  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.
