EEG

There are many theories regarding the basis of the electroencephalogram (EEG) instrument. The most common one, also with the best documentation, is that Albert Grass adapted instruments used to measure earthquakes to detect the small electrical signals from the brain. Grass formed Grass Instruments in Braintree, Mass, and for many years it was the major vendor for electroencephalogram devices.

 Modern day EEG equipment—a 32-channel digital amplifier

The EEG is used, most commonly, to help diagnose seizures (some units had a strobe light as an accessory to trigger seizures), epilepsy, dizziness, brain injuries, headaches, sleep disorders, and Alzheimer’s. As part of the organ-harvesting protocols in hospitals, it is also used to confirm brain death. The EEG cannot read thoughts or feelings; it does not send electrical signals to the brain, but it will detect responses to outside stimuli.

 Modern day EEG equipment—a screenshot with synchronized digital video.

The basic problem with EEG is the small magnitude of the electrical signals from the brain. The standard amplification factor is 10,000. (Remember, an electrocardiogram (ECG) has a base amplification of 1,000.) Another factor is the various signal locations, with more than 20 electrodes used for the testing; the most common ECG test uses only 10 or fewer electrodes. With that many electrodes on a patient, the amplifier must have a high common mode rejection ratio to prevent cross talk between the signals. The amplifier’s typical frequency response is 0–50 Hz; some of the newer devices do have selectable frequency responses up to 90 Hz. In most cases, the electrodes used in diagnostic EEG work are reusable and can have problems over time. The electrodes are about 1¼4 inch in diameter and have a thin will attached, which then goes to a junction block on the cable. Keeping these wires straight and undamaged is not an easy task.

In the past, needle electrodes were used for EEG. Along with the potential for infections, the needles could become magnetized and distort the signals.

Technology Makes Readings Easier
Originally, the EEG was recorded on paper using an ink pen driven by a galvanometer. The channel width was 28 mm with a base speed of 30 mm per second on the chart. Most machines had, and have, a multitude of speeds. One problem with the ink on paper was that the pens would clot and the tech would have to clean them. (There are many stained shirts owned by the biomeds who cleaned those pens.)

Another function that had to be done was to “lap” the pens. This involved flattening the tip, and it was generally done by placing a fine emery paper under the pen with the pen rotating through its full side-to-side excursion using the position control on the console. Again, there were many stained shirts from this procedure, and a simple test could use 500 or more sheets of paper.

The paper chart has been largely replaced with electronic data storage, which is displayed on a monitor. This technology has many benefits, including the ability to scan data quickly, lower costs, and not stain shirts.

A modified version of the EEG is becoming common in the operating room to measure the depth of anesthesia of the patient undergoing surgery. This is not a true EEG, in that only three or so channels are monitored by the unit—not the 17-plus channels that are used in diagnostic EEG. As these instruments evolve over the next few years, they will probably expand the number of channels monitored.

Troubleshooting
When troubleshooting EEG systems, the common problem points are the electrodes, cables, and—if it is a paper system—the chart drive mechanism. Amplifier failures are rare. Some machines are not as tolerant as others on line-voltage problems and grounding. On some older units, you may need an additional ground on the chassis, as the power cord ground will have a higher resistance then the desired 0.5 ohm.

Review Questions

1. Which of the following will an EEG not detect?
a) thoughts
b) stimuli
c) vision levels
d) ECG signals

2. The typical gain of an EEG amplifier is ________.
a) 10,000
b) 1,000
c) 100
d) 100,000

3. On a paper chart system, the chart speed is typically ________.
a) 2.5 cm per second
b) 50 mm per second
c) 3 mm per second
d) 30 mm per second

Answers: 1-a, 2-a, 3-d

David Harrington, PhD, is director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, and a member of 24×7’s editorial advisory board.


Corrections to 24×7 November 2005 ICC Prep Article: “Collimators and X-Ray Film”An old trick by instructors is to present wrong information to see if the students challenge it. Well, in the “Collimators and X-Ray Film” article that appeared in the November issue of 24×7, some bad information was presented. Unfortunately, it was not done as part of an education process but as a mistake on my part, for which I apologize.First, there was the statement that copper or aluminum are used as filter materials, but copper has not been used in years.For some reason, we reversed the fact that when film is exposed to light it turns black, not clear. Also, I was informed that the 12:1 grid is most common.Sorry for the mistakes, and please let me know when I make them in the future. —DH