Instrumentation and Calibration Flashcards

An EEG technologist is performing a biological calibration. The patient is asked to open and close their eyes, look left and right, and clench their jaw. What is the primary purpose of this procedure?

Answer: To verify the integrity of the recording system by generating physiological artifacts and confirming correct channel derivations.

Biological calibration (bio-cal) is essential to ensure the entire recording pathway is functioning correctly. By having the patient perform specific actions that are known to create distinct physiological artifacts (like eye movements and muscle tension), the technologist can confirm that electrodes are correctly placed and connected to the appropriate channels in the montage, and that the amplifiers are displaying these signals as expected.

A differential amplifier's ability to reject signals that are common to both of its inputs is measured by which of the following specifications?

Answer: Common Mode Rejection Ratio (CMRR)

The Common Mode Rejection Ratio (CMRR) is a critical specification for a differential amplifier. It quantifies how well the amplifier can cancel out identical signals (common mode) present at both input terminals, while amplifying the difference between them. A high CMRR is crucial in EEG to eliminate widespread electrical noise and interference.

During an EEG recording, a technologist notices significant 60 Hz artifact across all channels. Which of the following instrument settings should be adjusted to specifically target and reduce this type of interference?

Answer: Notch Filter

The Notch Filter is specifically designed to attenuate a very narrow band of frequencies. In North America, this is set to 60 Hz to eliminate interference from the AC electrical power lines. It should be used judiciously as it can sometimes distort clinically relevant activity near that frequency.

According to the American Clinical Neurophysiology Society (ACNS) guidelines, interelectrode impedances should be within what range for a standard EEG recording?

Answer: Between 100 Ohms and 10,000 Ohms (10 kOhms)

ACNS guidelines state that interelectrode impedances should be under 10,000 Ohms (10 kOhms) but over 100 Ohms. Impedances that are too high can lead to increased noise and artifact, while impedances below 100 Ohms may suggest a salt bridge or short circuit between electrodes. While values under 5 kOhms are often recommended, the acceptable range is up to 10 kOhms.

A technologist changes the sensitivity setting from 7 µV/mm to 10 µV/mm. How will this affect the appearance of a 70 µV spike on the display?

Answer: The spike will appear shorter.

Sensitivity is the ratio of input voltage to the pen or trace deflection (µV/mm). At 7 µV/mm, a 70 µV spike would cause a 10 mm deflection (70 µV / 7 µV/mm = 10 mm). When the sensitivity is changed to 10 µV/mm, the same 70 µV spike will cause a 7 mm deflection (70 µV / 10 µV/mm = 7 mm). Therefore, increasing the numerical value of the sensitivity setting decreases the amplitude of the displayed waveform, making it appear shorter.

Which of the following filter combinations would be most appropriate for routine clinical EEG recording to visualize cortical activity while minimizing slow-wave artifact and high-frequency noise?

Answer: Low-Frequency Filter (LFF) at 1 Hz, High-Frequency Filter (HFF) at 70 Hz

Standard settings for routine EEG are a Low-Frequency Filter (LFF) of 1 Hz and a High-Frequency Filter (HFF) of 70 Hz. This 'band-pass' allows the typical range of cerebral frequencies (1-70 Hz) to be displayed while attenuating very slow activity (like sweat artifact) and very high-frequency activity (like muscle artifact). Setting the LFF too high (e.g., 5 Hz) can filter out clinically relevant delta activity, and setting the HFF too low (e.g., 15 or 35 Hz) can filter out fast activity like spikes or beta.