Multi-Channel SQUID-Based Ultra-Low Field Magnetic Resonance Imaging in Unshielded Environment

Magnetic Resonance Imaging (MRI) is the best method for non-invasive imaging of soft tissue anatomy. A conventional MRI relies on 1.5-3 T fixed strength magnetic fields, with parts-per-million homogeneity, requiring large and expensive magnets. MRI can be done at ultra-low magnetic fields (ULF) with Larmor frequencies of a few kHz with much more modest magnetic system requirements. However the ULF regime requires a very sensitive detection system. A candidate detection system is based on SQUID gradiometers. A conventional SQUID gradiometer based detection system requires effective shielding from all ambient electromagnetic noise. Large shielded structures, such as magnetically shielded or eddy-current rooms, can be used for proof-of-principles experiments but do not lead to practical deployable instruments. Our goal is to develop a technique in which a SQUID-based detector array could be deployed without the limitation imposed by the requirement for a shielded structure. We have tested a 7-channel ULF MRI system located in unshielded environment inside a modern physics laboratory. It was possible to significantly suppress most of the electromagnetic interference by subtracting the signal from a one-channel reference magnetometer located nearby. We believe that the influence of the pre-polarization coil produces kHz-range frequency noise in gradiometer channels that is very well correlated with the signal from the magnetometer.