Noise cancellation by a whole-cortex SQUID MEG system

We report on the noise cancellation performance of several whole-cortex MEG systems operated under diverse noise conditions, ranging from unshielded environments to moderately shielded rooms. The noise cancellation is performed by means of spatial filtering using high order gradiometers (2nd or 3rd), which can be formed optionally either by SQUID electronics firmware or by software. The spatial dependence of the gradiometer responses was measured in an unshielded environment and compared with simulations to yield an estimate of the system common mode magnitudes relative to field, 1st, and 2nd gradients. High order gradiometers were also formed when the system was operated in moderately shielded rooms. The combination of the spatial filtering and room shielding resulted in very high combined noise rejections approaching that of high quality shielded rooms. Examples of noise cancellation under various conditions are shown.<>