Biomedical Engineering Reference
In-Depth Information
dewar. Another merit is that the patient can be measured in a lying position,
which is more relaxing compared to the conventional vertical type.
System Configuration.
The array of SQUID sensors is installed in the
liquid helium dewar, which keeps the sensors at around 4.2 K. The dewar is
placed inside the magnetically shielded room (MSR) in order to eliminate the
environmental noise. The sensors are operated with flux-locked loop (FLL)
electronics, which are located outside the MSR. The magnetic signals sensed
by SQUID are transformed to voltage signals and processed through analog
signal processors, and are acquired and stored by a data acquisition (DAQ)
system.
The data are collected by the analyzing software, which operates on Win-
dows NT on the host computer. Data analysis such as equivalent current
dipole (ECD) estimation is done on this console. Operators can easily com-
pile reports with the analyzed results using common word processing, desk-
top publishing software, spreadsheet software, or graphing according to their
preferences. The operation of the SQUID sensor, such as the lock or unlock
mode, is also controlled through this console. The post-processing software
and the acquired data are transportable to other PCs so that the users can
do further analysis using their own desktop or laptop Windows PCs. The
stimulation apparatus for the subjects is also provided and can be controlled
through the console. Another peripheral instrument is the automatic liquid
refilling system, which reduces the operator's burden. The subjects lie down
on a bed in a relaxed position to have their MEG responses to various sti-
muli measured. Figure 3.52 shows an overview of the system. We describe
the components of the systems in the following sections.
Sensor and Electronics.
A coaxial type first-order gradiometer is adopted
for the SQUID sensor. The sensing coil and reference coil have a diameter
of 15.5 mm and the baseline between the two coils is 50 mm. The magnetic
signal is transferred to a double-washer SQUID placed about 75 mm from the
sensing coil. There is small possibility of flux trapping caused by magnetic
materials that some subjects might bring into the tail of the dewar carelessly,
such as eyeglasses.
We have employed an external feedback coil configuration [67]. The feed-
back current to compensate the input magnetic signal is fed to the sensing
loop, the reference, the coil, and the input coil, so that the field to be mea-
sured suffers no disturbance. Consequently, the cross-talk between adjacent
sensors is reduced. To measure the cross-talk, two gradiometers are placed
in parallel and adjacent to each other at a separation of 23 mm. The level
of cross-talk is measured by monitoring one of the SQUID outputs when a
sinusoidal flux of known amplitude, 2.0
Φ
0
, is applied to another SQUID,
both being in FLL mode [68]. The result is excellent,
−
93
.
3 dB, thanks to
the external feedback coil configuration.
