Electronic Theory and Instrumentation in Electroneurodiagnostics
John Finkbeiner, R. EEG/EP T./R. NCS T.
Modified from the following web site as composed by the author:
Electricity and Electronics
The following is for the study and understanding of instrumentation and theory in electroneurodiagnostic studies. As much as possible, I will stick to those concepts needed to be studied for any of the registration examinations.
All magnets have two poles, north and south. Actually though, these two poles are a convention to easily visualize the idea that there is a flow of magnetism which flows in a circular field around the magnet. Magnets generate small electric fields.
Electricity produces magnetism, and magnetism produces electricity. To demonstrate this, by using a coil of wire and a natural magnet, the movement of the coil across the magnet at right angles to it produces alternating current in the coil of wire. On the other hand, a coil of wire which has electricity “running through it” produces a magnetic field; this is the basis of how an electric motor operates.
Atoms are the smallest, naturally occurring building block of nature; these building blocks are referred to as elements. Each atom consists of three particles: protons, neutrons, and electrons.
In most every atom, protons and neutrons exist together in the center of the atom, called the nucleus. Protons are positive, neutrons have no charge, and electrons are negative. The number of electrons and protons are generally the same.
Electrons circle around the nucleus, much like the planets circle around the sun. Like planets moving around the sun, the electrons maintain their place in their orbits. Unlike the solar system, however, each ring, known as a shell, has a specific number of electrons in it. These shells are called valence shells.
Atoms acquire their names by the number of each of their particles. For instance, hydrogen occupies the first place on the atomic chart because it has 1 electron.
It is possible for one of the electrons in the outermost ring to become unstable enough to move to another atom. This leaves that atom “unbalanced.” This type of atom is referred to as an ion.
The non-stabile atom now needs to attract another electron, so that it can become stable again. The movement of ions, to over-simplify the concept, is what creates electricity.
In order for current to be possible, it must have a conductor, and a source of energy or power. This is not a functionally useful circuit, however. It becomes useful when a load, or some type of resistor or resistance, is placed in the circuit. For instance, a light bulb, which takes on the function of a resistor, turns a straightforward flow of current into a useful tool; the light bulb can now light.
Because the current is meeting resistance as it flows toward the light bulb, this unbalances the circuit; there is resistance in half the circuit, and much less resistance in the other. Again, to oversimplify the point, nature always seeks balance, and therefore the current continues to flow back to the power source.
Direct Current and Alternating Current
There are two types of currents, direct current (D.C.) and alternating current (A.C.). D.C. flows directly from negative pole to the positive pole of the circuit. A.C. travels first forward, then back, alternating in charge as it does so. D.C. can be seen in a flashlight battery: the current flows from the negative pole of the battery, through the conductor, lights the bulb, and then back to the battery. A.C. current, on other hand, is useful in motors. Going back to the concept of the magnet moving over the coil of wire, when the coil alternates in charge, it becomes relatively negative, it attracts the positive pole of the magnet. When the current switches directions, it repels the magnet, which allows the magnet to swing away. By constantly switching charges back and forth, the motor turns.
A naturally occurring A.C. current occurs in the nervous system. Nerve cells, which at one millisecondare are negative at the cell body, quickly become relatively positive as the nervous impulse travels away from the cell body. After the discharge, the nerve cell repolarizes, and the cycle begins again.
Note that, traditionally, we think of current as flowing from positive to negative poles in a circuit. Since electrons are negative, however, they flow toward the positive, and therefore the current flows from negative to positive.
There are several elementary concepts and building blocks of electricity. Some basic shorthand, then, is:
V or E or EMF: stands for voltage. Voltage is the pressure, or force, behind electricity. This force is also known as electromotive force. This idea leads to the abbreviations; V stands for voltage, E or EMF stands for electromotive force. E, EMF, or V all stand for the same concept, and can be used interchangeably.
W stands for watts, which is defined as the time it takes for energy to create a given force
I stands for current, which is the outcome of volts time watts, or V * A.
A stands for amperes, and it is the measure of I or current.
R stands for resistance, which is defined as the resistance to current flow
C stands for capacitance, which is the temporary storage of electricity in a thing, and its sudden discharge.
Z stands for impedance
Watts / Volts / Current
To put these concepts together, think about a water wheel at an old mill. The flow of water puts pressure on the wheel to turn – this force is called volts. The time it takes for the water to force the wheel into motion is called watts. If the force is high, the wheel will turn, but if the speed of the force over time, the wheel will turn slowly. If, on the other hand, the force is low but the speed is high, the wheel will turn, but again, weakly. If the speed, or rate, and the power, or force, are both high, the wheel will spin rapidly and with great force.
The above paragraph, then, can be expressed in the following equation:
I = W x V
In other words, the amount of current is the product of volts times watts. High voltage does not necessarily hurt a person in and of itself, neither does high wattage. What can become lethal is the combination of high voltage and high wattage.
Resistance is inherent in any circuit. The concept of resistance also says that the amount of resistance in a circuit is dictated by the conductor of the electricity, for instance, copper or silver. Revisiting the illustration concerning the mill wheel, all mill wheels have naturally occurring resistance to turning.
The measure of resistance in a circuit is Ohms. The formula for measuring the amount of impedance is called Ohm’s Law; and is represented by the formula R = V/I, or stated a different way: E=I*R.
The concept of resistance can be stated as impedance. Resistance is the word used in reference to resistance to direct current; while impedance is the resistance to alternating current. These terms are sometimes used interchangeably, this is not correct usage. In electroneuodiagnostic technology resistance in a circuit (the amount of resistance between the patient’s scalp and the conducting media or electrode) is always stated in impedance. The primary reason for impedance being the measurement used is because checking resistance is uncomfortable to the patient. This is because when one checks the amount of resistance provided between the patient and the recording electrode, a very small amount of current is sent through a wire, to measure the relative resistance between it and other wires being used.
Different schema are employed to determine resistance; one company lets the computer determine which electrode has the lowest relative resistance to others, and uses that wire as a reference point, most either use a standard wire – such as a ground wire, while other companies let the user choose which wire will be the reference used to measure the amount of impedance in a current.
As a side note, A.C. is used in determine the resistance between skin and electrode. D.C. used in this manner tends to produce discomfort, it stings or hurts.
Capacitance is the quality that stores a charge, and then releases that charge at a fixed rate. While the rate or the amount of charge which occur in nature are more variable.
Capacitance is less commonly encountered, except in special circumstances. A good example is a person, on a dry day, shocking someone else by touching them after walking across a carpet.
Inductance occurs when 2 wires cross at a 90 degree angle. One of the wires carries active alternating current, while the other carries no voltage. When a sudden shift of current appears, current is then induced, or begun, in the wire carrying not voltage. This is because of the 90 degree angle.
Remembering that A.C. always shifts between negative and positive charge, this creates an electromagnetic effect which is always changing charges, and therefore induces A.C. in the non-active wire.
The effect of inductance is to produce 60 Hz interference at the output. To rectify the situation, make sure that all wires are parallel. This is especially important during a nerve conduction or somatosensory evoked potential study. In these cases, the stimulator cord being next to the recording electrode will produce artifact.
Electronic circuitry can be said to be the ability to manipulate electricity and current flow. In electroneurodiagnostics, the most relevant concepts are resistance, capacitance, and inductance. In the field of electronics, resistors and capacitors are actual electronic components, which embody the concepts of resistance and capacitance, while inductance is a thing made noticeable by modern technology.
These are electronic components which produce a specific amount of resistance. Each resistor is manufactured to a specific number of Ohms. These resistors can then be used in a circuit to create a desired effect.
Before proceeding further, it is important to understand concepts called series and parallel. This concept can be used in different scenarios, such as determining which light switch(es) turn lights off and on, but here we apply it to resistors in a circuit. It is used, in this context, to determine, subject to the user’s determination, the amount of resistance in a circuit.
A circuit utilizing the concept of series means that each component is daisy chained, one component connected to the next.
A circuit designed in series looks like this:
Assume that the 3 resistors each measure 1,000 ohms. The effective resistance in the circuit is 3,000 ohms. The effect of a resistor in series, then, is said to be additive. It is demonstrated by the formula R = R1 + R2 + R3…..
Now let us assume that the resistors are wired in parallel. Now the circuit looks like this:
The formula now looks like this:
The most common use of resistors in parallel is the ability to select only one of the resistors in the series, thus giving better control over the circuit.
A capacitor is an electronic component which has the ability to store charge, and release it a fixed rate. It is measured in farads. The two most salient features of a capacitor are:
· It takes time for charge to build up in the capacitor, and time for the release, depending on the rating of the capacitor. When the capacitor discharges, it is much like water flowing between containers, it seeks it's own level. Furthermore, as long as there is water in this model (or current flow), the charge never reaches zero volts.
· A capacitor displays great resistance to low frequency, and less resistance to a high frequency.
· Because of the way a capacitor is built, it let high frequencies pass, and offers resistance to lower frequencies.
Functionally the same as capacitance, this is a term used in reference to leakage current. In short, this means that electricity exists where it is not supposed to be. The subject will be discussed under electrical safety.
Electronic Circuits Relevant to Electroneurodiagnostics
Low Frequency Filter
A low frequency filter is a circuit which attenuates frequencies equal to, or less than, the stated filter frequency setting.
The word “attenuate” means smaller than, or less than. However, in this context, the extrapolation of “less than what?” is made.
Refer to the diagram below: .
The scale on the left of the diagram represents the amount of attenuation that a signal will have upon entering a low frequency filter circuit. This will be abbreviated LFF. The dotted line extending across the diagram at 30% represents the point of significant attenuation. To oversimplify, this is the point at which the signal attenuates, or becomes smaller than it’s potential 100% height, noticeably.
The scale across the bottom represents hertz (Hz)
To illustrate, a 1 Hz signal entering a 1 Hz LFF circuit will be attenuated by 30%. A signal with a higher frequency, say 5 Hz, will have no or no significant attenuation. A frequency less than 1 Hz will be significantly attenuated.
While some refer to this effect as signal “cut off”, this author prefers the term attenuation.
High Frequency Filter
This is a direct opposite to the low frequency filter – it is a circuit which attenuates at or above the stated frequency filter setting.
Referring to the diagram above, note that a 70 Hz signal is entering a 70 Hz HFF. This means that the signal will be significantly attenuated. A signal of 35 Hz entering the filter will not be affected. A signal of 80 Hz, however, will be significantly attenuated.
60 Hz Rejection Filter
Also referred to a notch filter because of it’s distinctive frequency curve, as seen below:
Note that any frequency at 60 Hz, or slightly above or below that frequency will significantly attenuated, or will not be apparent at all.
The time constant is the time it takes for a given frequency to decay from a waves peak amplitude to baseline.
You will often hear time constant and low frequency filter used interchangeably. In reality, the time constant actually has an effect on all frequencies. While EEG and Polysomnographic technologists are the most cognizant of time constants, it is an important concept governing all electrodiagnostic studies.
The formula to express the function of this circuit is:
TC = C X R
Put into more functional terms, when a D.C. current is input to a circuit, the resistor controls how fast the capacitor charges. The capacitor then discharges at a predetermined rate, depending on the rating of the capacitor.
Again, in more practical terms, time constant is the time it takes for a capacitor to charge 63%, or discharge 37%. Tradiitionally, this appears in the mechanical calibration signal on an EEG machine. This can also be seen in the older EP and NCS machines.
This is the apparent shifting of a wave in time, either in terms of latency or otherwise, earlier or later than its real occurrence. Remembering that a capacitor has the effect of passing low frequencies at slower rate than high frequencies. Because of this, there is a shift in the occurrence of a slow wave as compare to a wave occurring at the same time at a higher frequency.
A transformer is a piece of equipment which uses inductance and alternating current to perform it’s function; that function is to change the amount of electrical current, either raising or lowering it’s voltage.
In any transformer, there are two coils, mounted next to each so as to take advantage of inductance occurring between them. At the outset, only one of the coils holds a charge, but because of inductance, the second could takes up the charge.
In a circuit using inductance, the amount of current generated is determined by the amount of turns in a coil. By giving each one of the coils a set amount of turns, one is able to generate a certain amount of current which is predetermined.
A practical example of this mechanism is to assume that the electricity coming in to the frist coil is coming in at 120 V, A.C. The second coil, however, has less turns in it, and therefore the output from that coil is “stepped down.”
It is possible to also step up the amount of current by adding more turns of wire to the second coil of wire in the series.
The most important concept in dealing with electrical safety issues is to remember that electrical current always takes the path of least resistance.
Leakage current, also referred to as stray current, is defined as stray inductance and/or stray capacitance.
There is always some current that exists between the electrodiagnostic equipment circuitry and the chassis of that machine. There can also be leakage current between the chassis of the machine and the power outlet. Leakage current is measured when power is being put through the circuit, but no ground is operable. In a hospital setting this will be done by the clinical engineering.
Acceptable leakage current in a electroneurodiagnostic machine is considered to be 100uV.
While still inductance, stray inductance refers to leakage current in the context of electrical safety. A setting in which this principle would most likely come into play would be that such as an ICU. In theory, it is possible for a power cord to run parallel to recording lead, and through a ground fault, induce current in the recording lead. Making sure that you are using a shielded power cord can prevent this.
Having the same net effect as stray inductance, this occurs when enough charge builds up without a vehicle for discharge, and consequently discharges through the path of least resistance.
A ground is a wire which is used to shunt off excess current to a diversion of some sort. Originally in the field of electricity, this was the earth itself.
In the field of electroneurodiagnostics, there are 3 types of grounds.
The first is an earth ground; this is made possible by the 3 prong plug. The third plug connects to the building ground which, in theory, connects to the earth itself.
The second ground is an interface – it connects the earth ground to the working of the machine itself. In the world of digital technology, this second type of ground often taken care of by an isolation transformer. The basis of this equipment is a transformer. In a transformer, since inductance is used to ass the current, there is not a wire which directly connects the chassis of the machine to an earth ground.
The third is the ground wire which appears in all modalities of testing. This wire is also referred to as a common ground; because this one wire is actually connected to all of the amplifiers in the machine. (As explained later, each differential amplifier, the amplifiers used in electroneurodiagnosis, utilizes input 1, and input 2, and ground.) If this ground is not present, the machine is incapable of giving accurate readings, even though it may appear that it is doing so. Applying a ground where there was none before will convince the user immediately that a change takes place when a ground is not present.
Everything else not-withstanding, all patient care equipment must be grounded. Therefore, a three prong power cord must be used, and should plug into a hospital grade power receptacle. All power cords should be a 3 prong "hospital grade" plug.
In any evoked potential testing or nerve conduction study, one must always put the ground between the recording electrode and the point of stimulation.
A ground fault occurs any time the circuit is incapable of shunting excess current to an earth ground.
A ground loop can occur when two grounds are attached to a common point.
To illustrate, assume that a patient is receiving both an ENDT study and an ECG. Further, let us also say that both machines have patient grounds connected to the patient, and that the machines are plugged into outlets with different earth grounds.
If the machine on the same circuit as the ECG machine fails, a ground surge could becomes present. In that case, the ground on the patient's leg would conduct pass current from the ECG ground located in the leg, and pass this to the ground located on the END machine. Should enough current pass through heart, the resulting shock could have devastating effects.
Note that in modern medical care, wiring is specifically installed to accommodate double grounding. Because of the amount and nature of the equipment used, it becomes necessary to allow people to plug equipment into disparate socket, using multiple patient grounds.
Actually a version of the ground loop, this occurs when a patient has 2 grounds attached during a test. This has the potential to become a ground loop; that is, a fault in the machine and one of the ground wires may create a current flow in the patient.
Some Rules for Electrical Safey
· Do not use and extension cord: stray inductance may be created, as well as a loss of voltage should the cord be long enough.
· Use hospital grade power receptacles to plug your machine in. (In a hospital, there will be a second type of plug, it is colored red. These power outlets are on backup; if the power goes off, the receptacle stays live.)
· No bare wires.
· All patient related electronic/electrical devices must be inspected for safety. In a hospital setting, inspections are carried out in accordance with the protocols of the BioMedical department. A general guideline used in some institutions is once every 6 months to a year. There should be a seal showing the date of inspection when this is done.
· Make sure the patient is disconnected from the equipment before turning equipment on, and before powering off.
This is the representation of the amplitude of a given signal.
In selecting the sensitivity, it customary to decide on the correct value in accordance with the modality being used. For instance, the traditional value of 7μV/mm used in EEG would be totally inappropriate for the ECG noted during polysomnography, where the sensitivity used is usually closer to 100 - 150μV/mm.
The theory behind sensitivity is that each amplitude value is selected by deciding how many microvolts will be represented for a given distance of screen or paper real estate.
Assume that you have a sensitivity of 5. In the original use of this term, 5 refers to the number of volts represented by a pen deflection of 1 mm (on paper) A pen deflection of 2 millimeters, then showed a value of 10μV. Thus, one can see that there is an inverse relationship – the smaller number means that the pen must travel less distance to show a larger amplitude.
Put another way, assume that the voltage entering the amplifier is 100μV. At a sensitivity of 10, the pen would swing 10 millimeters, at a voltage of 20, the pen would swing 5 millimeters.
In the modern world, however, display screens no longer use the same display parameters as the old EEG machines; which could count on EEG paper made to exact measurements of width and height. For that reason, modern day machines show amplitude by noting the amount of micvolts / millivolts on a given amount of space on the machine. It is more prevalent today to let the user get the exact amplitude by using digital cursors.
For more, see the section below entitled Horizontal resolution.
Every monitor used has a pre-defined resolution, all ENDT programs automatically adjust this resolutionto maintain accuracy. A common resolution, for example, would be 1024 x 768. The number on the left always represents the width of the monitor, the number on the right, the height. These numbers are the number of dots on the screen, called pixels, which make up the image on the screen. No matter how fine the engineering behind the machine, this number actually sets the physical limits of accurate representation. This is why manufacturers have a minimum resolution for their machines.
Analog to Digital Conversion (ADC)
When a signal is first input into a machine, it enters as an analog signal. An analog signal is a "physically variable" current. This current is a dynamic, unbroken, and varying voltage. As the signal enters the machine, it is sampled by the computer at a predetermined speed. The computer converts each sampled analog value into a numeric value, representing a voltage. Each of the voltages is now written to a storage medium as a discrete number. The analog waveform is now said to be digitized; the computer displays this digitized waveform by "connecting the dots." The more data points there are in a given second, the more accurate this new representation of the analog wave becomes.
This method allows sophisticated display management not possible with an analog machine, such as adjusting sensitivity after the fact.
This is the rate, referred to in the above paragraph, at which the analog signal is sampled. It is common for most machines in the field to sample at 200 Hz, this being considered the minimum rate to accurately represent a given waveform.
This theorem states the minimum sampling rate needed to accurately represent waveforms. The minimum sampling rate is 2 ½ times higher than the highest high frequency filter setting. If the highest high frequency is 70 Hz, then the machine should sample at no less than 175 Hz.
This occurs when the number of data points which have been sampled by the computer is too small. When the computer connects the data points, the signal is no longer a true representation of the input signal, it becomes distorted.
Sweep Time (Horizontal resolution)
Also referred to as the X-axis, time window, or time base, this is the amount of time represented on the screen of a machine, as noted in an evoked potential or NCS study. Usually, the screen is divided into 10 equal parts, each part marked off with the name of the increment in time.
For instance, a screen appears which has the number 5 msec. which appears by a wave, or at the bottom of the screen. This means that each division of the screen is 5 ms, and that the full width of the windows is 50 milliseconds.
In EEG and PSG, the display is referred to as paper speed (the term is a throwback to the days of ink and paper, when the speed of the paper determined the physical representation of the duration of a wave), since the waves are always moving across the screen during testing; very much unlike the evoked potential family of tests. In the evoked potential family, the waves are always stable and stationary.
Amplitude Representation (Vertical resolution)
Sometimes referred to as gain (a misnomer), screen gain (because it digitally “maginifies” the image seen), or sensitivity, the amplitude of a wave on a machine is usually demarcated with a number followed by μV or mV. (Microvolts or millivolts) Each channel on the screen can have it’s own separate amplitude.
Unlike EEG or PSG machines, evoked potential machines have their own twist on amplitude representation. In a nerve conduction study, for example, the middle of the screen represents zero volts, with the upper part of the screen representing the amount of “positive” wave deflection, while the lower half the amount of “negative” wave deflection. Usually, however, each wave gets it’s own channel in which each wave from peak to peak is measured in terms of an absolute number of microvolts or millivolts.
Unlike the display of time, each data point is represented by a binary value, expressed in bits. In the language of binary, 4 bits represents a value of 16, 8 bits have a possible value of 256. An 8 bit representation then, gives a possible 256 distinct amplitude representations. The more vertical space on the screen, and the less the number of other channels, the more accurate the representation of amplitude.
Signal to Noise Ratio
In any electronic system, there is inherently some “noise.” The amount of signal to noise is expressed in dB.
In electroneurodiagnostics, “noise” can also mean biological, mechanical, or electrical artifact. The simplest way, in an evoked potential machine, to improve the signal to noise ratio, is to average the signal.
Averaging, which only takes place on evoked potential and nerve conduction study machines, takes advantage of several facts:
Every data point is always displayed at the exact same place in time on the screen each time it is displayed, each wave can be held in memory and redisplayed at any time; even after having been written to a storage medium, and each data point can be arithmetically averaged with other data points occurring at the exact same time/place in the time window.
One can improve the signal to noise ratio of a wave through retaining the wave in memory, and averaging each data point in the waveform with the other waves which have come before it.’
By doing this, any waves which were randomly appearing waves due to biological, physiological, or electrical artifact gradually disappear because their significance in the formation of the waveform becomes less and less. The more stable the waveform, the less averages are needed; a good example of this is a sensory nerve action potential. The smaller the waveform, or the less stable, such as a brainstem auditory evoked potential, the more averages are needed to stabilize the wave form.
As the number of averages increases, the waves become more stable, and change less. This possibility of change increases significantly with the occurrence of the square of the number of stimulation.
If the number of stimulations is 4, then in order to see a significant change in the average, the number of averages has to be 16. After that, the number of waves averaged goes up to 256. The more the number of averages, then, the longer it will take before another “significant” change can occur.
Inputs are grouped together in a systematic order, no matter what the modality of electroneurodiagnostic testing. All amplifiers in any electrodiagnostic machine are differential amplifiers.
There must be 3 inputs: Input 1, Input 2, and Ground. In a differential amplifier, the amplifier outputs the difference between input 1 and input 2; the ground
is used as a reference to help determine the output.
One set of these inputs is referred to as a channel. The number of channels varies according to the machine and modality. For instance, an EEG machine has no fewer than 16 channels (modern EEG machines have at least 32), while an NCS machine requires 2 channels.
One of the inputs is always a ground, which may occasionally be referred to as a "common" input. This one ground electrode connects with all of the other amplifier grounds.
The other two inputs are variously labeled, depending on the make of the machine and/or modality of the machine. Labels used are: input one and input two, + and -, G1 and G2 (now obsolete nomenclatures), G+ and G-, and active and reference.
To illustrate how a differential amplifier works, suppose input 1 sees +35uV, and input 2 sees +20uV. The amplifier will output 15uV. Let us now say that input 1 sees a voltage of +35uV, and input 2 sees a voltage of -35uV. According to the world of a differential amplifier, which sees the voltage difference in absolute voltage terms, the output voltage is 70uV. In just the same way, if input 1 is -40uV and input 2 is -60uV, then the output voltage is 20uV.
Using the above illustration, the machine uses these calculations to tell the wave which way to go – in other words, should the wave deflect up, no change, or down because of the varying voltages. This leads us to the following set of rules:
If input 1 is more negative than input 2, the wave deflects up
If input 1 is more positive than input 2, the wave deflects down
If input 2 is more negative than input 1, the wave deflects down
If input 2 is more positive than input 1, the wave deflects up
Remembering the first rule lets one use logic to determine the other 3 rules.
As an aside, some use the saying “If iNput 2 is more Negative than input 1, the pen goes dowN.” As above, memorizing the first of the four concepts lets you logically figure out the other three.
Anode and Cathode
The terms anode and cathode are also used in chemistry and physics. In electrodiagnostic technology, they are used more commonly in reference to stimulator probes. The use can also be seen in recording electrodes, this is not as common, and is confined to the evoked potentials and nerve conductions.
The cathode is negative. Some might think of this as a cat being a negative thing, because they make some people sneeze. The anode is positive.
Gain is the direct ratio by which the original signal has been amplified. In EEG, for instance, the signal is amplified by a factor of 10,000, so the gain is said to be 10,000 over 1. Evoked potentials studies, especially nerve conduction studies, have a higher gain. The formula for gain is: output voltage / Input voltage = gain.
It is common to erroneously refer to sensitivity as gain; gain is the amount of electronic amplification, sensitivity is the determination of what amplitude displayed.
Signal to Noise Ratio
In any electronic system, there is inherently some “noise.” The amount of signal to noise is expressed in dB.
In electroneurodiagnostics, averaging is used to reduce the amount of noise in a given signal.
Common Mode Rejection
Assume that input 1 and input 2 see identical signals. These two signals are said to be in phase. When this happens, it is referred to as a common mode signal.
Common mode rejection is the ability of the electroneurodiagnostic equipment to minimize or abolish this activity. This ability is expressed as a ration, called the common mode rejection ratio, or CMRR. The higher the ratio – e.g.: 100:1 - the better the ability of the machine to reject this phenomenon.
By far the most common scenario in which CMR becomes important is a 60 Hz signal which appears in both inputs. In these cases, the most common cause is a loose electrode. When this occurs, one side of the differential amplifier becomes higher in impedance than the other side; i.e.: input 1 has an input impedance of 15,000 ohms, whereas input 2 has an input impedance of 1,000 ohms. This inequality in impedance also causes an inequality of output; the higher impedance electrode now “takes over” the signal. In cases of mild imbalance, the CMR circuit is able to hold in check the 60 Hz activity from obliterating biological signal in the effected channel of the tracing. If the impedance imbalance is extreme enough, the 60 Hz activity will appear anyway; that activity will actually be amplified.
It is for this reason that the ACNS Guidelines of Electroencephalography, Evoked Potentials, and Polysomnography state that the impedance difference between input 1 and input 2 should be less than 5 K ohms. The optimum impedance for a recording is to have all electrodes less than or equal to 5,000 ohms.
When speaking of impedance as a routine fact in the electroneurodiagnostic field, we are actually referring to input impedance. Differential amplifiers use high input impedance amplifiers. This may seem counterintuitive, since we are forever trying to lower the impedance of electodes. The fact remains, however, that we consider 1,000 ohms, and in the field of electronics, low impedance is more likely to be seen as 2, 4, or 8, even 50 ohms. In fact, a measurement of input impedance of 100 ohms is considered to be a sign that there is probably an error in the technologists set-up; the most common scenario being a salt bridge, or the connecting of two electrodes by a conductive substance.
Digital Theory Revisited
Because each data point is stored as a numeric values, this makes it possible to digital manipulate the waveforms appearing on the screen. The following maneuvers is take advantage of the fact that, in a digital system, each digital value stored comes from one distinct electrode, not the combination of input 1 and input 2, as seen in an older analog system.
When creating a montage in any machine, the montage is created by subtracting the voltage of input 1 from the voltage of input 2. This exact methodology works in the digital world as well, exactly duplicating the analog world. Unlike the analog world, however, when can go back at any time, during or after the recording, and change the montage, by simply choosing a different array of input 1s and input 2s.
As we have already seen, a filter is a wave of attenuating certain frequencies, either selectively or in different groups.
There are two generally different kinds of filters. One is active filtering, which refers to the analog method we have spoken of already. The second kind is passive, which is a reference to digital filtering.
Passive filters can be made to mimic active filters, without the disadvantages. There are 3 kinds of filters:
These filters do not attenuate certain frequency bands, but rather cut off any frequencies after or before a given frequency. While this property has the advantage of having no phase shift because of the absence of a time constant, it does not give a representation of the electrical activity as we know it.
This is the strength of the stimulation being delivered to the patient. In visual evoked responses, this refers to the brightness and contrast of the stimulation used. In auditory evoked responses, is the loudness of the stimulation. In electrical stimulation, it is the strength of the electrical stimulation. Lastly, in magnetic stimulation, it is the strength of the magnetic field.
The duration of the stimulus is measured in micro, milliseconds, herz, or cycles per second; it is the measure of how long a particular stimulation lasts.
This is the measure of how often a stimulus is given. All evoked potential studies have a repetition rate. There are two types, the first is the ability to give the patient a single, discrete, stimulation; as done in nerve conduction studies. The other is repetitive stimulation, during which the rate can be determined. The rate is somewhat machine dependent, but commonly can range from 0.5 Hz to as high as even 70 Hz. Cerebral evoked potential machines can usually go as high as 70 - 80 Hz.
This is setting determines how many stimulations will be given in a set. For instance, a train of 10 means that 10 stimulations will be given at a given rate. In a nerve conduction study, the rate is 1 and the train is 1. During repetitive nerve stimulation, though, the machine might be set at a rate of 3 Hz, using a train of 5. The machine would then give 5 stimulations at a speed of 3 Hz.
In constant current, the voltage is changed based on the specific impedance in the stimulator circuitry In this way a controlled amount of current is used to stimulate the patient based on this impedance in the stimulators circuit. The output of this stimulator is expressed in milliampres. The customary output range for constant current stimulators is 0 - 100 milliampres.
This is the inverse of constant current. The amount of voltage is changed according to the impedance of the stimulator patient's skin and subacute tissue. The level of stimulation is measured in volts. The customary output range for constant voltage stimulators is 0 - 300 volts.
Because of the dependence in this circuit on biological impedance, it is necessary to make sure that the level of impedance between the stimulator and the patient's skin is as low as possible. If the impedance remains high, a higher will cause a higher stimulation intensity to become necessary in order to produce the same amount of stimulation.
Either system may be employed; modern machines allow the user to stimulate with either method. The stimulation type used then, is laboratory dependent.
While surface stimulation is by far the most common, there are two main applications in which this technique becomes useful.
The first is during intra-operative monitoring. In this scenario, sub-dermal stimulation is employed for two reasons. The first is that is easier for the technologist; it can be left alone for long periods of time without interrupting the actual surgery. One has to be careful, however, to not burn the skin, especially if stimulation is taking place curing electro-cuatery.
The second is during nerve conduction studies. Sub-dermal stimulation is used, albeit rarely, when there is too much fatty tissue, or some thickening of the derma which prevents stimulation from reaching the nerve appropriately. It is used very occasionally in more accurate placement of a lesion. This last technique is now being rarely used, since the true purpose of this technique is called "inching", and any lesion can be more accurately assessed by the surgeon during surgery.
Cathode and Anode
As noted earlier, in electrodiagnostic technology, these terms are usually used in reference to stimulator probes.
In a bipolar stimulator, the cathode is the probe on the stimulator which is closest to the nerve or area being stimulated. In NCS convention, it is usually the black electrode; occasionally referred to as the active stimulating electrode.
The anode is the probe on the stimulator which is furthest away from that nerve or area being stimulated.
Anodal block occurs when the cathode is reversed from the usual placement, sitting physically in the way of the direction of stimulation. In this scenario, the voltage, or negativity, created by the anode will act to block the stimulation impulse being delivered by the cathode. This has two effects: the first is to lengthen the time from the stimulation to the response. The second, is the drop in amplitude of the response.
Magnetic stimulation is an instrument capable of delivering controlled impulses of magnetic fields. The shape and size of the fields can be determined by the shape of the stimulator being used; there are stimulators optimized for cortical and extremity use.
This modality can be used in place of direct electrical stimulation. The advantages are that locations can be stimulated heretofore not possible. For instance, the motor cortex can be directly stimulated, directly measuring the entire path for different areas. One can stimulate the peripheral nerves using this method, although there is some debate as to the efficacy of the technique in terms of being able to effectively direct the stimulation to the area wanted to stimulate.
In standard clinical practice, they are currently not seen much.
All electrodes can be used either as stimulating or recording electrodes.
Usually an electrode about 10mm in diameter, a cup shape with a hole in the top, usually used for the introduction of various electrically conductive substances. Pediatric versions down to 2 mm sizes can be seen. These electrodes can be used with either collodion or paste conductive substances.
Sub-dermal needles range in length from 2 cm to even as large as 100 mm are made. The needles used in electromyography usually have a non-conductive sheath over parts of the needle to help refine the area of muscle being recorded. There are two types
A single needle with one single wire attached.
A needle containing two electrodes, one which goes straight down the middle of the needle, the other is attached to the outside sheath.
Actually a very fine wire which is introduced through the zygomatic arch, aimed towards the anterior temporal lobe.
Used in nerve conduction studies, is a spring loaded clamp or a spring which is looped around the patient’s finger. It is used in the stimulation or recording of a patient’s digits.
Adhesive pads / strips
Stick on electrodes which can be used
in just about every modality except scalp electrodes, where they become
ineffective. Manufacturers make them in different shapes, sizes, qualities,
and purpose of use.
Intra-cranial grids or strips
Flat disks which are embedded in plastic which can be autoclaved. They can come premade in various shapes, commonly a strip of 2, 4, 6, 8, or other number in a strip, or a grid, up to 8 electrodes long by 8 electrodes wide. These electrodes are placed on the surface of the brain.
Electrodes which are long cannulas, which have different numbers of electrical contacts on the outside of them. These electrodes are inserted deep into the brain, when grids or strips are not appropriate.
Small balls mounted on the end of a malleable wire, used to insert through the nostrils and coming to rest on the naso-pharyngeal area of the throat.
After all of the above, the actual response is the wave produced by the preceeding.
Different machines have different abilities. Commonly, machines can display up to 32 waves on the screen. Machines have the following abilities:
- Single tracing
- Overlay: two tracings shown together, in the same space, as if on the same channel
- Overwrite: when one response replaces the other
- Addition: this happens with more than one wave on the screen. The waveforms are actually added, and another waveform is produced, enhancing any commonalities in the waveforms
- Subtraction: when one waveform is subtracted from the other, enhancing the lack of commonality
- Grand average: when more than one averaged response is averaged with other averaged responses.