The use of a squid third order spatial gradiometer to measure magnetic fields of the brain.

It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend to pursue this direction with software systems. Some of the data presented suggest that components of MEG evoked activity may change independently of EEG. One interpretation which may derive from this is that the same current dipoles are probably not responsible for the entire configuration of evoked fields. This interpretation is consistent with EEG evidence which indicates that analogous components in the evoked potential may vary independently as a function of stimulus parameters and information processing. Perhaps a model of magnetic dipoles due to small current loops would be more compatible with the electrophysiological data.

Advanced electronics for the CTF MEG system.

Development of the CTF MEG system has been advanced with the introduction of a computer processing cluster between the data acquisition electronics and the host computer. The advent of fast processors, memory, and network interfaces has made this innovation feasible for large data streams at high sampling rates. We have implemented tasks including anti-alias filter, sample rate decimation, higher gradient balancing, crosstalk correction, and optional filters with a cluster consisting of 4 dual Intel Xeon processors operating on up to 275 channel MEG systems at 12 kHz sample rate. The architecture is expandable with additional processors to implement advanced processing tasks which may include e.g., continuous head localization/motion correction, optional display filters, coherence calculations, or real time synthetic channels (via beamformer). We also describe an electronics configuration upgrade to provide operator console access to the peripheral interface features such as analog signal and trigger I/O. This allows remote location of the acoustically noisy electronics cabinet and fitting of the cabinet with doors for improved EMI shielding. Finally, we present the latest performance results available for the CTF 275 channel MEG system including an unshielded SEF (median nerve electrical stimulation) measurement enhanced by application of an adaptive beamformer technique (SAM) which allows recognition of the nominal 20-ms response in the unaveraged signal.

Biomagnetometers for unshielded and well shielded environments.

The paper describes magnetometers for operation in well shielded and unshielded environments. For unshielded environments, the noise cancellation is accomplished by spatial filtering using higher-order gradiometers which are formed in software. The theory of the software formation of high-order gradiometers has been successfully tested experimentally to the second-order gradient level, using a seven-channel prototype system. An unshielded, open environment system, designed to operate with third-order noise cancellation is presently under construction. It is a whole head system, consisting of a minimum of 64 sensing channels, and is expected to be operational by late 1991. In the opposite extreme, a magnetometer is being developed for operation in a well shielded environment of a whole body superconducting shield. The high temperature superconducting shield will operate in liquid nitrogen, while the magnetometer detection system will operate in liquid helium. The high level of shielding will allow detection of biomagnetic signals using magnetometers. The whole body high temperature superconducting shielded system is presently under development and a half scale working shield already exists. The complete whole body shielded system is expected to be operational in 1994/95.