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In biomagnetic measurements, such as magnetoencephalography (MEG) and magnetocardiography (MCG), very weak magnetic fields are measured. Therefore, biomagnetic measurements are mostly carried out inside a walk-in relatively high performance magnetically shielded room (MSR). Generally speaking, the amplitude of MCG signals is higher than that of MEG's ignals by about one order, so the rooms for MCG can be designed with lower performance, and constructed cheaply and light enough for any floor in a building. A trial MSR we developed incorporated active compensation combined with a simplified MSR of one permalloy layer and one pair of cancellation coils to enable moderate performance at low cost mainly for MCG. A 3-D magnetic field analysis of the simplified MSR was carried out to obtain a guide for designing the system, and the results compared with a system consisting of only one pair of cancellation coils. We found that firstly, the distribution of the magnetic noise generated by a line current source was diminished and rendered uniform by the MSR compared with the case without the MSR, enabling the distance between reference and measuring sensors were to be larger. Secondly, the magnetic field distributions were made more uniform when the cancellation coils were outside the MSR than when inside it. The phase shift when the cancellation coils were inside the MSR was lower than when outside it. Thirdly, the phase shift by measurements when the cancellation coils were inside the MSR was found to be less than that when they were outside, as predicted, but that when inside was greater than 10 degrees although shifts of less than this had been predicted.

Evaluation of the Magnetic Field Distribution & Phase Shift inside a Simplified Magnetically Shielded Room for Active Shielding System