First experiences with a multichannel software gradiometer recording normal and tangential components of MEG

We describe a 28-channel magnetometer that operates as a 22-channel 'software gradiometer'. Gradiometer function is achieved by subtraction of weighted signals representing the noise field along three orthogonal axes as detected by six compensation channels. The instrument measures both normal and tangential field components; the user can select a total of 22 normal and/or tangential measuring sites from two arrays of 16 sites each. First experiences indicate that the combination of a shielded room with a software suppression of the residual field entering the room is amply sufficient to allow biomagnetic measurements in a hospital environment.