Biomedical Engineering Reference
In-Depth Information
of the head was measured with a normal subject and the amplitudes were
compared between the vertical dewar and the horizontal one.
Figure 3.56 shows the time variation of the magnetic field in 5 s caused
by small magnetic object, a 2 mm square part of a subway ticket, attached to
the forehead of the subject. These are not the noise signals that appear in an
ordinary MEG measurement, but they are a good indication of the relative
value of the vibration of the head position of the subjects. The subject was
asked to stay still during the measurements in a vertical-type dewar case
and a horizontal dewar case. The vibration was about 65 pT peak-to-peak
for the horizontal case, but 320 pT peak-to-peak for the vertical case. The
improvement factor is about 5. The factor for a longer time span, such as
20 minutes, would be larger. The result for a seriously ill patient is imagined
to be much larger, because it is hard to maintain a straight and rigid pose
in a vertical system. Recently, swinging type dewars which allow patients to
lie in a supine mode have been described [70-72], and those would show a
similar improvement.
It is practical for the operator to refer to the helium consumption rate in
terms of the necessary amount including loss during transfer, rather than in
terms of the boiling-off rate at the time in-between the refills. If the cycle for
refill is frequent, the overhead loss is not negligible. The necessary amount
of liquid helium to maintain this system is 100 liter/week or 5200 liter/year,
and the refill cycle is one refill per week.
User Friendliness.
As described in the previous sections, the systems have
some merit as far as “user friendliness” is concerned. We now summarize the
issue from this point of view, adding some other functions which have not
been referred to before.
Operator Friendliness.
The liquid helium refill operation with the LHe stor-
age dewar can be done outside the MSR with the door closed. The transfer
tube is installed in the dewar with one port sticking out of the MSR. The
operator connects the extension tube to be inserted in the 100 liter helium
container. There is no need to open the door of the MSR or to jolt the dewar
to be connected to the LHe container outside the MSR.
An automatic helium gas pressurizing system makes it easier to transfer
liquid from the container to the dewar. A heater inserted in the container
heats up the liquid helium and the vaporized gas forces the liquid helium
to transfer from 100 liter container to the dewar. The level gauge monitors
the liquid helium in the dewar and automatically stops the heating up when
the dewar is filled. There is no need for an extra helium gas cylinder or
an operator's rubber balloon. The whole process, including setting-up, is
accomplished in about 45 min.
Parameter tuning for the DOIT type FLL circuit is done automatically.
It takes only 30 s and saves 3 hours compared to manual tuning. Flux-locked
