The invention relates to a method of determining the spatial field distribution and/or the spatial position of a useful field source, producing the field distribution by means of a multi-channel field measuring device which comprises a plurality of spatially distributed sensors which generate measured values which contain on the one hand useful measured values stemming from the useful field source and on the other hand noise-measured values stemming from at least one noise field source, correction values being formed and superposed on the measured values so that compensated measured values are obtained which are compared with similarly compensated, mathematically derived reference values which are produced by a fictitious reference field source by means of the field sensors, the method thus determining the reference field source for which the pattern of compensated reference values corresponds best to the pattern of compensated measured values.
Using suitable known sensors, such a method enables the measurement of electrical or magnetic field distributions or the localization of associated field sources. For example, magnetoresistive elements constitute suitable magnetic sensors. Sensors utilizing the rotation of the magnetization in ferromagnetic films are also highly sensitive. SQUIDs are the sensors customarily used for measurements in the medical field.
The method of the invention is preferably used for the measurement of biomagnetic fields which are produced by currents flowing in the body, notably in the head or the heart.
A method of the kind set forth is known from DE-A-43 04 516.
Because on the useful fields to be measured there are usually superposed much stronger noise fields from noise field sources which are remote in comparison with the useful field source to be examined, the measured values contain comparatively large noise-measured values in addition to the desired useful measured values. Therefore, steps must be taken so as to suppress the noise measured values. In the known method noise signal vectors are derived from a pattern of measured values determined during a given period of time, said noise signal vectors being used to correct the measured values. It is assumed that the noise signal only slightly, resembles the useful signal. The comparison of the measured values with calculated field values is based on a corrected measured signal vector and a corrected source signal vector.
If the use of complex, heavy and expensive shielding devices is to be avoided, the steps taken for noise field compensation must be particularly effective. This holds notably if gradiometer devices are not or cannot be used for medical SQUID measurements, as is the case for example for SQUIDs ceramic high-temperature superconductors. This is because the manufacture of the superconducting connections required for conventional gradiometer devices is problematic.
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