Elekta Neuromag NM21283A, Руководство пользователя

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Principles of SQUID operation 6
NM21283A-A 27
C H A P T E R 6
Principles of SQUID operation
This chapter explains the operating principles of a SQUID sensor and
readout electronics. It gives the necessary background information that is
needed in manual tuning of the sensors.
6.1 Operation of a SQUID magnetometer
The Superconducting QUantum Interference Device (SQUID) is the only
sensor with sufficient sensitivity for biomagnetic measurements. The
SQUID is a transducer that converts magnetic flux into electric signal.
The basic structure of a SQUID magnetometer is illustrated in Figure 6.1.
The external magnetic field is not sensed directly by the SQUID; rather, it
is coupled to the SQUID detector by means of a flux transformer. The flux
transformer consists of two coils: a pickup coil that gathers the flux, and a
signal coil that couples it into the SQUID. This has the advantage of
increasing the field sensitivity by increasing the effective sensor area
(magnetic flux equals field times sensor area). Vectorview uses two kinds
of pickup loops: magnetometers and gradiometers. Magnetometer loops
are simple rectangular loops. Gradiometer loops comprise two rectangular
coils side by side, wound in opposite sense. This arrangement is “near-
sighted” which greatly improves the rejection of environmental magnetic
noise.
The SQUID is formed out of a superconducting ring, interrupted by two
weak links or so called Josephson junctions. Without these interruptions
the external magnetic fields, such as those generated by the brain, would
have no detectable effect on the superconducting ring. These links consist
of a microscopical isolating layer that is thin enough to ensure that the
Figure 6.1 Schematic of a SQUID sensor
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SQUID Flux transformer
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