Biomedical Engineering Reference
In-Depth Information
3.5.1
Field Observation
We have developed a compact low-Tc three-axis superconducting quantum
interference device magnetometers (SQUID magnetometer) for geophysical
applications at low frequency from dc up to 1 kHz, such as for the search
for magnetized metals buried under the ground, or for moving magnetized
objects, and for measurements of geomagnetic fields. The SQUIDs used here
are relaxation oscillation SQUIDs (ROSs), which have a larger voltage output
compared to that of standard the SQUIDs used for the magnetoencephalo-
gram (MEG) system, so that greater dynamic performance is obtained for
stable operation in the field.
Currently, the major application of low-Tc superconducting quantum in-
terference devices (SQUIDs) is for magnetoencephalograms (MEG) to detect
extremely small magnetic fields in the order of
100 fT generated in a human
brain [81]. We have developed 160-channel systems for human brain research
and clinical diagnosis, which can help brain researchers and medical doctors
to investigate brain functions without harming patients [82]. In that case,
the SQUIDs are operated in a magnetically shielded room (MSR) to prevent
environmental noise.
While being widely applied for MEG systems, SQUIDs have also been
expected to function as promising sensors for geophysical applications. In
particular, the unrivaled sensitivity in the range at extremely low frequency
(ELF), lower than
∼
1 kHz, enables one to obtain more detailed properties
of the Earth deep below the surface. In the 1980s, Clarke et al. proposed
remote-reference magnetotellurics (MTs) by using SQUID magnetometers,
and indicated their availability for geological survey [83-85]. Besides this
use in MT methods, SQUIDs have a big potential in geophysics. Recently,
some geophysicists have been observing electromagnetic phenomena related
to seismic and volcanic activity [86,87].
It is expected that SQUID magnetometers will yield new results, not ob-
tainable with conventional sensors, not only in geophysics but also in astro-
∼
z
72 mm
x
y
Fig. 3.66.
A lateral X-ray image of the subject and the tail of the cryostat
