Biomedical Engineering Reference
In-Depth Information
mode operation, the data acquisition operation, stimulus control, and data
reports can be done on a single PC, allowing the operator to remain in focused
on a single monitor.
Owner Friendliness.
A small MSR is sucient for installation because the
dewar is compact: it is 0.89 m in height and needs no gantry. Prospective
owners of the system need not be concerned about available space.
The necessary liquid He amount to maintain the system is 100 liter/week
or 5200 liter/year, and the refill cycle is 1 refill/week for this system.
An upgrade plan is easy to realize because the components of the system
are based on a 16-channel unit. It is easy to add on another 16 channels or
more. It is also easy to install additional sensors.
Subject/Patient Friendliness.
Conventional MEG dewars are of a vertical
type and their height is more than 1 m. The subjects/patients must sit be-
neath the dewar and put their heads into the concave-shaped tail: they must
sit with their necks straight and stiff during measurement. Especially for pa-
tients in a serious condition, it is a hardship to be measured beneath such
a vertical dewar. The dewars are settled on a gantry in order to adjust the
alignment of the angle of the head and the tail of the dewar. The gantry
structure causes some vibration problems. This might give the subject an
uncomfortable feeling under the heavy weight.
When using this horizontal dewar, the subject can lie down during the
MEG measurement. The reclining type of dewar is gantry free and the sub-
jects/patients feel less weight. The swinging type dewars of commercial com-
panies [70-72] have similar merits in relaxing subjects in a supine mode, but
those dewars still have gantry structures.
An Example of MEG Measurement.
Figure 3.57 shows an example of
the MEG response and the estimated equivalent current dipole of a normal
subject, a right-handed 35-year old male, with electrical stimulation to his
right-hand thumb. Measurement is done with a filtering band of 3-500 Hz,
a sampling rate of 2 kHz and 400
synchronized averaging. The time chart
is recorded with an origin time at the moment of stimulation. The signals
measured with 160 channel sensors are superimposed on the same chart, in
the upper left of Fig. 3.57. A clear response at 20.5 ms after the stimulus is
observed with a maximum amplitude of about 110 fT. The large signal at
0 ms is an artifact due to the stray current from the electrodes attached to
the subject for stimulation. The artifact does not cause deterioration of the
20.5 ms response because the latency is separated far enough. The contour
map of the isomagnetic field at 20.5 ms around the head is also displayed
in the upper right in reference to the position of the SQUID sensors. The
increment for the contour is 10 fT/step. The dark lines are positive, which
means that the flux goes from the inside to the outside of the head. The gray
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