Advantages and limitations of magnetic source imaging

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Abstract
The term "magnetic source image" (MSI) describes the distribution of neuronal activity in the brain that can be deduced from measurements of the field pattern it produces across the scalp. The signals which provide the basis for an MSI are obtained from the magnetoencephalogram (MEG) which is conventionally recorded with superconducting detectors. Advances in MSI techniques during the past decade have revealed numerous aspects of the functional organization of human sensory systems that were previously unknown. In addition, studies of spontaneous signals, such as those in the alpha bandwidth, have identified specific cortical areas that support rhythmic activity. Extensions of this approach to cognitive research are able to determine the active cortical areas where spontaneous activity is suppressed when a person is engaged in a task such as mental imagery and auditory memory recall. Because only the component of the intracellular current tangential to the overlying skull contributes to the extracranial field, a confined source — modeled as a current dipole — has a characteristic field pattern that simplifies the pattern recognition problem of identifying the underlying sources. This advantage is illustrated by the identification of simultaneously active sources in auditory primary and association cortex. Their separate localization makes it possible to characterize their functional differences. Because the source strength in an MSI may be inferred without knowledge of the electrical conductivities of intervening tissue, it is also possible to estimate the extent of cortical envolvement. From the tangential source strength in an MSI, it is possible in most cases to determine thetotal source strength by taking account of the orientation of the cortical surface. This provides an objective, quantitative measure of the strength of neuronal activity. At present, the major limitation in more extensive use of MSI is the cost of instrumentation. While it requires no contact with the head, and measurements can commence within a few minutes of the arrival of the subject or patient, the present cost of a large array of sensors is two to three million dollars.