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3.1 Montages should be designed in conformity with Guideline 6: A Proposal for Standard Montages to Be Used in Clinical Electroencephalography. It is desirable that at least some montages in all laboratories be uniform to facilitate communication and comparison. Digital systems allow reformatting of montages to provide optimal display of activity at the time of interpretation. To permit this flexibility, initial recording must be made from a referential montage; but the system reference itself cannot easily be reformatted. For this reason the digital recording reference should be an additional electrode (or combination of electrodes), and not one of those in the 10:10 or 10:20 system. An additional electrode between Cz and Pz is commonly used. The use of linked ears as a digital recording reference is specifically discouraged.
3.2 The record should have written on it as a minimum the name and age of the patient, the date of the recording, an identification number, and the name or initials of the technologist.
Identifications should be made at the time of recording. Failure to do so may result in errors that have adverse medical and legal consequences. A Basic Data Sheet, attached to every record, should include the time of the recording, the time and date of the last seizure (if any), the behavioral state of the patient, a list of all medications that the patient has been taking, including premedication given to induce sleep during EEG, and any relevant additional medical history.
3.3 Appropriate calibrations should be made at the beginning and end of every EEG re-cording. If feasible, a recording with all channels connected to the same pair of electrodes should follow at the beginning. However, this biological calibration may not be possible with all digital systems. At the outset, all channels should be adjusted, if necessary, so that they respond equally and correctly to the calibration signal. When doubt as to correct functioning of any amplifier exists, a repeat calibration run should be made.
The calibration is an integral part of every EEG recording. It gives a scaling factor for the interpreter, and tests the EEG machine for sensitivity, high and low-frequency response, noise level, and pen alignment and damping. It also gives information about the competence and care of the technologist. Calibration voltages must be appropriate for the sensitivities used.
In addition to the standard square-wave calibration, the biologic calibration (“bio-cal”) may at times be of additional help in detecting errors in the montage selection process or in the pen-writing mechanism. For this purpose, an anteroposterior (fron to occipital) derivation should be used, since it can include fast and alpha range patterns as well as eye movement activity in the delta range. In digital systems that lack full provision for instrumental and biological calibration, the first 30 seconds of recording should be observed by the technologist from the primary system-reference montage.
3.4 The sensitivity of the EEG equipment for routine recording should be set in the range of 5—10 μV/ mm of pen deflection.
Sensitivity is defined as the ratio of input voltage to pen deflection. It is expressed in microvolts per millimeter (μV/mm). A commonly used sensitivity is 7 μV/mm, which, for a calibration signal of 50 μV, results in a deflection of 7.1 mm.
If the sensitivity is decreased (for example, from 7 to 10 μV/mm), the amplitude of the writeout of a given EEG on the paper also decreases. Conversely, if the sensitivity is increased (for example, from 7 to 5 μV/ mm), the amplitude of the writeout of a given EEG increases.
When the sensitivity is less than 10 μV/mm (for example, 20 μV/mm), significant low-amplitude activity may become indiscernible. If the sensitivity is greater than 5 μV/mm (for example, 3 μV/mm), normal EEG activity may overload the system, causing a squaring off of the peaks of the writeout onto the paper or overlapping of traces on the computer monitor.
Note that a sensitivity of 5 μV/mm means that, to obtain a pen deflection of 1 mm, a 5-uV input voltage is required (and correspondingly, to obtain a 10-mm deflection, an input of 50 μV is required). If the sensitivity is decreased to 10 μV/mm, the same 1-mm pen deflection now requires a larger input, i.e., 10 μV rather than 5 μV (and correspondingly, a 10-mm pen deflection now requires an input of 100 uV rather than 50 μV). Thus, as the sensitivity is increased, its numerical value becomes smaller. Conversely, as the sensitivity is decreased, its numerical value becomes larger. This perhaps seemingly paradoxical relationship is actually a logical consequence of the definition of sensitivity as input voltage per unit of pen deflection. With digital systems, this straightforward physical relationship is lost. Because the dimensions of computer monitors will vary, clear scale markers must be available as part of the display.
The operation of EEG amplifiers can also be expressed as gain, defined as the ratio of the output voltage to the input voltage. For example, if an EEG input signal of 10 μV is amplified to 1.0 V in order to move the mechanical pens of an electroencephalograph, then the gain is 1.0/.00001 = 100,000. The gain of an analog or digital system is not as obvious to the user as the sensitivity.
During calibration for routine recordings, the recorded signals should not be distorted but should be large enough to permit measurement to better than ±5% between any of the signals on the different channels.
No matter which sensitivity (within the above limits) is chosen prior to the recording, appropriate adjustments should be made whenever EEG activity encountered is of too high or low amplitude to be recorded properly.
3.5 For standard recordings, the low-frequency filter should be no higher than 1 Hz (—3 dB) corresponding to a time constant of at least 0.16 s. The high-frequency filter should be no lower than 70 Hz (-3 dB). Note, however, that to display frequencies as high as 70 Hz, a computer monitor would need a horizontal resolution of at least 1400 pixels in the data display area. Interpreters should be aware that some loss of high-frequency resolution will otherwise occur, along with the possibility of lower-frequency distortion due to spatial aliasing.
A low-frequency filter setting higher than 1 Hz should not be used routinely to attenuate slow-wave artifacts in the record. Vital information may be lost when pathologic activity in the delta range is present. Similarly, a setting lower than 70 Hz for the high-frequency filters can distort or attenuate spikes and other pathologic discharges into unrecognizable forms and can cause muscle artifact to resemble spikes. Production of a record with lost or inaccurate information is poor medical practice.
It must be emphasized, however, that judicious use of the low- or high-frequency filters—with appropriate annotation on the record—can emphasize or clarify certain types of patterns in the record. These filter controls, therefore, should be used selectively and carefully.
3.6 The 60-Hz (notch) filter can distort or attenuate spikes; it therefore should be used only when other measures against 60 Hz interference fail.
3.7 A paper speed of 3 cm/s, or digital display of 10 seconds/page, should be utilized for routine recordings. A paper speed of 1.5 cm/s, or 20 seconds/page, is sometimes used for EEG recordings in newborns or in other special situations.