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Calibration of a vector-MEG helmet system.

The MEG system Argos 500, recently installed at the University of Ulm, is designed for clinical application and routine use, to allow investigation of a large number of patients per day. To reach this goal, the system design meets the requirements of reliability, high field sensitivity, minimal set-up overhead before each measurement and an easy-to-handle user interface. The sensor system consists of a 163 vector-magnetometer array oriented and located in a suitable way to cover the whole head of the patient. Four additional triplets are available as references to build software gradiometers. To use this system at a high performance level, it must be properly calibrated, with these goals: to determine the actual geometry of the sensors array, which can deviate from the design specifications, and to determine the actual sensitivity of each sensor. The calibrating source consists of 31 coils placed at the corners of a head-size dodecahedron. Various details of the calibration system and process are presented here.