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room makes the procedure cumbersome with the present technology.

Biomagnetism offers a tool to study processes where electrical function is important. Promising results have been obtained in the fields of cardiology and epilepsy. Sites of origin for heart arrhythmias (e.g., the accessory path for Wolff-Parkinson-White syndrome) can be identified by electrical activity in abnormal anatomical locations. Non-invasive localization allows treatment by guiding an ablation catheter directly to the correct site. MSI can also be used in the surgical treatment of intractable epilepsy to locate the epileptigenic focus and functional areas of the brain that must be conserved during surgery. A potential use in neuroscience is the spatial and temporal study of functional processing areas in the brain in response to auditory, visual, and somatosensory stimuli. Biomagnetism also offers a research tool for studying schizophrenia as well as Parkinson's and Huntington's syndromes. The function of peripheral nerves can be studied, and prenatal magnetocardiography can be carried out.

A major strength of MSI is that it can resolve events separated by milliseconds, whereas other methods such as functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), positron emission tomography (PET), and single photon emission computed tomography (SPECT) have time resolutions of seconds to many minutes, depending on the information sought. A weakness of MSI is that any magnetic field distribution on the surface of the head can be explained by an infinite number of current distributions inside the head. Thus, a successful source analysis is dependent on the availability of additional information suitable for constraining the inverse problem to be solved.

Because of their extreme weakness, on the order of femto-teslas, biomagnetic fields require sensitive sensors. Up to now only superconducting quantum interference devices (SQUIDs) have been able to achieve the required sensitivity below image. In order to separate the biomagnetic fields from much larger external interferences, screening chambers made from soft magnetic and conductive material are necessary. The two technical areas most in need of development are simplification of the detector array and creation of methods to remove interference without the use of an expensive (e.g., $500,000) enclosure. An active screen applying a closed control loop with a sensor for the interfering field and compensating current loops could be an attractive alternative to the passive screen.

SQUIDs based on high-temperature superconductor technology could operate with inexpensive liquid nitrogen. Current technology has already led to devices with a sensitivity of about image, which is still an order of



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