EEG equipment
EEG has changed greatly in recent years. For many years, machines were fairly standard, analog machines with paper display. Sixteen channels was the norm, though additional channels helped with localization. Most of the recent advance has been in the conversion to digital machines. These usually use a CRT display, although there are a few older machines which would print on paper. Now, paper print is mainly used for only selected portions of the record rather than the entire record. Digital vs analog display
EEG equipment is either analog or digital. Analog machines are commonly in use, although digital machines are of increased popularity. Analog machines show a direct representation of the cerebral signal amplified many times to be displayed on paper. Analog filters change the frequency components of the displayed signal prior to writing on paper. Digital EEG machines produce display output which looks similar to the output of an analog machine, but the signal handling is very different. Cerebral signal is
amplified then digitized, as discussed in Chapter 2. Digital filters alter the frequency components of the signal. The display is usually on a computer monitor although the results can be written on paper. In the interest of conservation of paper and ease of interpretation, electronic display is preferable to paper display. Electronic display allows for alteration of filters, gain, and montage “on-the-fly”.
Digital machines have several advantages over analog machines:
Figure 3-4: Diagram of analog vs digital EEG recording.
A: Flow chart of the function of analog EEG. The initial 10x amplifier is to elevate the magnitude of the signal so that there is sufficient signal voltage to reduce apparent artfat and signal degradation.
B: Flow chart of the function of digital EEG. The analog-to- digital converter, montage generator, and digital filters are part of the computer which is the core of the machine. The display may also be integral to the computer, but can also be an external, secondary high-resolution display.
• Flexible display montage • Continuously changeable gain • Reduced storage assets
• Lack of mechanical distortion
Digital interpretation should be used only with caution and not as a substitute for visual interpretation. Quantitative differences may be significant mathematically yet
meaningless clinically. Similarly, mathematical analysis may miss findings of great clinical importance. No hardware and software combination is close to threatening human interpretive abilities.
Display montage: Most digital machines record the potential from each electrode so that the montage can be altered by the reader. A spike seen on a longitudinal bipolar montage can then be viewed on a transverse bipolar montage. This can help localization, but the reader has to realize that multiple views of the same event do not represent multiple occurrences of the event.
Gain: High-amplitude transients can peg the pens of an analog display, so some
information is lost. Digital machines allow the gain to be changed, so that high and low- amplitude epochs will not be lost.
Storage assets: A busy EEG lab can generate tons of EEG paper. Although optical disk, CD, and DVDs cost money to record, the amount of information which can be placed on a disk results in a lower cost for archiving the data. Unfortunately, not all of the EEG machines use the same format for storage of the digital data, so that playing the information in the future can require having
compatible machines. This problem could be solved by having the data stored in a standardized format. Mechanical distortion: EEG machines have mechanical
distortion including arc distortion, inertial distortion, and overshoot. Arc distortion is due to the fact that the pen of a paper display moves around a pivot point, therefore, a perfectly vertical signal would produce an arc response (Figure 3-
Figure 3-5: Mechanical distortion.
A: An ideal response of EEG amplifier, filter, and display to a square-wave calibration pulse.
B: Mechanical distortion on paper because of overshoot of the pens. The inertia of the moving pen carries the recording beyond the target of the display. This error can be reduced by adjustment of pen pressure and by electrical corrective feedback.
C: Mechanical distortion when the compensatory
mechanisms described in B are excessive. The peak of the potetial is rounded.
5). Inertial distortion is due to the fact that the pens have mass and pressure on the paper. Therefore, it takes more energy to get the pen moving vertically than it does to keep it moving. Although the paper is moving horizontally, getting it to move vertically requires more energy. The other extreme of inertial distortion is potential overshoot of the pen when it is moving vertically. In the absence of some corrective maneuvers, the pen overshoots its vertical target, termed overshoot. Mechanisms to compensate for these mechanical distortions on paper recordings include electrical compensation of the amplifiers and adjusting pen pressure. Damping is the term used to describe the
compensatory mechanisms for inertial distortion. Digital machines are immune to these mechanical distortions.
Number of channels
The effective minimum number of channels for routine recording is 8, however, most neurophysiologists require 16 and 21 adds additional channels to montages which can greatly help localization. ICU monitoring for patients in status epilepticus can be performed with 4 channels, however, a baseline recording with a complete set of electrodes should be performed for evaluation of the discharge and background. Factors to consider in purchasing EEG equipment
The number of factors considered in making a decision is hugs, however, there are a few factors which deserve special mention.
Cost: Cost of the machine is a small part of the expense of an EEG lab. The space and technical help are costs which dwarf the cost of the EEG machine. Therefore, it makes little sense to skimp on machine features for the sake of small amount of money. Technical familiarity: Technicians should ideally use equipment which functions similarly between facilities. Therefore, decisions about office machinery should consider equipment which is available or to be acquired by other offices and medical facilities where the technicians will work. Physicians also will be using equipment in different facilities, so identical or similar function is highly desirable.
Service: EEG equipment occasionally needs service, so service needs to be considered. Guarantees of function and short-time to repair are important. Discussion of the local service facilities with other offices can be illuminating. An EEG lab which is
nonfunctional for a week can lost a substantial part of the profit margin and referral base. Portability: At least one of the hospital EEG machines must be portable so that ICU, ER, and OR recordings can be made. There are special requirements for equipment which will be brought into the OR, but most modern machines meet these requirements. This has to specified, however, if use in the OR is at all anticipated.
Reading stations: This is a moot point for paper EEG, but digital EEG can only be interpreted at workstations. Number, location, and availability of workstations should be
considered. For example, in one of our hospitals, EEG data is retained on a drive in the machine, so when a portable study is being performed in the OR or ICU while
disconnected from the network, EEGs performed on that machine cannot be read. This is not acceptable.