Optimization and Configuration of SQUID Sensor Arrays for a MEG-MRI System

The idea of using a large-scale superconducting quantum interference device array for simultaneous detection of both magneto-encephalography (MEG) and magnetic resonance images of the brain at ultra-low field (ULF MRI) is extremely attractive. It could reasonably improve the superposition of images from the two different modalities. Adding a ULF MRI capability to MEG implies the addition of coils for generation of fields and gradients. In addition there is a difference between the optimization criteria for pickup coils. MEG pick-up coils should be small enough to avoid smoothing of spatially sharp field distributions. In the case of ULF MRI the spatial resolution is defined by the applied gradients and the voxel signal-to-noise ratio but not by the pick-up coil diameter. Thus, ULF MRI systems may need fewer pick-up coils of larger size to cover the same area of interest. One approach is a hybrid design with different sizes and quantities of pick-up coils for recording of MEG and MRI signals. We describe a configuration of the 80-channel SQUID array that consists of 64 MEG and 16 MRI magnetometers. We also describe performance of gradiometers in comparison with magnetometers.