SQUIDs in neuro- and cardiomagnetism

In this paper, an adequate account of the theory and practice of SQUIDs and of magnetoencephalography (MEG) and magnetocardiography (MCG) is given. MEG and MCG are two completely non-invasive imaging techniques, suitable for basic and clinical studies of human subjects. Large SQUID arrays, operating at liquid helium temperatures, are employed for detecting and localizing the magnetically active regions, modelled by current dipoles, in the working brain or heart. The measurements are usually performed in magnetically shielded rooms. Time resolution of both methods is better than 1 ms and spatial uncertainty 5. . . 6 mm. The evoked brain signals are as small as 40 fT, one billionth of the earths geomagnetic field. The dc SQUID is discussed with a detailed description of the ideas behind and construction of the modern VTT superconducting sensor. A shorted account is given on the PTB SQUID. Commercial multi-SQUID instruments, briefly described, are now available from three manufacturers. Seven examples of brain studies, including two clinical applications, are discussed. Cardiomagnetic instrumentation is described and the use of MCG in detecting heart abnormalities is presented with three examples. The advantages and drawbacks of modern imaging techniques, including MEG, MCG, MRI, fMRI, and PET, are compared and the future of MEG and MCG is discussed. Eventually, when it becomes feasible to use high-T_c SQUIDs in MEG and MCG, the high cost of magnetic measurements can be reduced.