1. Technical Field
The present invention relates to a biomagnetic measurement apparatus, including a SQUID (Superconducting Quantum Interference Device) fluxmeter that measures weak magnetic signals generated from the heart, the brain, etc. of adults, children, fetuses, or the like. In particular, the present invention relates to the biomagnetic measurement apparatus having means for detecting and avoiding a condition of saturation, when an integrator of a fluxmeter operation circuit is saturated with noise and thereby magnetic measurements cannot be conducted.
2. Background Art
The SQUID fluxmeter has experienced problems such as magnetic field measurement errors, which are caused by changes of output signals of an integrator when phase changes or amplitude changes of amplifier outputs occur due to variations of temperature, power source, or other factors. In order to solve this problem, JP Patent Publication (Kokai) No. 9-257895 A (1997) discloses, in a fluxmeter using SQUID, a magnetometer comprising a comparator that compares an output from a SQUID with preset upper and lower limits. This magnetometer notifies the SQUID or a drive circuit of abnormalities when the output is beyond the upper or lower limits.
In addition to phase changes and amplitude changes of amplifier outputs, which are regarded as problems according to JP Patent Publication (Kokai) No. 9-257895 A (1997), when the SQUID fluxmeter detects further noise greater than a biomagnetic signal, an integrator of a fluxmeter operation circuit becomes saturated and a problem arises wherein a magnetic field lock of a magnetic sensor is released. When the magnetic field lock is released, the output of the fluxmeter does not revert back, even after the causative noise for releasing the lock disappears, and it is impossible to continue the measurement. Therefore, it is necessary to discover that the magnetic field lock is released as early as possible, to notify an operator of the above saturated state (detection of the release of the magnetic field lock), to take required measures to resolve the saturated state, for example, manually or automatically, and to conduct operations for the magnetic field lock again, thereby creating a state for enabling magnetic measurements.