Instrumentation

, Ali T. Ghouse² and Raghav Govindarajan³

(1)

Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada

(2)

McMaster University Department of Medicine, Hamilton, ON, Canada

(3)

Department of Neurology, University of Missouri, Columbia, MO, USA

All electrodiagnostic instruments, analog or digital, should meet the specifications recommended by the American Association of Neuromuscular and Electrodiagnostic Medicine. Digital instruments allow for the use of automatic cursor placements, amplitudes, area measurements, averaging, frequency analysis, interference pattern analysis, and trigger delay lines; they also have the ability to change the display sensitivity even after the waveform is captured. Embedded programs allow the use of sophisticated functions, such as motor unit estimates and report generation. The instruments should be user-friendly.

To obtain accurate information, the electrodiagnostic technician and consultant should be familiar with the sources and magnitudes of the various equipment errors, and the techniques that can be used to minimize the adverse effects. The electrodiagnostic medicine consultant must know how and when to override the computer-generated response and be in control of the technology.

Electricity and Charge

The basis of electricity is charge and charge flow. Electrical equipment in much of the world is typically powered by a 110-V, 60-cps line current. Within an electromyography (EMG) machine, the line voltage is lowered to a direct current voltage between 5 and 15 V that is used to power the amplifiers, filters, and computer circuits.

Current is measured in amps and is defined as the amount of charge (coulombs) flowing per unit of time in seconds. The usual charge carriers are negative (electrons).

The bases of instrument design are voltage, current, and impedance. An analogy can be drawn between these entities and the characteristics of water flowing down a river:

Voltage (V): Voltage is the potential energy per unit charge (joules/coulombs) (steepness of the river grade determines the current).
Current (C): Current is the charge flow per second (amount of river water flowing per second).
Impedance (I): Impedance is the ratio of voltage to current (friction between the water and the river bed).

Filters

The most important function of a filter is noise attenuation. Every electric device that conducts an electric charge changes the signal, that is, filters it. An ideal device should eliminate frequencies that constitute noise while allowing the frequencies that correspond to physiological signals. Filters are designed with capacitors and resistors. Capacitors are devices (like valves) that impede current flow by an amount that depends on the frequency content of the current. Resistors supply a constant impedance to current flow.

High-Pass Filters

Allow high frequencies, e.g., 20 Hz, to pass through. They are set to stop low frequencies, done by placing a capacitor in the signal path.

Low-Pass Filters

Allow low frequencies to pass through. They are set to stop high frequencies, done by placing a capacitor between the signal path and ground.

Amplifiers

Amplifiers convert a low-voltage waveform to a higher-voltage waveform. Two amplifiers can be combined to create a differential amplifier that will improve the signal-to-noise ratio. The input leads should be close to each other so that the environmental noise will be the same in both leads. Ideally the two leads will carry the same noise, but only the active lead will carry the signal. A measure of how identical the two amplifiers are is the common mode rejection ratio (CMRR), which is the ratio between the gain of neurophysiological potential and the gain of 60-cycle noise.

Electrodes

Three electrodes are always attached to the patient during nerve conduction studies (NCS) and EMG; an active electrode, a reference electrode, and a ground electrode.

The recording electrode (G1) is shown as a black solid circle.
The reference electrode (G2) is shown as a red or white open circle.
Ground electrodes are placed between the stimulating and recording points.

Disposable needle electrodes are popular because of their lower maintenance and because of concerns about infection. Reusable needle electrodes have the disadvantage of bending, and of becoming corroded, barbed, or excessively thin, and possible fraying of the Teflon coating, with increased risk of artifacts.

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