Magnetic resonance imaging at ultra-low fields (ULF MRI) is a promising new imaging method that uses SQUID sensors to measure the spatially encoded precession of pre-polarized nuclear spin populations at a microtesla-range measurement field. In this work, a seven-channel SQUID system designed for simultaneous 3D ULF MRI and magnetoencephalography (MEG) is described. The system includes seven second-order SQUID gradiometers, characterized by magnetic field resolutions of 1.2 - 2.8 fT/rtHz. It is also equipped with five sets of coils for 3D Fourier imaging with pre-polarization. Essential technical details of the design are discussed. The system's ULF MRI performance is demonstrated by multi-channel 3D images of a preserved sheep brain acquired at 46 microtesla measurement field with pre-polarization at 40 mT. The imaging resolution is 2.5 mm x 2.5 mm x 5 mm. The ULF MRI images are compared to images of the same brain acquired using conventional high-field MRI. Different ways to improve imaging SNR are discussed.
@article{arxiv.0705.0661,
title = {SQUID-based instrumentation for ultra-low-field MRI},
author = {V. S. Zotev and A. N. Matlashov and P. L. Volegov and A. V. Urbaitis and M. A. Espy and R. H. Kraus},
journal= {arXiv preprint arXiv:0705.0661},
year = {2007}
}