Biomedical Engineering Reference
In-Depth Information
9.3
Data Acquisition and Preprocessing for the OR
Voltage signals, like the EEG, are always measured as a difference in potential
between two points, thus a bioelectric amplifier has two signal inputs, a plus and a
minus. Bioelectric amplifiers also have a third input for a reference electrode, which
is discussed later. Because the electrical activity of the cortex is topographically het-
erogeneous, it is generally advantageous to measure this activity at several locations
on the scalp. In diagnostic neurology, several systems of nomenclature for electrode
placement have evolved. The most commonly used at present is the International
10-20 system [20].
Practicality in the operating room environment requires an absolute minimum
of time be spent securing scalp electrodes. Production pressure in the form of social
and economic incentives to minimize time between surgeries will react negatively to
time-consuming electrode montages and doom products that require them to failure
unless absolutely required for clinical care. Nonresearch, intraoperative EEG moni-
toring for drug effect is now performed exclusively with preformed strips of elec-
trodes containing one or two channels. Many of these strips are designed to self-prep
the skin, eliminating the time-consuming separate step of local skin cleaning and
abrasion.
Note that this environment is quite distinct from that of the neurology clinic
where diagnostic EEGs are seldom recorded with fewer than 16 channels
(plus-minus pairs of electrodes) in order to localize abnormal activity. Monitoring 8
or 16 channels intraoperatively during carotid surgery is often recommended,
although there is a paucity of data demonstrating increased sensitivity for the detec-
tion of cerebral ischemia when compared with the 2- or 4-channel computerized sys-
tems more commonly available to anesthesia personnel. Although regional changes
in EEG occur during anesthesia [21, 22], there is little evidence that these topo-
graphic features are useful markers of clinically important changes in anesthetic or
sedation levels [23], and most monitors of anesthetic drug effect use only a single
frontal channel.
9.3.1 Amplifiers
As noted previously, the EEG signal is but one of several voltage waveforms present
on the scalp. Although all of these signals may contain interesting information in
their own right, if present, they distort the EEG signal. An understanding of the
essential characteristics of specific artifacts can be used to mitigate them [24]. A
well-designed bioelectric amplifier can remove or attenuate some of these signals as
the first step in signal processing. For example, consider power-line radiation. This
artifact possesses two characteristics useful in its mitigation: At its very low fre-
quency, it is in the same amplitude phase over the entire body surface and it is a sin-
gle characteristic frequency (50 or 60 Hz). Because EEG voltage is measured as the
potential difference between two electrodes placed on the scalp, both electrodes will
have the same power-line artifact (i.e., it is a common-mode signal). Common-mode
signals can be nearly eliminated in the electronics stage of an EEG machine by using
a differential amplifier that has connections for three electrodes: plus (+), minus (-),
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