NOISE CANCELLATION BY A WHOLE-CORTEX SQUID MEG SYSTEM

We report on the noise cancellation performance of several whole-corte x MEG systems operated under diverse noise conditions, ranging from un shielded environments to moderately shielded rooms. The noise cancella tion is performed by means of spatial filtering using high order gradi ometers (2nd or 3rd), which can be formed optionally either by SQUID e lectronics firmware or by software. The spatial dependence of the grad iometer responses was measured in an unshielded environment and compar ed with simulations to yield an estimate of the system common mode mag nitudes relative to field, 1st, and 2nd gradients. High order gradiome ters were also formed when the system was operated in moderately shiel ded rooms. The combination of the spatial filtering and room shielding resulted in very high combined noise rejections approaching that of h igh quality shielded rooms. Examples of noise cancellation under vario us conditions will be shown.