Spatial Fourier Technique for Calibrating Gradiometers
One of the main features of the biomagnetic technique is the possibility to identify and locate sources of bioelectric activity with an uncertainty of a few millimeters. The use of multichannel systems not only eases this procedure shortening the time of the measurement session but also enables simultaneous measurement at different points in space, making possible the observation of non periodic phenomena. However, a multichannel system can introduce an important source of error in the localization result, if the calibration factor (field/voltage) of each channel is not measured with an extreme accuracy (Costa Ribeiro et al, 1988). The prospect of using planar gradiometers in biomagnetism imposes further difficulties to adapt the calibration procedures normaly used for axial gradiometers. In this work we propose a very general calibration method valid for all kinds of gradiometers and arrays. It is based on a spatial Fourier technique and in the fact that the gradiometer performs discrete spatial filtering.
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