Biomedical Engineering Reference
In-Depth Information
(a)
(b)
z
x
y
Fig. 3.65. a
The configuration of the composite gradiometer.
b
The configuration
of the probe of the system and its
x
-
y
-
z
coordination
Instrumentation.
We used composite SQUID gradiometers in the system
to observe not only the radial components of the biomagnetic fields from
the subjects but also the tangential components. Each combination sensor is
composed of one axial-type gradiometer and two planar-type gradiometers,
located perpendicular to each other as shown in Fig. 3.65a; therefore the
three-dimensional components of the magnetic field vectors can be observed.
The pickup of axial type is a round coil with a diameter of 15.5 mm. The
planar-type pickup is a square coil with a 14 mm side length. Both gradio-
meters have a baseline length of 50 mm.
Eight composite gradiometers arranged in a 2
×
4 matrix are attached to
a flat plane as shown in Fig. 3.65b.
The SQUIDs are driven by the direct offset integration technique (DOIT)
[69], which is useful for the reduction of both cost and complexity, especially
in a multi-channel magnetometer system.
The array of composite gradiometers is installed in a glass fiber reinforced
plastic (GFRP) cryostat. The cryostat has a capacity of 28 liters of liquid
helium and can maintain the superconductivity of the sensors for about 84
hours without refilling. The thickness of the bottom of the cryostat is less
than 7 mm.
It is necessary to identify the exact values of the location and direction
of the sensors relative to the subjects for precise measurement of the spatial
distribution of the field.
