6 Netherlands
13.7 Norway
13.8 Poland
13.9 South Korea
13.10 Spain
13.11 Sweden
13.12 Taiwan
13.13 United Kingdom
13.14 United States
14 See also
15 References
15.1 Further reading
16 External links
//
[edit] Inertial cavitation
Inertial cavitation was first studied by Lord Rayleigh in the late 19th century, when he considered the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low pressure, it may rupture and form a cavity. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration. A fast-flowing river can cause cavitation on rock surfaces, particularly when there is a drop-off, such as on a waterfall. Other ways of generating cavitation voids involve the local deposition of energy, such as an intense focused laser pulse (optic cavitation) or with an electrical discharge through a spark. Vapor gases evaporate into the cavity from the surrounding medium; thus, the cavity is not a perfect vacuum, but has a relatively low gas pressure. Such a low-pressure cavitation bubble in a liquid begins to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapor within increases. The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand kelvin, and the pressure several hundred atmospheres.
Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles that are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefaction in the liquid is insufficient for a Rayleigh like void to occur. High-power ultrasonics usually utilize the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces, liquids, and slurries.
The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid.
In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can nucleate. This surface can be provided by the sides of a container, by impurities in the liquid, or by small undissolved microbubbles within the liquid. It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake's threshold.
The vapor pressure here differs from the meteorological definition of vapor pressure, which describes the partial pressure of water in the atmosphere at some value less than 100% saturation. Vapor pressure as relating to cavitation refers to the vapor pressure in equalibrium conditiond and can therefore be more accurately defined as the equilibrium (or saturated) vapour pressure.
[edit] Noninertial cavitation
Noninertial cavitation is the process in which small bubbles in a liquid are forced to oscillate in the presence of an acoustic field, when the intensity of the acoustic field is insufficient to cause total bubble collapse. This form of cavitation causes significantly less erosion than inertial cavitation, and is often used for the cleaning of delicate materials, such as silicon wafers.
[edit] Cavitation damage


Cavitation damage to a Francis turbine.
Cavitation is, in many cases, an undesirable occurrence. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency.
When the cavitation bubbles collapse, they force liquid energy into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. The noise created by cavitation is a particular problem for military submarines, as it increases the chances of being detected by passive sonar.
Although the collapse of a cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.
After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the fluid flow and create crevasses that act as nucleation sites for additional cavitation bubbles. The pits also increase the components' surface area and leave behind residual stresses. This makes the surface more prone to stress corrosion.[1]

[edit] Hydrodynamic Cavitation
Hydrodynamic cavitation describes the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure. Cavitation will only occur if the pressure declines to some point below the saturated vapor pressure of the liquid. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation.
Hydrodynamic cavitation can be produced by passing a liquid through a constricted channel at a specific velocity or by mechanical rotation through a liquid. In the case of the constricted channel and based on the specific (or unique) geometry of the system, the combination of pressure and kinetic energy can be created when the hydrodynamic cavitation cavern downstream of the local constriction generating high energy cavitation bubbles.
The process of bubble generation, subsequent growth and collapse of the cavitation bubbles results in very high energy densities, resulting in very high temperatures and pressures at the surface of the bubbles for a very short time. The overall liquid medium environment, therefore, remains at ambient conditions. When uncontrolled, cavitation is damaging; however, by controlling the flow of the cavitation the power is harnessed and non-destructive. Controlled cavitation can be used to enhance chemical reactions or propogate certain unexpected reactions because free radicals are generated in the process due to disassociation of vapors trapped in the cavitating bubbles.
[edit] Chemical engineering applications
In industry, cavitation is often used to homogenize, or mix and break down, suspended particles in a colloidal liquid compound such as paint mixtures or milk. Many industrial mixing machines are based upon this design principle. It is usually achieved through impeller design or by forcing the mixture through an annular opening that has a narrow entrance orifice with a much larger exit orifice. In the latter case, the drastic decrease in pressure as the liquid accelerates into a larger volume induces cavitation. This method can be controlled with hydraulic devices that control inlet orifice size, allowing for dynamic adjustment during the process, or modification for different substances. The outer surface of this type of mixing valve, upon which the cavitation bubbles are driven against to cause their implosion, undergoes tremendous stress, and is often constructed of super-hard or tough materials such as stainless steel, Stellite, or even polycrystalline diamond (PCD).
Cavitating water purification devices have also been designed, in which the extreme conditions of cavitation can break down pollutants and organic molecules. Spectral analysis of light emitted in sonochemical reactions reveal chemical and plasma-based mechanisms of energy transfer. The light emitted from cavitation bubbles is termed sonoluminesence.
Hydrophobic chemicals are attracted underwater by cavitation as the pressure difference between the bubbles and the liquid water forces them to join together. This effect may assist in protein folding.[2]
[edit] Biomedical application
Cavitation plays an important role for the destruction of kidney stones in shock wave lithotripsy. Currently, tests are being conducted as to whether cavitation can be used to transfer large molecules into biological cells (sonoporation). Nitrogen cavitation is a method used in research to lyse cell membranes while leaving organelles intact. Cavitation plays a key role in non-thermal noninvasive fractionation of tissue for treatment of a variety of diseases.[3] Cavitation also probably plays a role in HIFU, a thermal noninvasive treatment methodology for cancer.[4]
[edit] Cleaning application
In industrial cleaning applications, cavitation has sufficient power to overcome the particle-to-substrate adhesion forces, loosening contaminants. The threshold pressure required to initiate cavitation is a strong function of the pulse width and the power input. This method works by generating controlled acoustic cavitation in the cleaning fluid, picking up and carrying contaminant particles away so that they do not reattach to the material being cleaned.
[edit] Pumps and propellers
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.
As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.
Cavitation in pumps may occur in two different forms:
[edit] Suction cavitation
Suction cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.
[edit] Discharge cavitation
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high velocity. This velocity causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.
Discharge cavitation in joint fluid is thought to cause the popping sound produced by bone joint cracking, for example by deliberately cracking one's knuckles.
[edit] Cavitation in engines
Some bigger diesel engines suffer from cavitation due to high compression and undersized cylinder walls. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall, which will eventually let cooling fluid leak into the cylinder and combustion gases to leak into the coolant.
It is possible to prevent this from happening with the use of chemical additives in the cooling fluid that form a protective layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself.
From about the 1980's, new designs of smaller petrol engines also displayed cavitation phenomenon. One answer to the need for smaller and lighter engines was a smaller coolant volume and a correspondingly higher coolant velocity. This gave rise to rapid changes in flow velocity and therefore rapid changes of static pressure in areas of high heat transfer. Where resulting vapour bubbles collapsed against a surface, they had the effect of first disrupting protective oxide layers (of cast aluminum materials) and then repeatedly damaging the newly formed surface, preventing the action of some types of corrosion inhibitor (such as silicate based inhibitors). A final problem was the affect that increased material temperature had on the relative electrochemical reactivity of the base metal and its alloying constituents. The result was deep pits that could form and penetrate the engine head in a matter of hours when the engine was running at high load and high speed. These effects could largely be avoided by the use of organic corrosion inhibitors or (preferably) by designing the engine head in such a way as to avoid certain cavitation inducing conditions.
[edit] Vascular plants
Cavitation occurs in the xylem of vascular plants when the tension of water within the xylem becomes so great that dissolved air within the water expands to fill either the vessel elements or tracheids. Plants are generally able to repair cavitated xylem in a number of ways. For plants less than 50cm tall, root pressure can be sufficient to redissolve air. For larger plants, they must repair cavitation by importing solutes into the xylem; this causes water to enter as well, which can then redissolve the air. In some trees, the sound of the cavitation is clearly audible, particularly in summer, when the rate of evapotranspiration is highest. Deciduous trees shed leaves in the autumn partly because cavitation increases as temperatures decrease.
[edit] Marine life
Just as cavitation bubbles form on a fast-spinning boat propeller, they may also form on the tails and fins of aquatic animals. The effects of cavitation are especially important near the surface of the ocean, where the ambient water pressure is relatively low and cavitation is more likely to occur.
For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed.[5] Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the painful bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create an air film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.
Cavitation is not always a limitation for sea life; some animals have found ways to use it to their advantage when hunting prey. The pistol shrimp snaps a specialized claw to create cavitation, which can kill small fish. The mantis shrimp (type smasher) uses cavitation as well in order to stun, smash open, or kill the shellfish that it feasts upon.
[edit] Coastal erosion
In the last half-decade, coastal erosion in the form of inertial cavitation has been generally accepted.[6] Air pockets in an incoming wave are forced into cracks in the cliff being eroded, then the force of the wave compresses the air pockets until the bubble implodes, becoming liquid, giving off various forms of energy that blast apart the rock.
[edit] List of cavitation tunnels
See also: Water tunnel (hydrodynamic)
[edit] Canada
National Research Council—Institute for Ocean Technology Cavitation Tunnel,[7] St. Johns, Newfoundland.
[edit] France
"Tunnel de Cavitation" Ecole Navale,[8] Lanveoc.
"Grand Tunnel Hydrodynamique" Bassin d'Essais des Carènes,[9] Val de Reuil.
[edit] Germany
Multiple cavitation tunnels at the Versuchsanstalt für Wasserbau und Schiffbau,[10] Berlin.
Large Cavitation tunnel at Hamburg Ship Model Basin,[11] Hamburg.
[edit] India
Cavitation Tunnel of the Naval Science and Technology Labs at Visakhapatnam.
[edit] Iran
Applied Hydrodynamics Laboratory, Iran University of Science and Technology,[12] Narmak, Tehran.
Marine Engineering Laboratory, Sharif University of Technology,[13][14] Azadi Av., Tehran.
[edit] Netherlands
Vapor
From Wikipedia, the free encyclopedia
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This article is about the chemical use. For computer usage, see Vaporware.


Water condenses into visible droplets after evaporating from a cup of hot tea
A vapor (American spelling) or vapour (see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature.[1] This means that the vapor can be condensed to a liquid or to a solid by increasing its pressure, without reducing the temperature.
For example, water has a critical temperature of 374°C (or 647 K) which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water is known as water vapor and will condense to liquid if its partial pressure is increased sufficiently.
A vapor may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas pressure will equal the equilibrium vapor pressure of the liquid (or solid).[2]
Contents
[hide]
1 Properties
2 Vapor pressure
3 Examples
4 Measuring vapor
5 Vapors of flammable liquids
6 See also
7 References
//
[edit] Properties
Vapor refers to a gas phase at a temperature where the same substance can also exist in the liquid or solid state, below the critical temperature of the substance. If the vapor is in contact with a liquid or solid phase, the two phases will be in a state of equilibrium. The term gas refers to a compressible fluid phase. Fixed gases are gases for which no liquid or solid can form at the temperature of the gas (such as air at typical ambient temperatures). A liquid or solid does not have to boil to release a vapor.
Vapor is responsible for the familiar processes of cloud formation and condensation. It is commonly employed to carry out the physical processes of distillation and headspace extraction from a liquid sample prior to gas chromatography.
The constituent molecules of a vapor possess vibrational, rotational, and translational motion. These motions are considered in the kinetic theory of gases.
[edit] Vapor pressure
Main article: Vapor pressure


Liquid-Vapor Equilibrium
The vapor pressure is the equilibrium pressure from a liquid or a solid at a specific temperature. The equilibrium vapor pressure of a liquid or solid is not affected by the amount of contact with the liquid or solid interface.
The normal boiling point of a liquid is the temperature at which the vapor pressure is equal to one atmosphere (unit).[3]
For two-phase systems (e.g., two liquid phases), the vapor pressure of the system is the sum of the vapor pressures of the two liquids. In the absence of stronger inter-species attractions between like-like or like-unlike molecules, the vapor pressure follows Raoult's Law, which states that the partial pressure of each component is the product of the vapor pressure of the pure component and its mole fraction in the mixture. The total vapor pressure is the sum of the component partial pressures.[4]
The physical chemistry behind distillation is based on manipulating the equilibrium occurring between the liquid and vapor phases of a molecule in solution.
[edit] Examples


Water vapor is responsible for humidity
Perfumes contain chemicals that vaporize at different temperatures and at different rate in scent accords known as notes.
Atmospheric water vapor is found near the earth's surface, and may condense into small liquid droplets and form meteorological phenomena such as fog, mist and haar.
Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states.
[edit] Measuring vapor
Since it is in the gas phase, the amount of vapor present is quantified by the partial pressure of the gas. Also, vapors obey the barometric formula in a gravitational field just as conventional atmospheric gases do.
[edit] Vapors of flammable liquids
Flammable liquids do not burn when ignited. It is the vapor cloud above the liquid that will burn if the vapor's concentration is between the lower explosive limit and upper explosive limit of the flammable liquid.
[edit] See also
Holography
Holography (from the Greek, Â-holos whole +-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store and retrieve information. Holograms are common in science-fiction, most notably Star Trek, Star Wars, and Red Dwarf.
Overview
Holography was invented in 1948 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971. He received patent GB685286 on the invention. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, but the field did not really advance until the invention of the laser in 1960.
Several types of holograms can be made. The very first holograms were "transmission holograms", which were viewed by shining laser light through them. A later refinement, the "rainbow transmission" hologram allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission holograms incorporate a reflective foil backing which provides the light from "behind" to reconstruct their imagery. Another kind of common hologram is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer.
One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers - typically used by the millions in DVD recorders and other applications, but sometimes also useful for holography. These cheap, compact, solid-state lasers can compete well with the large, expensive gas lasers previously required to make holograms, and are already helping to make holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.
Technical description
The difference between holography and photography is best understood by considering what a Black & White (B&W) photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.
However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image (albeit, in most cases, a monochromatic one, though colour holograms are possible).
Hologram recording process
To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film.
Hologram reconstruction process
Once the film is processed, if illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase. Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move their viewpoint and see the image rotate exactly as the original object would.
Because of the need for interference between the reference and object beams, holography typically uses a laser to produce them. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.
The coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram. Small pen laser pointers tend to have a smaller coherence length and were considered too small to do holography. That has been shown to be incorrect, and people have successfully made small holograms with laser pens. Large analogue holograms cannot be made with laser pens due to their lower power (typically 1mW to 5mW). Digital holography does not suffer from this problem.
Other applications of holograms include metrology and optical computing.
Holography in art
Salvador Dalí claims to have been the first to employ holography artistically. He was certainly the first and most notorious Catalan surrealist to do so, but the 1972 New York exhibit of Dali holograms had been preceded by "the first holographic art exhibition [which] was held at the Cranbrook Academy of Art in Michigan in 1968. The second took place at the Finch College gallery in New York in 1970 and attracted national media attention." (source: http://www.holophile.com/history.htm ). A vastly entertaining account of a 1973 Dali holography project (with Alice Cooper as subject) can be found here: http://www.alicecoopertrivia.pwp.blueyonder.co.uk/people/p-dali.php
Holographic data storage
See the main article at holographic memory.
Holography can be applied to a variety of uses other than recording images. Holographic data storage is a technique that can store information at high density inside crystals (à la HAL 9000) or photopolymers. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.
Using currently available SLM's can produce about 1000 different images a second at 1024 X 1024 bit resolution. With the right type of media (probably polymers rather than something like LiNbO3), this would result in about 1 Gigabit per second writing speed. Read speeds can surpass this and experts believe 1 Terabit per second readout is possible.
In 2005, the company Optware has produced a 120 mm disc that uses holographic surface to store data to a possible 1TB (terabyte). See Holographic Versatile Disc, for more information.
How Holographic Memory Will Work
by Kevin Bonsor
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Please copy/paste the following text to properly cite this HowStuffWorks article:
Bonsor, Kevin. "How Holographic Memory Will Work." 08 November 2000. HowStuffWorks.com. 07 May 2009.
Inside this Article
1. Introduction to How Holographic Memory Will Work
2. A Little Background
3. The Basics
4. Desktop Holographic Data Storage
5. Lots More Information
6. See all Memory articles
Computer Videos

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Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released, which enabled the storage of full-length movies on a single disc.
Holographic Memory Image Gallery
In a holographic memory device, a laser beam is split in two, and the two resulting beams interact in a crystal medium to store a holographic recreation of a page of data. See more pictures of holographic memory.
CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data, which is equivalent to about one hour and 15 minutes of music, but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. A double-sided, double-layer DVD can hold 15.9 GB of data, which is about eight hours of movies. These conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilities, scientists are now working on a new optical storage method, called holographic memory, that will go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area.
Three-dimensional data storage will be able to store more information in a smaller space and offer faster data transfer times. In this article, you will learn how a holographic storage system might be built in the next three or four years, and what it will take to make a desktop version of such a high-density storage system.

Next Page
The Basics
Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept. Here are the basic components that are needed to construct an HDSS:
Blue-green argon laser
Beam splitters to spilt the laser beam
Mirrors to direct the laser beams
LCD panel (spatial light modulator)
Lenses to focus the laser beams
Lithium-niobate crystal or photopolymer
Charge-coupled device (CCD) camera
When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal -- the data is stored as a hologram.

Images courtesy Lucent TechnologiesThese two diagrams show how information is stored and retrieved in a holographic data storage system.
An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.
The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.
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Inside this Article
1. Introduction to How Holographic Memory Will Work
2. A Little Background
3. The Basics
4. Desktop Holographic Data Storage
5. Lots More Information
Heart & Vascular Health > Echocardiogram
Echocardiogram

By Robin Parks, MS
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Test Overview
An echocardiogram (also called an echo) is a type of ultrasound test that uses high-pitched sound waves that are sent through a device called a transducer. The device picks up echoes of the sound waves as they bounce off the different parts of your heart. These echoes are turned into moving pictures of your heart that can be seen on a video screen.
The different types of echocardiograms are:
Transthoracic echocardiogram (TTE). This is the most common type. Views of the heart are obtained by moving the transducer to different locations on your chest or abdominal wall.
Stress echocardiogram. During this test, an echocardiogram is done both before and after your heart is stressed either by having you exercise or by injecting a medicine that makes your heart beat harder and faster. A stress echocardiogram is usually done to find out if you might have decreased blood flow to your heart (coronary artery disease, or CAD).
Doppler echocardiogram. This test is used to look at how blood flows through the heart chambers, heart valves, and blood vessels. The movement of the blood reflects sound waves to a transducer. The ultrasound computer then measures the direction and speed of the blood flowing through your heart and blood vessels. Doppler measurements may be displayed in black and white or in color.
Transesophageal echocardiogram (TEE). For this test, the probe is passed down the esophagus instead of being moved over the outside of the chest wall. TEE shows clearer pictures of your heart, because the probe is located closer to the heart and because the lungs and bones of the chest wall do not block the sound waves produced by the probe. A sedative and an anesthetic applied to the throat are used to make you comfortable during this test.
Echo can be used as part of a stress test and with an electrocardiogram (EKG) to help your doctor learn more about your heart.
Why It Is Done
Transthoracic echocardiogram (TTE)
This test is done to:
Look for the cause of abnormal heart sounds (murmurs or clicks), an enlarged heart, unexplained chest pains, shortness of breath, or irregular heartbeats.
Check the thickness and movement of the heart wall.
Look at the heart valves and check how well they work.
See how well an artificial heart valve is working.
Measure the size and shape of the heart's chambers.
Check the ability of your heart chambers to pump blood (cardiac performance). During an echocardiogram, your doctor can calculate how much blood your heart is pumping during each heartbeat (ejection fraction). You might have a low ejection fraction if you have heart failure.
Detect a disease that affects the heart muscle and the way it pumps, such as cardiomyopathy.
Look for blood clots and tumors inside the heart.
A transthoracic echocardiogram may also be used to:
Look for congenital heart defects or to check the effectiveness of previous surgery to repair a congenital heart defect.
Check how well your heart works after a heart attack.
Identify the specific cause of heart failure.
Look for a collection of fluid around the heart (pericardial effusion) or a thickening of the lining (pericardium) around the heart.
Stress echocardiogram
A stress echo may be done to:
Identify and monitor reduced blood flow to heart muscle (ischemia). This is usually more apparent after some form of stress, such as exercise or medicine.
Doppler echocardiogram
A Doppler echocardiogram can be done during a transthoracic echocardiogram (TTE), a transesophageal echocardiogram (TEE), or a stress echocardiogram to:
Measure the speed at which blood travels through the heart.
Measure the blood pressure and speed of blood flow through the heart valves.
Transesophageal echocardiogram (TEE)
Transesophageal echocardiogram (TEE) may be done to:
Monitor heart function during surgery.
Check how well an artificial heart valve works.
Look for masses or blood clots in the upper left chamber (left atrium) of the heart.
Identify abnormal blood flow between the chambers of the heart (cardiac shunt).
Help find out if you have endocarditis.
Guide procedures done during cardiac catheterization.
Help find out if you have a tear in the aorta (aortic dissection).
How To Prepare
Transthoracic echocardiogram (TTE) and Doppler echocardiogram
You do not need any special preparation for a transthoracic or Doppler echocardiogram.
Stress echocardiogram
Do not eat heavily for a few hours before a stress echo. This will help prevent nausea, which can occur while exercising with a full stomach or from the injection of dobutamine.
Wear flat, comfortable shoes (no bedroom slippers or sandals) and loose, lightweight shorts or sweatpants for an exercise stress echo.
Transesophageal echocardiogram (TEE)
Do not eat or drink for at least 6 hours before the TEE.
If you have dentures or dental prostheses, you may need to remove them before the test.
Before TEE, you will be given a sedative. You will not be able to drive for at least 12 hours after the procedure. Be sure to make arrangements in advance for someone to pick you up after the test.
Before an echocardiogram, you will typically be asked to sign a consent form. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will indicate. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .
How It Is Done
An echocardiogram may be done in a hospital, clinic, or doctor's office. It can also be done at your bedside in the hospital.
You will need to remove any jewelry and clothes above your waist (you may be allowed to keep on your underwear if it does not interfere with the test). You may be given a cloth or paper covering to use during the test.
A transthoracic echocardiogram (TTE), Doppler echocardiogram, and stress echocardiogram are performed by a specially trained ultrasound technician. A transesophageal echocardiogram (TEE) is performed by a cardiologist with the help of assistants.
Transthoracic echocardiogram (TTE) and Doppler echocardiogram
You will lie on your back or on your left side on a bed or table. Small metal discs (electrodes) will be taped to your arms and legs to record your heart rate during the test. For more information, see the medical test Electrocardiogram.
A small amount of gel will be rubbed on the left side of your chest to help pick up the sound waves. A small instrument (transducer) that looks like a microphone is pressed firmly against your chest and moved slowly back and forth. This instrument sends sound waves into the chest and picks up the echoes as they reflect off different parts of the heart. The echoes are sent to a video monitor that records pictures of your heart for later viewing and evaluation. The room is usually darkened to help the technician see the pictures on the monitor.
At times you will be asked to hold very still, breathe in and out very slowly, hold your breath, or lie on your left side. The transducer is usually moved to different areas on your chest that provide specific views of your heart.
The test usually takes from 30 to 60 minutes. When the test is over, the gel is wiped off and the electrodes are removed.
Exercise stress echocardiogram
An echo without activity or stress will be done before you start exercising. This is called the baseline, and after it is established you will exercise for a specific amount of time. You will either walk on a treadmill or pedal a stationary bicycle while being monitored by an EKG machine. For more information, see the test topic Exercise Electrocardiogram.
You will then lie on a bed or table, and another echocardiogram will be done. At times you will be asked to hold very still, breathe in and out very slowly, hold your breath, or lie on your left side. The transducer is usually moved to different areas on your chest that provide specific views of your heart. You may receive an injection of saline in a vein (IV) to help your doctor assess your heart function. An IV contrast material may be used if it is difficult to get good views of your heart because of conditions such as obesity or chronic lung disease. IV contrast may also be used when a person is on a mechanical ventilator.
An exercise stress echo takes about 30 to 60 minutes.
Dobutamine stress echocardiogram
Sometimes medicine called dobutamine is used instead of exercise to stress your heart. For this test, you will lie on your back or left side on a bed or examination table, and a baseline echocardiogram will be done. EKG electrodes will be taped to your arms and legs to record your heart rate during the test.
Next, the technician cleans the site on your arm where the medicine will be injected, and an intravenous (IV) line will be placed in a vein in your arm.
After the IV is started, you will be given the dobutamine, which increases your heart rate and causes your heart to work harder. Echocardiogram images will be taken while you receive the dobutamine. Your peak heart rate is reached in about 15 minutes. At times you will be asked to hold very still, breathe in and out very slowly, hold your breath, or lie on your left side. After your peak heart rate is reached, the medicine will be stopped and your heart rate will return to normal (in about 1 to 3 minutes). More echocardiogram images will be taken when your heart rate returns to normal.
A dobutamine stress echo takes about an hour.
Transesophageal echocardiogram (TEE)
Your throat may be numbed with an anesthetic spray, gargle, or lozenge to relax your gag reflex and to ease insertion of the probe. Shortly before the procedure begins, an IV line will be placed in a vein in your arm. Medicine to decrease saliva and stomach secretions may be given through the IV. A pain medicine and sedative will be given to you through the IV in your arm during the procedure. You should feel relaxed and drowsy but still alert enough to cooperate.
Your heart rate, breathing rate, and blood pressure will be monitored throughout the procedure. Also, a small device used to measure the amount of oxygen in your blood (pulse oximeter) may be attached to your finger or earlobe.
You will be asked to lie on your left side with your head tilted slightly forward. A mouth guard may be inserted to protect your teeth from the probe. Then the lubricated tip of the probe will be guided into your mouth while your doctor gently presses your tongue out of the way. You may be asked to swallow to help move the tube along. It may be helpful to remember that the instrument is no thicker than many foods you swallow. When the probe is in your esophagus, it will be moved down gently to the level of your upper right heart chamber (atrium), and ultrasound images will be taken. You will not feel or hear the sound waves during the test. You may receive an IV injection of saline or contrast dye to help your doctor assess your heart function.
During the procedure, try not to swallow unless requested. An assistant may remove the saliva from your mouth with a suction device, or you can just let the saliva drain from the side of your mouth. A transesophageal echo is generally painless, though you may feel nauseated and uncomfortable while the probe is in your throat.
The test takes about 2 hours. The probe will be in place in your esophagus for about 10 to 20 minutes.
How It Feels
Transthoracic echocardiogram (TTE) and Doppler echocardiogram
You will not have pain from the echocardiogram. Gel is put on your chest for the ultrasound. It may feel cool. The handheld ultrasound device is pressed firmly against your chest, but it does not cause pain. You will not hear or feel the sound waves.
You may feel uncomfortable from lying still or from the transducer pressing on your chest. If you need to take a break, tell the technician.
Although most people to do not experience any discomfort from ultrasound tests, if you have severe difficulty breathing or cannot lie flat for a long examination, you may not be able to have an entire echo study. Talk to your doctor or the technician performing your echo about any concerns you have.
Dobutamine stress echocardiogram
You may have a brief, sharp pain when the intravenous (IV) needle is placed in a vein in your arm.
If medicine to stress your heart is used, you may have symptoms of mild nausea, headache, dizziness, flushing, or chest pain (angina). These symptoms only last a few minutes.
Transesophageal echocardiogram (TEE)
During the test:
You may notice a brief, sharp pain when the intravenous (IV) needle is placed in a vein in your arm.
The anesthetic sprayed into your throat may taste bitter and will make your tongue and throat feel numb and swollen. Some people report that they feel as if they cannot breathe at times because of the probe in their throat, but this is a false sensation caused by the anesthetic. There is always plenty of breathing space around the probe in your mouth and throat. Remember to relax and take slow, deep breaths.
You may gag and feel nauseous, bloated, or have mild belly cramps when the probe is moved. If the discomfort is severe, alert your doctor with an agreed-upon signal or a tap on the arm. Even though you won't be able to talk during the procedure, you can still communicate.
The IV medicines will make you feel sleepy. Other side effects—such as heavy eyelids, trouble speaking, a dry mouth, or blurred vision—may last for several hours after the test. You probably will not be able to remember much of the test.
After the test:
You may have a tickling, dry throat; slight hoarseness; or a mild sore throat. These symptoms may last for 2 to 3 days. Throat lozenges and warm saltwater gargles can help relieve these symptoms.
Do not drink alcohol for 24 hours.
Contact your doctor immediately if you have:
Difficulty swallowing or talking.
Shortness of breath or a fast heartbeat.
Chest pain.
Risks
An echocardiogram is safe, because the test uses only sound waves to evaluate your heart. These high-frequency sound waves have not been shown to have any harmful effects.
Transthoracic echocardiogram (TTE) and Doppler echocardiogram
There are no known risks from a transthoracic or Doppler echocardiogram. During a transthoracic echo, the technician may have to press hard on your chest with the transducer. Tell the technician if you feel any pain or discomfort.
Stress echocardiogram
A stress echocardiogram can cause dizziness, low blood pressure, shortness of breath, nausea, irregular heartbeats, and heart attack.
Transesophageal echocardiogram (TEE)
A transesophageal echocardiogram (TEE) can sometimes cause:
Nausea.
Mouth and throat discomfort.
Minor bleeding.
Trouble breathing.
Slow or abnormal heartbeats.
Insertion of the probe may tear or puncture your esophagus. This is rare.
This test is not recommended if you have:
Had recent radiation treatment to your neck or chest.
Serious problems with your esophagus, such as a very narrow esophagus, dilated (engorged) veins in the esophagus that could rupture and bleed (esophageal varices), or severe arthritis of your neck.
Trouble swallowing.
A bleeding disorder, such as hemophilia.
Results
An echocardiogram is a type of ultrasound test that uses high-pitched sound waves that are sent through a device called a transducer. The device picks up echoes of the sound waves as they bounce off the different parts of your heart. These echoes are turned into moving pictures of your heart that can be seen on a video screen.
Results are usually available within a week. If the test is done by a cardiologist, the results may be available immediately after the test.
Echocardiogram
Normal:
The heart chambers and walls of the heart are of normal size and thickness, and they move normally.

Heart valves are working normally, with no leaks or narrowing. There is no sign of infection.

The amount of blood pumped from the left ventricle with each heartbeat (ejection fraction) is more than 55%.

There is no excess fluid in the sac surrounding the heart, and the lining around the heart is not thickened.

There are no tumors and blood clots in the heart chambers.

Abnormal:
Heart chambers are too big. The walls of the heart are thicker or thinner than normal. A thin heart wall may mean poor blood flow to the heart muscle or an old heart attack. A thin, bulging area of the heart wall may indicate a bulge in the ventricle (ventricular aneurysm). The heart muscle walls do not move normally because of a decreased blood supply from narrowed coronary arteries.

One or more heart valves do not open or close properly (are leaking) or do not look normal. Signs of infection are present.

The amount of blood pumped from the left ventricle with each heartbeat (ejection fraction) is less than 55%.

There is fluid around the heart (pericardial effusion). The lining around the heart is too thick.

A tumor or blood clot may be found in the heart.

What Affects the Test
You may not be able to have the test or the results may not be helpful if you are:
Overweight, have a thick chest or large breasts, or have lung disease, such as chronic obstructive pulmonary disease (COPD). In these situations, other heart tests may be done. For more information, see the medical tests Cardiac Blood Pool Scan, Electrocardiogram (EKG), Exercise Electrocardiogram, and Cardiac Catheterization.
Not able to lie still during the test.
Not able to stand having a probe in your throat during a transesophageal echo (TEE).
What To Think About
An echocardiogram provides detailed information about how well the heart is working and possible causes of chest pain, shortness of breath, lightheadedness, and swelling.
A transesophageal echocardiogram (TEE) can be used to monitor your heart function during heart surgery, such as coronary artery bypass graft (CABG) surgery. TEE also can be used to guide some procedures done during a cardiac catheterization. For more information, see the medical test Cardiac Catheterization.
References
Other Works Consulted
Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.
Credits
Electrocardiography
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"ECG" redirects here. For other uses, see ECG (disambiguation).
Not to be confused with echocardiogram, electroencephalogram, or EEG.


12 Lead ECG of a 26-year-old male.


Image showing patient with 10 leads connected
Electrocardiography (ECG or EKG) is the recording of the electrical activity of the heart over time via skin electrodes.[1] It is a noninvasive recording produced by an electrocardiograph. The etymology of the word is derived from electro, because it is related to electrical activity, cardio, Greek for heart, graph, a Greek root meaning "to write".
Electrical impulses in the heart originate in the sinoatrial node and travel through conducting system to the heart muscle.The impulses stimulate the muscle fibres to contract and thus producing the systole. The electrical waves can be measured at selectively placed electrodes (electrical contacts) on the skin. Electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An ECG displays the voltage between pairs of these electrodes, and the muscle activity that they measure, from different directions, also understood as vectors. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle. It is the best way to measure and diagnose abnormal rhythms of the heart,[2] particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by levels of dissolved salts (electrolytes), such as potassium, that are too high or low.[3] In myocardial infarction (MI), the ECG can identify damaged heart muscle. But it can only identify damage to muscle in certain areas, so it can't rule out damage in other areas.[4] The ECG cannot reliably measure the pumping ability of the heart; for which ultrasound-based (echocardiography) or nuclear medicine tests are used.
Contents
[hide]
1 History
2 ECG graph paper
3 Filter selection
4 Leads
4.1 Limb
4.2 Augmented limb
4.3 Precordial
4.4 Ground
5 Waves and intervals
5.1 P wave
5.2 QRS complex
5.3 PR/PQ interval
5.4 ST segment
5.5 T wave
5.6 QT interval
5.7 U wave
6 Clinical lead groups
7 Axis
8 Electrocardiogram heterogeneity
8.1 Background
8.2 Research
8.3 Future applications
9 See also
10 References
11 Additional images
12 External links
//
[edit] History
Alexander Birmick Muirhead is reported to have attached wires to a feverish patient's wrist to obtain a record of the patient's heartbeat while studying for his Doctor of Science (in electricity) in 1872 at St Bartholomew's Hospital.[5] This activity was directly recorded and visualized using a Lippmann capillary electrometer by the British physiologist John Burdon Sanderson.[6] The first to systematically approach the heart from an electrical point-of-view was Augustus Waller, working in St Mary's Hospital in Paddington, London.[7] His electrocardiograph machine consisted of a Lippmann capillary electrometer fixed to a projector. The trace from the heartbeat was projected onto a photographic plate which was itself fixed to a toy train. This allowed a heartbeat to be recorded in real time. In 1911 he still saw little clinical application for his work.
An initial breakthrough came when Willem Einthoven, working in Leiden, The Netherlands, used the string galvanometer that he invented in 1903[4]. This device was much more sensitive than both the capillary electrometer that Waller used and the string galvanometer that had been invented separately in 1897 by the French engineer Clément Ader.[8]
Einthoven assigned the letters P, Q, R, S and T to the various deflections, and described the electrocardiographic features of a number of cardiovascular disorders. In 1924, he was awarded the Nobel Prize in Medicine for his discovery.[9]
Though the basic principles of that era are still in use today, there have been many advances in electrocardiography over the years. The instrumentation, for example, has evolved from a cumbersome laboratory apparatus to compact electronic systems that often include computerized interpretation of the electrocardiogram.[10]
[edit] ECG graph paper


One second of ECG graph paper
Timed interpretation of an ECG was once incumbent to a stylus and paper speed. Computational analysis now allows considerable study of heart rate variability. A typical electrocardiograph runs at a paper speed of 25 mm/s, although faster paper speeds are occasionally used. Each small block of ECG paper is 1 mm². At a paper speed of 25 mm/s, one small block of ECG paper translates into 0.04 s (or 40 ms). Five small blocks make up 1 large block, which translates into 0.20 s (or 200 ms). Hence, there are 5 large blocks per second. A diagnostic quality 12 lead ECG is calibrated at 10 mm/mV, so 1 mm translates into 0.1 mV. A calibration signal should be included with every record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is two large squares on ECG paper.
[edit] Filter selection
Modern ECG monitors offer multiple filters for signal processing. The most common settings are monitor mode and diagnostic mode. In monitor mode, the low frequency filter (also called the high-pass filter because signals above the threshold are allowed to pass) is set at either 0.5 Hz or 1 Hz and the high frequency filter (also called the low-pass filter because signals below the threshold are allowed to pass) is set at 40 Hz. This limits artifact for routine cardiac rhythm monitoring. The high-pass filter helps reduce wandering baseline and the low-pass filter helps reduce 50 or 60 Hz power line noise (the power line network frequency differs between 50 and 60 Hz in different countries). In diagnostic mode, the high-pass filter is set at 0.05 Hz, which allows accurate ST segments to be recorded. The low-pass filter is set to 40, 100, or 150 Hz. Consequently, the monitor mode ECG display is more filtered than diagnostic mode, because its passband is narrower.[11]
[edit] Leads


Graphic showing the relationship between positive electrodes, depolarization wavefronts (or mean electrical vectors), and complexes displayed on the ECG.
The word lead has two meanings in electrocardiography: it refers to either the wire that connects an electrode to the electrocardiograph, or (more commonly) to a combination of electrodes that form an imaginary line in the body along which the electrical signals are measured. Thus, the term loose lead artifact uses the former meaning, while the term 12 lead ECG uses the latter. In fact, a 12 lead electrocardiograph usually only uses 10 wires/electrodes. The latter definition of lead is the one used here.
An electrocardiogram is obtained by measuring electrical potential between various points of the body using a biomedical instrumentation amplifier. A lead records the electrical signals of the heart from a particular combination of recording electrodes which are placed at specific points on the patient's body.
When a depolarization wavefront (or mean electrical vector) moves toward a positive electrode, it creates a positive deflection on the ECG in the corresponding lead.
When a depolarization wavefront (or mean electrical vector) moves away from a positive electrode, it creates a negative deflection on the ECG in the corresponding lead.
When a depolarization wavefront (or mean electrical vector) moves perpendicular to a positive electrode, it creates an equiphasic (or isodiphasic) complex on the ECG. It will be positive as the depolarization wavefront (or mean electrical vector) approaches (A), and then become negative as it passes by (B).
There are two types of leads—unipolar and bipolar. The former have an indifferent electrode at the center of the Einthoven’s triangle (which can be likened to the ‘neutral’ of a wall socket) at zero potential. The direction of these leads is from the “center” of the heart radially outward. These include the precordial (chest) leads and augmented limb leads—VR, VL, & VF. The bipolar type, in contrast, has both electrodes at some potential, with the direction of the corresponding lead being from the electrode at lower potential to the one at higher potential, e.g., in limb lead I, the direction is from left to right. These include the limb leads—I, II, and III.
Note that the colouring scheme for leads varies by country.
[edit] Limb
Leads I, II and III are the so-called limb leads because at one time, the subjects of electrocardiography had to literally place their arms and legs in buckets of salt water in order to obtain signals for Einthoven's string galvanometer. They form the basis of what is known as Einthoven's triangle.[5] Eventually, electrodes were invented that could be placed directly on the patient's skin. Even though the buckets of salt water are no longer necessary, the electrodes are still placed on the patient's arms and legs to approximate the signals obtained with the buckets of salt water. They remain the first three leads of the modern 12 lead ECG.
Lead I is a dipole with the negative (white) electrode on the right arm and the positive (black) electrode on the left arm.
Lead II is a dipole with the negative (white) electrode on the right arm and the positive (red) electrode on the left leg.
Lead III is a dipole with the negative (black) electrode on the left arm and the positive (red) electrode on the left leg.

Lead I

Lead II

Einthoven's triangle (captions in Dutch, but relationships visible)
[edit] Augmented limb


Proper placement of the limb leads, color coded as recommended by the American Health Association [3]
Leads aVR, aVL, and aVF are 'augmented limb leads'. They are derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of 'Wilson's central terminal', which is derived by adding leads I, II, and III together and plugging them into the negative terminal of the ECG machine. This zeroes out the negative electrode and allows the positive electrode to become the "exploring electrode" or a unipolar lead. This is possible because Einthoven's Law states that I + (-II) + III = 0. The equation can also be written I + III = II. It is written this way (instead of I - II + III = 0) because Einthoven reversed the polarity of lead II in Einthoven's triangle, possibly because he liked to view upright QRS complexes. Wilson's central terminal paved the way for the development of the augmented limb leads aVR, aVL, aVF and the precordial leads V1, V2, V3, V4, V5, and V6.
Lead aVR or "augmented vector right" has the positive electrode (white) on the right arm. The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm.
Lead aVL or "augmented vector left" has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm.
Lead aVF or "augmented vector foot" has the positive (red) electrode on the left leg. The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg.
The augmented limb leads aVR, aVL, and aVF are amplified in this way because the signal is too small to be useful when the negative electrode is Wilson's central terminal. Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane.
In modern digital equipment, the augmented leads are derived from the limb leads by simple calculation:
aVR = -(I + II)/2
aVL = I - II/2
aVF = II - I/2
[edit] Precordial
The precordial leads V1, V2, V3, V4, V5, and V6 are placed directly on the chest. Because of their close proximity to the heart, they do not require augmentation. Wilson's central terminal is used for the negative electrode, and these leads are considered to be unipolar. The precordial leads view the heart's electrical activity in the so-called horizontal plane. The heart's electrical axis in the horizontal plane is referred to as the Z axis.
Leads V1, V2, and V3 are referred to as the right precordial leads and V4, V5, and V6 are referred to as the left precordial leads.
The QRS complex should be negative in lead V1 and positive in lead V6. The QRS complex should show a gradual transition from negative to positive between leads V2 and V4. The equiphasic lead is referred to as the transition lead. When the transition occurs earlier than lead V3, it is referred to as an early transition. When it occurs later than lead V3, it is referred to as a late transition. There should also be a gradual increase in the amplitude of the R wave between leads V1 and V4. This is known as R wave progression. Poor R wave progression is a nonspecific finding. It can be caused by conduction abnormalities, myocardial infarction, cardiomyopathy, and other pathological conditions.
Lead V1 is placed in the fourth intercostal space to the right of the sternum.
Lead V2 is placed in the fourth intercostal space to the left of the sternum.
Lead V3 is placed directly between leads V2 and V4.
Lead V4 is placed in the fifth intercostal space in the midclavicular line (even if the apex beat is displaced).
Lead V5 is placed horizontally with V4 in the anterior axillary line
Lead V6 is placed horizontally with V4 and V5 in the midaxillary line.
[edit] Ground
An additional electrode (usually green) is present in modern four-lead and twelve-lead ECGs. This is the ground lead and is placed on the right leg by convention, although in theory it can be placed anywhere on the body. With a three-lead ECG, when one dipole is viewed, the remaining lead becomes the ground lead by default.
[edit] Waves and intervals


Schematic representation of normal ECG
A typical ECG tracing of a normal heartbeat (or cardiac cycle) consists of a P wave, a QRS complex and a T wave. A small U wave is normally visible in 50 to 75% of ECGs. The baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave. The four deflections were originally named ABCDE but renamed PQRST after correction for artifacts introduced by early amplifiers.[citation needed]
[edit] P wave
During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG, which is upright in II, III, and aVF (since the general electrical activity is going toward the positive electrode in those leads), and inverted in aVR (since it is going away from the positive electrode for that lead). A P wave must be upright in leads II and aVF and inverted in lead aVR to designate a cardiac rhythm as Sinus Rhythm.
The relationship between P waves and QRS complexes helps distinguish various cardiac arrhythmias.
The shape and duration of the P waves may indicate atrial enlargement.
[edit] QRS complex


Various QRS complexes with nomenclature.
See also: Electrical conduction system of the heart
The QRS complex is a structure on the ECG that corresponds to the depolarization of the ventricles. Because the ventricles contain more muscle mass than the atria, the QRS complex is larger than the P wave. In addition, because the His/Purkinje system coordinates the depolarization of the ventricles, the QRS complex tends to look "spiked" rather than rounded due to the increase in conduction velocity. A normal QRS complex is 0.08 to 0.12 sec (80 to 120 ms) in duration represented by three small squares or less, but any abnormality of conduction takes longer, and causes widened QRS complexes.
Not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any combination of these waves can be referred to as a QRS complex. However, correct interpretation of difficult ECGs requires exact labeling of the various waves. Some authors use lowercase and capital letters, depending on the relative size of each wave. For example, an Rs complex would be positively deflected, while a rS complex would be negatively deflected. If both complexes were labeled RS, it would be impossible to appreciate this distinction without viewing the actual ECG.
The duration, amplitude, and morphology of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
Q waves can be normal (physiological) or pathological. Pathological Q waves refer to Q waves that have a height of 25% or more than that of the partner R wave and/or have a width of greater than 0.04 seconds. Normal Q waves, when present, represent depolarization of the interventricular septum. For this reason, they are referred to as septal Q waves, and can be appreciated in the lateral leads I, aVL, V5 and V6.
Q waves greater than 1/4 the height of the R wave, greater than 0.04 sec (40 ms) in duration, or in the right precordial leads are considered to be abnormal, and may represent myocardial infarction.
"Buried" inside the QRS wave is the atrial repolarization wave, which resembles an inverse P wave. It is far smaller in magnitude than the QRS and is therefore obscured by it.


Animation of a normal ECG wave.
[edit] PR/PQ interval
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It is usually 120 to 200 ms long. On an ECG tracing, this corresponds to 3 to 5 small boxes. In case a Q wave was measured with a ECG the PR interval is also commonly named PQ interval instead.
A PR interval of over 200 ms may indicate a first degree heart block.
A short PR interval may indicate a pre-excitation syndrome via an accessory pathway that leads to early activation of the ventricles, such as seen in Wolff-Parkinson-White syndrome.
A variable PR interval may indicate other types of heart block.
PR segment depression may indicate atrial injury or pericarditis.
Variable morphologies of P waves in a single ECG lead is suggestive of an ectopic pacemaker rhythm such as wandering pacemaker or multifocal atrial tachycardia
[edit] ST segment
Main article: Myocardial infarction
The ST segment connects the QRS complex and the T wave and has a duration of 0.08 to 0.12 sec (80 to 120 ms). It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the T wave. However, since it is usually difficult to determine exactly where the ST segment ends and the T wave begins, the relationship between the RT segment and T wave should be examined together. The typical ST segment duration is usually around 0.08 sec (80 ms). It should be essentially level with the PR and TP segment.
The normal ST segment has a slight upward concavity.
Flat, downsloping, or depressed ST segments may indicate coronary ischemia.
ST segment elevation may indicate myocardial infarction. An elevation of >1mm and longer than 80 milliseconds following the J-point. This measure has a false positive rate of 15-20% (which is slightly higher in women than men) and a false negative rate of 20-30%.[12]
[edit] T wave
The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period (or vulnerable period).
In most leads, the T wave is positive. However, a negative T wave is normal in lead aVR. Lead V1 may have a positive, negative, or biphasic T wave. In addition, it is not uncommon to have an isolated negative T wave in lead III, aVL, or aVF.
Inverted (or negative) T waves can be a sign of coronary ischemia, Wellens' syndrome, left ventricular hypertrophy, or CNS disorder.
Tall or "tented" symmetrical T waves may indicate hyperkalemia. Flat T waves may indicate coronary ischemia or hypokalemia.
The earliest electrocardiographic finding of acute myocardial infarction is sometimes the hyperacute T wave, which can be distinguished from hyperkalemia by the broad base and slight asymmetry.
When a conduction abnormality (e.g., bundle branch block, paced rhythm) is present, the T wave should be deflected opposite the terminal deflection of the QRS complex. This is known as appropriate T wave discordance.
[edit] QT interval
Main article: QT interval
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. Normal values for the QT interval are between 0.30 and 0.44 seconds.[citation needed] The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome. The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval for the heart rate. The QT interval represents on an ECG the total time needed for the ventricles to depolarize and repolarize.
The most commonly used method for correcting the QT interval for rate is the one formulated by Bazett and published in 1920.[13] Bazett's formula is , where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, this formula tends to be inaccurate, and over-corrects at high heart rates and under-corrects at low heart rates.
QTc may also be found via the following formula: QTc = QT + 1.75(Ventricular Rate - 60).
[edit] U wave


An electrocardiogram of an 18-year-old man showing U waves, most evident in lead V3.
The U wave is not always seen. It is typically small, and, by definition, follows the T wave. U waves are thought to represent repolarization of the papillary muscles or Purkinje fibers.[14] Prominent U waves are most often seen in hypokalemia, but may be present in hypercalcemia, thyrotoxicosis, or exposure to digitalis, epinephrine, and Class 1A and 3 antiarrhythmics, as well as in congenital long QT syndrome and in the setting of intracranial hemorrhage. An inverted U wave may represent myocardial ischemia or left ventricular volume overload.[15]
[edit] Clinical lead groups


Diagram showing the contiguous leads in the same color
Main article: Myocardial infarction
There are twelve leads in total, each recording the electrical activity of the heart from a different perspective, which also correlate to different anatomical areas of the heart for the purpose of identifying acute coronary ischemia or injury. Two leads that look at the same anatomical area of the heart are said to be contiguous (see color coded chart).
The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior (or diaphragmatic) surface.
The lateral leads (I, aVL, V5 and V6) look at the electrical activity from the vantage point of the lateral wall of left ventricle. The positive electrode for leads I and aVL should be located distally on the left arm and because of which, leads I and aVL are sometimes referred to as the high lateral leads. Because the positive electrodes for leads V5 and V6 are on the patient's chest, they are sometimes referred to as the low lateral leads.
The septal leads, V1 and V2 look at electrical activity from the vantage point of the septal wall of the ventricles.
The anterior leads, V3 and V4 look at electrical activity from the vantage point of the anterior surface of the heart.
In addition, any two precordial leads that are next to one another are considered to be contiguous. For example, even though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another.
Lead aVR offers no specific view of the left ventricle. Rather, it views the inside of the endocardial wall to the surface of the right atrium, from its perspective on the right shoulder.
[edit] Axis


Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.
The heart's electrical axis refers to the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane. It is usually oriented in a right shoulder to left leg direction, which corresponds to the left inferior quadrant of the hexaxial reference system, although -30o to +90o is considered to be normal.
Normal
-30o to 90o
Normal
Normal
Left axis deviation
-30o to -90o
May indicate left anterior fascicular block or Q waves from inferior MI.
Left axis deviation is considered normal in pregnant women and those with emphysema.
Right axis deviation
+90o to +180o
May indicate left posterior fascicular block, Q waves from high lateral MI, or a right ventricular strain pattern.
Right deviation is considered normal in children and is a standard effect of dextrocardia.
Extreme right axis deviation
+180o to -90o
Is rare, and considered an 'electrical no-man's land'.

In the setting of right bundle branch block, right or left axis deviation may indicate bifascicular block.
[edit] Electrocardiogram heterogeneity
Electrocardiogram (ECG) heterogeneity is a measurement of the amount of variance between one ECG waveform and the next. This heterogeneity can be measured by placing multiple ECG electrodes on the chest and by then computing the variance in waveform morphology across the signals obtained from these electrodes. Recent research suggests that ECG heterogeneity often precedes dangerous cardiac arrhythmias.
[edit] Background
There are over 350,000 cases of sudden cardiac death (SCD) in the United States each year, and over twenty percent of these cases involve people with no outward signs of serious heart disease. For decades, researchers have been attempting to come up with methods of identifying electrocardiogram (ECG) patterns that reliably precede dangerous arrhythmias. As these methods are found, devices are being created that monitor the heart in order to detect the onset of dangerous rhythms and to correct them before they cause death.
[edit] Research
Research being conducted[16] suggests that a crescendo in ECG heterogeneity, both in the R-wave and the T-wave, often signals the start of ventricular fibrillation. In patients with coronary artery disease, exercise increases T-wave heterogeneity, but this effect is not seen in normal patients. These results, when combined with other pieces of emerging evidence, suggest that R-wave and T-wave heterogeneity both have predictive value.
[edit] Future applications
In the future, researchers hope to automate the process of heterogeneity detection and to augment the clinical evidence supporting the validity of ECG heterogeneity as a predictor of arrhythmia. Someday soon, implantable devices may be programmed to measure and TRACK heterogeneity. These devices could potentially help ward off arrhythmias by stimulating nerves such as the vagus nerve, by delivering drugs such as beta-blockers, and if necessary, by defibrillating the heart.[17]
[edit] See also
Electroencephalography
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"EEG" redirects here. For other uses, see EEG (disambiguation).
"Brainwave" redirects here. For the comic book character, see Brainwave (comics).
Not to be confused with electrocardiography or ECG.

This article needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (March 2009)


Epileptic spike and wave discharges monitored with EEG.
Electroencephalography (EEG) is the recording of electrical activity along the scalp produced by the firing of neurons within the brain. In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from multiple electrodes placed on the scalp. In neurology, the main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study. A secondary clinical use of EEG is in the diagnosis of coma and encephalopathies. EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.
Derivatives of the EEG technique include evoked potentials (EP), which involves averaging the EEG activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). Event-related potentials refer to averaged EEG responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research.
Contents
[hide]
1 Source of EEG activity
2 Clinical use
3 Research use
4 Method
4.1 Limitations
4.2 EEG vs fMRI and PET
5 Normal activity
5.1 Comparison table
5.2 Wave patterns
6 Artifacts
6.1 Biological artifacts
6.2 Environmental artifacts
6.3 Artifact correction
7 Abnormal activity
8 History
9 Various uses
9.1 Games
10 Images
11 See also
12 References
13 External links
//
[edit] Source of EEG activity
The electrical activity of the brain can be described in spatial scales from the currents within a single dendritic spine to the relatively gross potentials that the EEG records from the scalp, much the same way that the economics can be studied from the level of a single individual's personal finances to the macro-economics of nations. Neurons, or nerve cells, are electrically active cells which are primarily responsible for carrying out the brain's functions. Neurons create action potentials, which are discrete electrical signals that travel down axons and cause the release of chemical neurotransmitters at the synapse, which is an area of near contact between two neurons. This neurotransmitter then fits into a receptor in the dendrite or body of the neuron that is on the other side of the synapse, the post-synaptic neuron. The neurotransmitter, when combined with the receptor, typically causes an electrical current within dendrite or body of the post-synaptic neuron. Thousands of post-synaptic currents from a single neuron's dendrites and body then sum up to cause the neuron to generate an action potential (or not). This neuron then synapses on other neurons, and so on.
It is generally accepted that the activity measured by EEG are electrical potentials created by the post-synaptic currents, rather than by action potentials. More specifically, the scalp electrical potentials that produce EEG are due to the extracellular ionic currents caused by dendritic electrical activity (whereas the fields producing magnetoencephalographic signals are associated with intracellular ionic currents).
Although post-synaptic potentials generate the EEG signal, it is not possible for a scalp EEG to assess the activity within a single dendrite or neuron. Rather, a surface EEG reading is the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation, radial to the scalp. Currents that are tangential to the scalp are not picked up by the EEG. The EEG therefore benefits from the parallel, radial arrangement of apical dendrites in the cortex. Because voltage fields fall off with the fourth power of the radius, activity from deep sources is more difficult to detect than currents near the skull.[1]
Scalp EEG activity shows oscillations at a variety of frequencies. Several of these oscillations have characteristic frequency ranges, spatial distributions and are associated with different states of brain functioning (e.g., waking and the various sleep stages). These oscillations represent synchronized activity over a network of neurons. The neuronal networks underlying some of these oscillations are understood (e.g., the thalamocortical resonance underlying sleep spindles), while many others are not (e.g., the system that generates the posterior basic rhythm).
[edit] Clinical use
A routine clinical EEG recording typically lasts 20-40 minutes (plus preparation time) and usually involves recording from 25 scalp electrodes. Routine EEG is typically used in the following clinical circumstances:
to distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants.
to differentiate "organic" encephalopathy or delirium from primary psychiatric syndromes such as catatonia
to serve as an adjunct test of brain death
to prognosticate, in certain instances, in patients with coma
At times, a routine EEG is not sufficient, particularly when it is necessary to record a patient while he/she is having a seizure. In this case, the patient may be admitted to the hospital for days or even weeks, while EEG is constantly being recorded (along with time-synchronized video and audio recording). A recording of an actual seizure (i.e., an ictal recording, rather than an inter-ictal recording of a possibly epileptic patient at some period between seizures) can give significantly better information about whether or not a spell is an epileptic seizure and the focus in the brain from which the seizure activity eminates.
Epilepsy monitoring is typically done
to distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants.
to characterize seizures for the purposes of treatment
to localize the region of brain from which a seizure originates for work-up of possible seizure surgery
Additionally, EEG may be used to monitor certain procedures:
to monitor the depth of anesthesia
as an indirect indicator of cerebral perfusion in carotid endarterectomy
to monitor amobarbital effect during the Wada test
EEG can also be used in intensive care units for brain function monitoring:
to monitor for non-convulsive seizures/non-convulsive status epilepticus
to monitor the effect of sedative/anesthesia in patients in medically induced coma (for treatment of refractory seizures or increased intracranial pressure)
to monitor for secondary brain damage in conditions such as subarachnoid hemorrhage (currently a research method)
If a patient with epilepsy is being considered for resective surgery, it is often necessary to localize the focus (source) of the epileptic brain activity with a resolution greater than what is provided by scalp EEG. This is because the cerebrospinal fluid, skull and scalp smear the electrical potentials recorded by scalp EEG. In these cases, neurosurgeons typically implant strips and grids of electrodes (or penetrating depth electodes) under the dura mater, through either a craniotomy or a burr hole. The recording of these signals is referred to as electrocorticography (ECoG}, subdural EEG (sdEEG) or intracranial EEG (icEEG)--all terms for the same thing. The signal recorded from ECoG is on a different scale of activity than the brain activity recorded from scalp EEG. Low voltage, high frequency components that cannot be seen easily (or at all) in scalp EEG can be seen clearly in ECoG. Further, smaller electrodes (which cover a smaller parcel of brain suface) allow even lower voltage, faster components of brain activity to be seen. Some clinical sites record from penetrating microelectrodes.
[edit] Research use

An early EEG recording, obtained by Hans Berger in 1924. The upper tracing is EEG, and the lower is a 10 Hz timing signal.
EEG, and its derivative, ERPs, are used extensively in neuroscience, cognitive science, cognitive psychology, and psychophysiological research. Many techniques used in research contexts are not standardized sufficiently to be used in the clinical context.
A different method to study brain function is functional magnetic resonance imaging (fMRI). Some benefits of EEG compared to fMRI include:
Hardware costs are significantly lower for EEG sensors versus an fMRI machine
EEG sensors can be deployed into a wider variety of environments than a bulky, immobile fMRI machine
EEG enables higher temporal resolution, on the order of milliseconds, rather than seconds
EEG is relatively tolerant of subject movement versus an fMRI (where the subject must remain completely still)
EEG is silent, which allows for better study of the responses to auditory stimuli
EEG does not cause claustrophobia
Limitations of EEG as compared with fMRI include:
Significantly less spatial resolution
Need to apply electrodes to the scalp (which may bother people with severe tactile sensitivities, e.g., some individuals with autism)
ERP studies require relatively simple paradigms, compared with block-design fMRI studies
EEG recordings have successfully obtained simultaneously with fMRI scans, though successful simultaneous recording requires that several technical issues be overcome, such as the presence of ballistocardiographic artifact, MRI pulse artifact and the induction of electrical currents in EEG wires that move within the strong magnetic fields of the MRI.
EEG also has some charateristics that compare favorably with behavioral testing:
EEG can detect covert processing (i.e., that which does not require a response)
EEG can be used in subjects who are incapable of making a motor response
Some ERP components can be detected even when the subject is not attending to the stimuli
As compared with other reaction time paradigms, ERPs can elucidate stages of processing (rather than just the final end result)
[edit] Method


Computer Electroencephalograph Neurovisor-BMM 40
In conventional scalp EEG, the recording is obtained by placing electrodes on the scalp with a conductive gel or paste, usually after preparing the scalp area by light abrasion to reduce impedance due to dead skin cells. Many systems typically use electrodes, each of which is attached to an individual wire. Some systems use caps or nets into which electrodes are embedded; this is particularly common when high-density arrays of electrodes are needed.
Electrode locations and names are specified by the International 10–20 system for most clinical and research applications (except when high-density arrays are used). This system ensures that the naming of electrodes is consistent across laboratories. In most clinical applications, 19 recording electrodes (plus ground and system reference) are used. A smaller number of electrodes are typically used when recording EEG from neonates. Additional electrodes can be added to the standard set-up when a clinical or research application demands increased spatial resolution for a particular area of the brain. High-density arrays (typically via cap or net) can contain up to 256 electrodes more-or-less evenly spaced around the scalp.
Each electrode is connected to one input of a differential amplifier (one amplifier per pair of electrodes); a common system reference electrode is connected to the other input of each differential amplifier. These amplifiers amplify the voltage between the active electrode and the reference (typically 1,000–100,000 times, or 60–100 dB of voltage gain). In analog EEG, the signal is then filtered (next paragraph), and the EEG signal is output as the deflection of pens as paper passes underneath. Most EEG systems these days, however, are digital, and the amplified signal is digitized via an analog-to-digital converter, after being passed through an anti-aliasing filter. Analog-to-digital sampling typically occurs at 256-512 Hz in clinical scalp EEG; sampling rates of up to 20 kHz are used in some research applications.
During the recording, a series of activation procedures may be used. These procedures may induce normal or abnormal EEG activity that might not otherwise be seen. These procedures include hyperventilation, photic stimulation (with a strobe light), eye closure, mental activity, sleep and sleep deprivation. During (inpatient) epilepsy monitoring, a patient's typical seizure medications may be withdrawn.
The digital EEG signal is stored electronically and can be filtered for display. Typical settings for the high-pass filter and a low-pass filter are 0.5-1 Hz and 35–70 Hz, respectively. The high-pass filter typically filters out slow artifact, such as electrogalvanic signals and movement artifact, whereas the low-pass filter filters out high-frequency artifacts, such as electromyographic signals. An additional notch filter is typically used to remove artifact caused by electrical power lines (60 Hz in the United States and 50 Hz in many other countries).
As part of an evaluation for epilepsy surgery, it may be necessary to insert electrodes near the surface of the brain, under the surface of the dura mater. This is accomplished via burr hole or craniotomy. This is referred to variously as "electrocorticography (ECoG)", "intracranial EEG (I-EEG)" or "subdural EEG (SD-EEG)". Depth electrodes may also be placed into brain structures, such as the amygdala or hippocampus, structures which are common epileptic foci and may not be "seen" clearly by scalp EEG. The electrocorticographic signal is processed in the same manner as digital scalp EEG (above), with a couple of caveats. ECoG is typically recorded at higher sampling rates than scalp EEG because of the requirements of Nyquist theorem—the subdural signal is composed of a higher predominance of higher frequency components. Also, many of the artifacts which affect scalp EEG do not impact ECoG, and therefore display filtering is often not needed.
A typical adult human EEG signal is about 10µV to 100 µV in amplitude when measured from the scalp [2] and is about 10–20 mV when measured from subdural electrodes.
Since an EEG voltage signal represents a difference between the voltages at two electrodes, the display of the EEG for the reading encephalographer may be set up in one of several ways. The representation of the EEG channels is referred to as a montage.
Bipolar montage
Each channel (i.e., waveform) represents the difference between two adjacent electrodes. The entire montage consists of a series of these channels. For example, the channel "Fp1-F3" represents the difference in voltage between the Fp1 electrode and the F3 electrode. The next channel in the montage, "F3-C3," represents the voltage difference between F3 and C3, and so on through the entire array of electrodes.
Referential montage
Each channel represents the difference between a certain electrode and a designated reference electrode. There is no standard position at which this reference is always placed; it is, however, at a different position than the "recording" electrodes. Midline positions are often used because they do not amplify the signal in one hemisphere vs. the other. Another popular reference is "linked ears," which is a physical or mathematical average of electrodes attached to both earlobes or mastoids.
Average reference montage
The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each channel.
Laplacian montage
Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes.
When analog (paper) EEGs are used, the technologist switches between montages during the recording in order to highlight or better characterize certain features of the EEG. With digital EEG, all signals are typically digitized and stored in a particular (usually referential) montage; since any montage can be constructed mathematically from any other, the EEG can be viewed by the electroencephalographer in any display montage that is desired.
The EEG is read by a neurologist, optimally one who has specific training in the interpretation of EEGs. This is done by visual inspection of the waveforms. The use of computer signal processing of the EEG—so-called quantitative EEG—is somewhat controversial when used for clinical purposes (although there are many research uses).
[edit] Limitations
EEG has several limitations. Most important is its poor spatial resolution. EEG is most sensitive to a particular set of post-synaptic potentials: those which are generated in superficial layers of the cortex, on the crests of gyri directly abutting the skull and radial to the skull. Dendrites which are deeper in the cortex, inside sulci, in midline or deep structures (such as the cingulate gyrus or hippocampus), or producing currents which are tangential to the skull, have far less contribution to the EEG signal.
The meninges, cerebrospinal fluid and skull "smear" the EEG signal, obscuring its intracranial source.
It is mathematically impossible to reconstruct a unique intracranial current source for a given EEG signal, as some currents produce potentials that cancel each other out. This is referred to as the inverse problem. However, much work has been done to produce remarkably good estimates of, at least, a localized electric dipole that represents the recorded currents.
[edit] EEG vs fMRI and PET
EEG has several strong sides as a tool of exploring brain activity; for example, its time resolution is very high (on the level of a single millisecond). Other methods of looking at brain activity, such as PET and fMRI have time resolution between seconds and minutes. EEG measures the brain's electrical activity directly, while other methods record changes in blood flow (e.g., SPECT, fMRI) or metabolic activity (e.g., PET), which are indirect markers of brain electrical activity. EEG can be used simultaneously with fMRI so that high-temporal-resolution data can be recorded at the same time as high-spatial-resolution data, however, since the data derived from each occurs over a different time course, the data sets do not necessarily represent the exact same brain activity. There are technical difficulties associated with combining these two modalities, including the need to remove the MRI gradient artifact present during MRI acquisition and the ballistocardiographic artifact (resulting from the pulsatile motion of blood and tissue) from the EEG. Furthermore, currents can be induced in moving EEG electrode wires due to the magnetic field of the MRI.
EEG can be recorded at the same time as MEG so that data from these complimentary high-time-resolution techniques can be combined.
[edit] Normal activity


One second of EEG signal
The EEG is typically described in terms of (1) rhythmic activity and (2) transients. The rhythmic activity is divided into bands by frequency. To some degree, these frequency bands are a matter of nomenclature (i.e., any rhythmic activity between 8-12 Hz can be described as "alpha"), but these designations arose because rhythmic activity within a certain frequency range was noted to have a certain distribution over the scalp or a certain biological significance.
Most of the cerebral signal observed in the scalp EEG falls in the range of 1-20 Hz (activity below or above this range is likely to be artifactual, under standard clinical recording techniques).
[edit] Comparison table
Comparison of EEG bands
Type
Frequency (Hz)
Location
Normally
Pathologically
Delta
up to 4
frontally in adults, posteriorly in children; high amplitude waves
adults slow wave sleep
in babies
subcortical lesions
diffuse lesions
metabolic encephalopathy hydrocephalus
deep midline lesions.
Theta
4 - 7 Hz

young children
drowsiness or arousal in older children and adults
idling
focal subcortical lesions
metabolic encephalopathy
deep midline disorders
some instances of hydrocephalus
Alpha
8 - 12 Hz
posterior regions of head, both sides, higher in amplitude on dominant side. Central sites (c3-c4) at rest .
relaxed/reflecting
closing the eyes
coma
Beta
12 - 30 Hz
both sides, symmetrical distribution, most evident frontally; low amplitude waves
alert/working
active, busy or anxious thinking, active concentration
benzodiazepines
Gamma
30 – 100 +

certain cognitive or motor functions

[edit] Wave patterns


delta waves.
Delta is the frequency range up to 3 Hz. It tends to be the highest in amplitude and the slowest waves. It is seen normally in adults in slow wave sleep. It is also seen normally in babies. It may occur focally with subcortical lesions and in general distribution with diffuse lesions, metabolic encephalopathy hydrocephalus or deep midline lesions. It is usually most prominent frontally in adults (e.g. FIRDA - Frontal Intermittent Rhythmic Delta) and posteriorly in children (e.g. OIRDA - Occipital Intermittent Rhythmic Delta).


theta waves.
Theta is the frequency range from 4 Hz to 7 Hz. Theta is seen normally in young children. It may be seen in drowsiness or arousal in older children and adults; it can also be seen in meditation.[3] Excess theta for age represents abnormal activity. It can be seen as a focal disturbance in focal subcortical lesions; it can be seen in generalized distribution in diffuse disorder or metabolic encephalopathy or deep midline disorders or some instances of hydrocephalus. On the contrary this range has been associated with reports of relaxed, meditative, and creative states.


alpha waves.
Alpha is the frequency range from 8 Hz to 12 Hz. Hans Berger named the first rhythmic EEG activity he saw, the "alpha wave." This is activity in the 8-12 Hz range seen in the posterior regions of the head on both sides, being higher in amplitude on the dominant side. It is brought out by closing the eyes and by relaxation. It was noted to attenuate with eye opening or mental exertion. This activity is now referred to as "posterior basic rhythm," the "posterior dominant rhythm" or the "posterior alpha rhythm." The posterior basic rhythm is actually slower than 8 Hz in young children (therefore technically in the theta range). In addition to the posterior basic rhythm, there are two other normal alpha rhythms that are typically discussed: the mu rhythm and a temporal "third rhythm". Alpha can be abnormal; for example, an EEG that has diffuse alpha occurring in coma and is not responsive to external stimuli is referred to as "alpha coma".


Sensorimotor rhythm aka mu rhythm.
Mu rhythm is alpha-range activity that is seen over the sensorimotor cortex. It characteristically attenuates with movement of the contralateral arm (or mental imagery of movement of the contralateral arm).


beta waves.
Beta is the frequency range from 12 Hz to about 30 Hz. It is seen usually on both sides in symmetrical distribution and is most evident frontally. Low amplitude beta with multiple and varying frequencies is often associated with active, busy or anxious thinking and active concentration. Rhythmic beta with a dominant set of frequencies is associated with various pathologies and drug effects, especially benzodiazepines. Activity over about 25 Hz seen in the scalp EEG is rarely cerebral (i.e., it is most often artifactual). It may be absent or reduced in areas of cortical damage. It is the dominant rhythm in patients who are alert or anxious or who have their eyes open.


gamma waves.
Gamma is the frequency range approximately 26–100 Hz. Because of the filtering properties of the skull and scalp, gamma rhythms can only be recorded from electrocorticography or possibly with magnetoencephalography. Gamma rhythms are thought to represent binding of different populations of neurons together into a network for the purpose of carrying out a certain cognitive or motor function.
"Ultra-slow" or "near-DC" activity is recorded using DC amplifiers in some research contexts. It is not typically recorded in a clinical context because the signal at these frequencies is susceptible to a number of artifacts.
Some features of the EEG are transient rather than rhythmic. Spikes and sharp waves may represent seizure activity or interictal activity in individuals with epilepsy or a predisposition toward epilepsy. Other transient features are normal: vertex waves and sleep spindles are transient events which are seen in normal sleep.
It should also be noted that there are types of activity which are statistically uncommon but are not associated with dysfunction or disease. These are often referred to as "normal variants." The mu rhythm is an example of a normal variant.
The normal EEG varies by age. The neonatal EEG is quite different from the adult EEG. The EEG in childhood generally has slower frequency oscillations than the adult EEG.
The normal EEG also varies depending on state. The EEG is used along with other measurements (EOG, EMG) to define sleep stages in polysomnography. Stage I sleep (equivalent to drowsiness in some systems) appears on the EEG as drop-out of the posterior basic rhythm. There can be an increase in theta frequencies. Santamaria and Chiappa cataloged a number of the variety of patterns associated with drowsiness. Stage II sleep is characterized by sleep spindles--transient runs of rhythmic activity in the 12-14 Hz range (sometimes referred to as the "sigma" band) that have a frontal-central maximum. Most of the activity in Stage II is in the 3-6 Hz range. Stage III and IV sleep are defined by the presence of delta frequencies and are often referred to collectively as "slow-wave sleep." Stages I-IV comprise non-REM (or "NREM") sleep. The EEG in REM (rapid eye movement) sleep appears somewhat similar to the awake EEG.
EEG under general anesthesia depends on the type of anesthetic employed. With halogenated anesthetics, such as halothane or intravenous agents, such as propofol, a rapid (alpha or low beta), nonreactive EEG pattern is seen over most of the scalp, especially anteriorly; in some older terminology this was known as a WAR (widespread anterior rapid) pattern, contrasted with a WAIS (widespread slow) pattern associated with high doses of opiates. Anesthetic effects on EEG signals are beginning to be understood at the level of drug actions on different kinds of synapses and the circuits that allow synchronized neuronal activity (see: http://www.stanford.edu/group/maciverlab/).
[edit] Artifacts
[edit] Biological artifacts
Electrical signals detected along the scalp by an EEG, but that originate from non-cerebral origin are called artifacts. EEG data is almost always contaminated by such artifacts. The amplitude of artifacts can be quite large relative to the size of amplitude of the cortical signals of interest. This is one of the reasons why it takes considerable experience to correctly interpret EEGs clinically. Some of the most common types of biological artifacts include:
Eye-induced artifacts (includes eye blinks and eye movements)
EKG (cardiac) artifacts
EMG (muscle activation)-induced artifacts
Glossokinetic artifacts
Eye-induced artifacts are caused by the potential difference between the cornea and retina, which is quite large compared to cerebral potentials. When the eye is completely still, this does not affect EEG. But there are nearly always small or large reflexive eye movements, which generates a potential which is picked up in the frontopolar and frontal leads. Involuntary eye movements, known as saccades, are caused by ocular muscles, which also generate electromyographic potentials. Purposeful or reflexive eye blinking also generates electromyographic potentials, but more importantly there is reflexive movement of the eyeball during blinking which gives a characteristic artifactual appearance of the EEG (see Bell's phenomenon).
Eyelid fluttering artifacts of a characteristic type were previously called Kappa rhythm (or Kappa waves). It is usually seen in the prefrontal leads, that is, just over the eyes. Sometimes they are seen with mental activity. They are usually in the Theta (4–7 Hz) or Alpha (8–13 Hz) range. They were named because they were believed to originate from the brain. Later study revealed they were generated by rapid fluttering of the eyelids, sometimes so minute that it was difficult to see. They are in fact noise in the EEG reading, and should not technically be called a rhythm or wave. Therefore, current usage in electroencephalography refers to the phenomenon as an eyelid fluttering artifact, rather than a Kappa rhythm (or wave).[4]
Some of these artifacts are useful. Eye movements are very important in polysomnography, and is also useful in conventional EEG for assessing possible changes in alertness, drowsiness or sleep.
EKG artifacts are quite common and can be mistaken for spike activity. Because of this, modern EEG acquisition commonly includes a one-channel EKG from the extremities. This also allows the EEG to identify cardiac arrhythmias that are an important differential diagnosis to syncope or other episodic/attack disorders.
Glossokinetic artifacts are caused by the potential difference between the base and the tip of the tongue. Minor tongue movements can contaminate the EEG, especially in parkinsonian and tremor disorders.
[edit] Environmental artifacts
In addition to artifacts generated by the body, many artifacts originate from outside the body. Movement by the patient, or even just settling of the electrodes, may cause electrode pops, spikes originating from a momentary change in the impedance of a given electrode. Poor grounding of the EEG electrodes can cause significant 50 or 60 Hz artifact, depending on the local power system's frequency. A third source of possible interference can be the presence of an IV drip; such devices can cause rhythmic, fast, low-voltage bursts, which may be confused for spikes.
[edit] Artifact correction
Recently, source decomposition techniques have been used to correct or remove EEG contaminates. These techniques attempt to "unmix" the EEG signals into some number of underlying components. There are many source separation algorithms, often assuming various behaviors or natures of EEG. Regardless, the principle behind any particular method usually allow "remixing" only those components that would result in "clean" EEG by nullifying (zeroing) the weight of unwanted components.
[edit] Abnormal activity
Abnormal activity can broadly be separated into epileptiform and non-epileptiform activity. It can also be separated into focal or diffuse.
Focal epileptiform discharges represent fast, synchronous potentials in a large number of neurons in a somewhat discrete area of the brain. These can occur as interictal activity, between seizures, and represent an area of cortical irritability that may be predisposed to producing epileptic seizures. Interictal discharges are not wholly reliable for determining whether a patient has epilepsy nor where his/her seizure might originate. (See focal epilepsy.)
Generalized epileptiform discharges often have an anterior maximum, but these are seen synchronously throughout the entire brain. They are strongly suggestive of a generalized epilepsy.
Focal non-epileptiform abnormal activity may occur over areas of the brain where there is focal damage of the cortex or white matter. It often consists of an increase in slow frequency rhythms and/or a loss of normal higher frequency rhythms. It may also appear as focal or unilateral decrease in amplitude of the EEG signal.
Diffuse non-epileptiform abnormal activity may manifest as diffuse abnormally slow rhythms or bilateral slowing of normal rhythms, such as the PBR.
[edit] History
A timeline of the history of EEG is given by Swartz.[5] Richard Caton (1842–1926), a physician practicing in Liverpool, presented his findings about electrical phenomena of the exposed cerebral hemispheres of rabbits and monkeys in the British Medical Journal in 1875. In 1890, Beck published an investigation of spontaneous electrical activity of the brain of rabbits and dogs which included rhythmic oscillations altered by light.
In 1912, Russian physiologist, Vladimir Vladimirovich Pravdich-Neminsky published the first EEG and the evoked potential of the mammalian (dog).[6] In 1914, Cybulsky and Jelenska-Macieszyna photographed EEG-recordings of experimentally induced seizures.
German physiologist and psychiatrist Hans Berger (1873–1941) began his studies of the human EEG in 1920. He gave the device its name and is sometimes credited with inventing the EEG, though others had performed similar experiments. His work was later expanded by Edgar Douglas Adrian. In 1934, Fisher and Lowenback first demonstrated epileptiform spikes. In 1935 Gibbs, Davis and Lennox described interictal spike waves and the 3 cycles/s pattern of clinical absence seizures, which began the field of clinical electroencephalography. Subsequently, in 1936 Gibbs and Jasper reported the interictal spike as the focal signature of epilepsy. The same year, the first EEG laboratory opened at Massachusetts General Hospital.
Franklin Offner (1911–1999), professor of biophysics at Northwestern University developed a prototype of the EEG which incorporated a piezoelectric inkwriter called a Crystograph (the whole device was typically known as the Offner Dynograph).
In 1947, The American EEG Society was founded and the first International EEG congress was held. In 1953 Aserinsky and Kleitmean describe REM sleep.
In the 1950s, William Grey Walter developed an adjunct to EEG called EEG topography which allowed for the mapping of electrical activity across the surface of the brain. This enjoyed a brief period of popularity in the 1980s and seemed especially promising for psychiatry. It was never accepted by neurologists and remains primarily a research tool.
[edit] Various uses
The EEG has been used for many purposes besides the conventional uses of clinical diagnosis and conventional cognitive neuroscience. Neurofeedback remains an important extension, and in its most advanced form is also attempted as the basis of brain computer interfaces. There are many commercial products substantially based on the EEG.
Honda is attempting to develop a system to move its Asimo robot using EEG, a technology which it eventually hopes to incorporate into its automobiles. [7]
EEGs have been used as evidence in trials in the Indian state of Maharastra. [8]
[edit] Games
Magnetoencephalography
From Wikipedia, the free encyclopedia
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Magnetoencephalography (MEG) is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices (SQUIDs). These measurements are commonly used in both research and clinical settings. There are many uses for the MEG, including assisting surgeons in localizing a pathology, assisting researchers in determining the function of various parts of the brain, neurofeedback, and others.
Contents
[hide]
1 History of the MEG
2 The basis of the MEG signal
2.1 Sensor types
3 Magnetic shielding
3.1 Magnetically Shielded Room (MSR)
3.2 Active shielding system
4 Source localization
4.1 The inverse problem
4.2 Magnetic source imaging
4.3 Dipole model source localization
4.4 Lead-field-based imaging approach
4.5 Independent Component Analysis (ICA)
5 MEG use in the field
5.1 Focal epilepsy
5.2 Cognitive neuroscience applications of MEG
6 Comparison with other imaging techniques
6.1 MEG vs. EEG
7 See also
8 Further reading
9 References
//
[edit] History of the MEG
The MEG was first measured by University of Illinois physicist David Cohen in 1968,[1] before the availability of the SQUID, using only a copper induction coil as the detector. To reduce the magnetic background noise, the measurements were made in a magnetically shielded room. However, the insensitivity of this detector resulted in poor, noisy MEG signals, which were difficult to use. Then, later at MIT, he built a better shielded room, and used one of the first SQUID detectors, just developed by James E. Zimmerman, a researcher at Ford Motor Company,[2] to again measure the MEG.[3] This time the signals were almost as clear as an EEG, and stimulated the interest of physicists who had begun looking for uses of SQUIDs. Thus, the MEG began to be used, so that various types of spontaneous and evoked MEGs began to be measured.
At first, only a single SQUID detector was used, to successively measure the magnetic field at a number of points around the subject’s head. This was cumbersome, and in the 1980s, MEG manufacturers began to increase the number of sensors in the Dewar to cover a larger area of the head, using a correspondingly larger Dewar. Present-day MEG Dewars are helmet-shaped and contain as many as 300 sensors, covering most of the head, as shown in the first figure. In this way, MEGs of a subject or patient can now be accumulated rapidly and efficiently.
[edit] The basis of the MEG signal
Synchronized neuronal currents induce very weak magnetic fields that can be measured on MEG. However, the magnetic field of the brain is considerably smaller at 10 fT (femtotesla) for cortical activity and 103 fT for the human alpha rhythm than the ambient magnetic noise in an urban environment, which is on the order of 108 fT. Two essential problems of biomagnetism arise: weakness of the signal and strength of the competing environmental noise. The development of extremely sensitive measurement devices, SQUIDs, facilitates analysis of the brain's magnetic field and confronts the aforementioned problems.


origin of the brain's magnetic field; the electric current also produces the EEG
The MEG (and EEG) signals derive from the net effect of ionic currents flowing in the dendrites of neurons during synaptic transmission. In accordance with Maxwell's equations, any electrical current will produce an orthogonally oriented magnetic field. It is this field which is measured with MEG. The net currents can be thought of as current dipoles which are currents defined to have an associated position, orientation, and magnitude, but no spatial extent. According to the right-hand rule, a current dipole gives rise to a magnetic field that flows around the axis of its vector component.
In order to generate a signal that is detectable, approximately 50,000 active neurons are needed.[4] Since current dipoles must have similar orientations to generate magnetic fields that reinforce each other, it is often the layer of pyramidal cells in the cortex, which are generally perpendicular to its surface, that give rise to measurable magnetic fields. Furthermore, it is often bundles of these neurons located in the sulci of the cortex with orientations parallel to the surface of the head that project measurable portions of their magnetic fields outside of the head. Researchers are experimenting with various signal processing methods to try to find methods that will allow deep brain i.e., non-cortical, signal to be detected, but as of yet there is no clinically useful method available.
It is worth noting that action potentials do not usually produce an observable field, mainly because the currents associated with action potentials flow in opposite directions and the magnetic fields cancel out. However, action fields have been measured from peripheral nerves.
[edit] Sensor types
There are at least three types of different measurement device in use to detect magnetic fields.
Magnetometers. Such a device detects magnetic field directly using a loop.
Axial gradiometer. Such a device consists of two magnetometers placed in series (i.e. one above the other). The result coming from the device is the difference in magnetic flux at that point in space (a.k.a. first spatial derivative).
Planar gradiometer. Such a device consists of two magnetometers placed next to each other. The result coming from the device is difference in flux between the two loops.
Each sensor type responds differently to certain spatial signals.

This section requires expansion.
[edit] Magnetic shielding
Because the magnetic signals emitted by the brain are on the order of a few femtoteslas (1 fT = 10 − 15 T), shielding from external magnetic signals, including the Earth's magnetic field, is necessary. Appropriate magnetic shielding can be obtained by constructing rooms made of aluminium and mu-metal for reducing high-frequency and low-frequency noise, respectively.


Entrance to MSR, showing the separate shielding layers
[edit] Magnetically Shielded Room (MSR)
A Magnetically Shielded Room (MSR) model consists of three nested main layers. Each of these layers is made of a pure aluminium layer plus a high permeability ferromagnetic layer, similar in composition to molybdenum Permalloy. The ferromagnetic layer is supplied as 1 mm sheets, while the innermost layer is composed of four sheets in close contact, and the outer two layers are composed of three sheets each. Magnetic continuity is maintained by overlay strips. Insulating washers are used in the screw assemblies so that each main layer is electrically isolated to help eliminate radio frequencies, which degrade SQUID performance. Electrical continuity of the aluminium is also maintained by aluminium overlay strips to allow AC eddy-current shielding which is important at frequencies greater than 1 Hz. The junctions of the inner layer are often electroplated with silver or gold to improve conductivity of the aluminium layers. [5]
[edit] Active shielding system
Active systems are designed for three dimensional noise cancellation. To implement an active system, low-noise fluxgate magnetometers are mounted at the center of each surface and oriented orthogonally to it. This negatively feeds a DC amplifier through a low-pass network with a slow falloff to minimize positive feedback and oscillation. Built into the system are shaking and degaussing wires. Shaking wires increase the magnetic permeability, while the permanent degaussing wires are applied to all surfaces of the inner main layer to degauss the surfaces. [1] Moreover, noise cancellation algorithms can reduce both low-frequency and high-frequency noise. Modern systems have a noise floor of around 2 to 3 fT per √Hz above 1 Hz.
[edit] Source localization
[edit] The inverse problem
Main article: Inverse problem
In order to determine the location of the activity within the brain, advanced signal processing techniques are used which use the magnetic fields measured outside the head to estimate the location of that activity's source. This is referred to as the inverse problem. (The forward problem is a situation where we know where the source(s) is (are) and we are estimating the field at a given distance from the source(s).) The primary technical difficulty is that the inverse problem does not have a unique solution, i.e., there are infinite possible "correct" answers, and the problem of finding the best solution is itself the subject of intensive research. Adequate solutions can be derived using models involving prior knowledge of brain activity.
The source models can be either overdetermined or underdetermined. An overdetermined model may consist of a few point-like sources, whose locations are then estimated from the data. The underdetermined models may be used in cases where many different distributed areas are activated; there are several possible current distributions explaining the measurement results, but the most likely is selected. It is believed by some researchers in the field that more complex source models increase the quality of a solution. However this may decrease the robustness of the estimation and increasing the effects of forward model errors. Many experiments use simple models, reducing possible sources of error and decreasing the computation time to find a solution. Localization algorithms make use of the given source and head models to find a likely location for an underlying focal field generator. An alternative methodology involves performing independent component analysis first in order to segregate sources without using a forward model,[6] and then localizing the separated sources individually. This method has been shown to improve the signal-to-noise ratio of the data by correctly separating non-neuronal noise sources from neuronal sources, and has shown promise in segregating focal neuronal sources.
Localization algorithms using overdetermined models operate by successive refinement. The system is initialized with a first guess. Then a computation loop is started, in which a forward model is used to generate the magnetic field that would result from the current guess, and the guess then adjusted to reduce the difference between this estimated field and the measured field. This process is iterated until convergence.
Another approach is to ignore the ill-posed inverse problem and estimate the current at a fixed location. This method makes use of beamforming techniques. One such approach is the second-order technique known as Synthetic Aperture Magnetometry (SAM), which uses a linear weighting of the sensor channels to focus the array on a given target location. Whereas SAM uses the temporal domain, and a non linear fitting of the dipole, other approaches use the fourier transform of the signals and a linear dipole fit. The so-approximated sources can be used to compute to estimate the synchronisation of large brain networks [7].
[edit] Magnetic source imaging
The estimated source locations can be combined with magnetic resonance imaging (MRI) images to create magnetic source images (MSI). The two sets of data are combined by measuring the location of a common set of fiducial points marked during MRI with lipid markers and marked during MEG with electrified coils of wire that give off magnetic fields. The locations of the fiducial points in each data set are then used to define a common coordinate system so that superimposing ("coregistering") the functional MEG data onto the structural MRI data is possible.
A criticism of the use of this technique in clinical practice is that it produces colored areas with definite boundaries superimposed upon an MRI scan: the untrained viewer may not realize that the colors do not represent a physiological certainty, because of the relatively low spatial resolution of MEG, but rather a probability cloud derived from statistical processes. However, when the magnetic source image corroborates other data, it can be of clinical utility.
[edit] Dipole model source localization
A widely accepted source-modeling technique for MEG involves calculating a set of Equivalent Current Dipoles (ECDs), which assumes the underlying neuronal sources are focal. This dipole fitting procedure is non-linear at over-determined as the number of unknown dipole parameters is less than the number of MEG measurements [8]. Automated multiple dipole model algorithms such as MUSIC (MUltiple SIgnal Classification) and MSST (MultiStart Spatial and Temporal) modeling are applied to analysis of MEG responses. The limitations of dipole models to characterize neuronal responses has three main drawbacks: (1) significant difficulties in localizing extended sources with ECDs, (2) problems with accurately estimating the total number of dipoles in advance, and (3) the sensitivity of dipole location, especially with respect to depth in the brain.
[edit] Lead-field-based imaging approach
Unlike multiple-dipole modeling, lead-field-based modeling divides the source space into a grid containing a large number of dipoles. The inverse problem is to obtain the dipole moments for the grid nodes [9]. As the number of unknown dipole moments is much greater than the number of MEG sensors, the inverse solution is highly underdetermined. To compensate for this, additional constraints are needed to reduce non-uniqueness of the solution. The primary advantage of this system is that no prior specification for source model must be made. Other strengths include relatively low computation load and smooth source time-courses, both of which lead to simple statistical comparison. A weakness is that the spatial resolution is quite poor, and tends to provide distributed statistical reconstruction models, despite having focal generators.
[edit] Independent Component Analysis (ICA)
Independent Component Analysis (ICA), is another signal processing solution that separates different signals that are statistically independent in time. It is primarily used to remove artifacts such as blinking, eye muscle movement, facial muscle artifacts, cardiac artifacts, etc. from MEG and EEG signals that may be contaminated with outside noise [10]. However, ICA has poor resolution of highly correlated brain sources due to its fundamental statistical independence.
[edit] MEG use in the field
In research, MEG's primary use is the measurement of time courses of activity. MEG can resolve events with a precision of 10 milliseconds or less, while fMRI, which depends on changes in blood flow, can at best resolve events with a precision of several hundred milliseconds. MEG also accurately pinpoints sources in primary auditory, somatosensory and motor areas, whereas its use in creating functional maps of human cortex during more complex cognitive tasks is more limited; in those cases MEG should preferably be used in combination with fMRI. It should be noted, however, that neuronal (MEG) and hemodynamic (fMRI) data do not necessarily agree and the methods complement each other. However, the two signals may have a common source: it is known that there is a tight relationship between LFP (local field potentials) and BOLD (blood oxygenation level dependent) signals. Since the LFP is the source signal of MEG/EEG, MEG and BOLD signals may derive from the same source (though the BOLD signals are filtered through the hemodynamic response).
In 2007, a group of researchers have reported on successful attempt to classify patients with multiple sclerosis, Alzheimer's disease, schizophrenia, Sjögren's syndrome, chronic alcoholism, facial pain, also distinguishing them from healthy controls, suggesting a possible use of MEG in diagnostics.[11]
[edit] Focal epilepsy
The clinical uses of MEG are in detecting and localizing epileptiform spiking activity in patients with epilepsy, and in localizing eloquent cortex for surgical planning in patients with brain tumors or intractable epilepsy. The goal of epilepsy surgery is to remove the epileptogenic tissue while sparing essential brain areas to avoid neurologic deficits [12]. Knowing the exact position of essential brain regions (such as the primary motor cortex and primary sensory cortex, visual cortex, and speech cortex) is of utmost importance. Direct cortical stimulation and somatosensory evoked potentials recorded on ECoG are considered the gold standard for localization of essential brain regions. These procedures can be performed either intraoperatively or from chronically indwelling subdural grid electrodes; however, they are both invasive to the patient.
MEG localizations of the central sulcus obtained from somatosensory evoked magnetic fields show strong agreement with these invasive recordings [13][14][15]. MEG studies assist in clarification of the functional organization of primary somatosensory cortex and to delineate the spatial extent of hand somatosensory cortex by stimulation of the individual digits. This agreement between invasive measures of localization of cortical tissue and MEG recordings implies the effectiveness of MEG analysis.
[edit] Cognitive neuroscience applications of MEG
MEG has also recently been used to study cognitive processes such as audition in fetuses[16] and language processing.
[edit] Comparison with other imaging techniques
MEG has been in development since the 1960s but has been greatly aided by recent advances in computing algorithms and hardware, and promises improved spatial resolution coupled with extremely high temporal resolution (better than 1 ms); since MEG takes its measurements directly from the activity of the neurons themselves its temporal resolution is comparable with that of intracranial electrodes.
MEG's strengths complement those of other brain activity measurement techniques such as electroencephalography (EEG), positron emission tomography (PET), and fMRI whose strengths, in turn, complement MEG. Other important strengths to note about MEG are that the biosignals it measures do not depend on head geometry as much as EEG does (unless ferromagnetic implants are present) and that it is completely non-invasive, as opposed to PET.
[edit] MEG vs. EEG
Although EEG and MEG are generated by the same neurophysiologic processes, there are important differences concerning the neurogenesis of MEG and EEG [17]. In contrast to electric fields, magnetic fields are less distorted by the resistive properties of the skull and scalp, which result in a better spatial resolution of the MEG. As Electric and magnetic fields are oriented perpendicular to each other, the directions of highest sensitivity, usually the direction between the field maxima, are orthogonal to each other. Whereas scalp EEG is sensitive to both tangential and radial components of a current source in a spherical volume conductor, MEG detects only its tangential components. This shows MEG selectively measures the activity in the sulci, whereas scalp EEG measures activity both in the sulci and at the top of the cortical gyri but appears to be dominated by radial sources.
Scalp EEG is sensitive to extracellular volume currents produced by postsynaptic potentials, MEG primarily detects intracellular currents associated with these synaptic potentials because the field components generated by volume currents tend to cancel out in a spherical volume conductor [18] The decay of magnetic fields as a function of distance is more pronounced than for electric fields. MEG is therefore more sensitive to superficial cortical activity, which should be useful for the study of neocortical epilepsy. Finally, MEG is reference-free which is in contrast to scalp EEG, where an active reference can lead to serious difficulties in the interpretation of the data.
[edit] See also