  Parallel imaging techniques have been widely used in high-field magnetic
resonance imaging (MRI). Multiple receiver coils have been shown to improve
image quality and allow accelerated image acquisition. Magnetic resonance
imaging at ultra-low fields (ULF MRI) is a new imaging approach that uses SQUID
(superconducting quantum interference device) sensors to measure the spatially
encoded precession of pre-polarized nuclear spin populations at
microtesla-range measurement fields. In this work, parallel imaging at
microtesla fields is systematically studied for the first time. A seven-channel
SQUID system, designed for both ULF MRI and magnetoencephalography (MEG), is
used to acquire 3D images of a human hand, as well as 2D images of a large
water phantom. The imaging is performed at 46 microtesla measurement field with
pre-polarization at 40 mT. It is shown how the use of seven channels increases
imaging field of view and improves signal-to-noise ratio for the hand images. A
simple procedure for approximate correction of concomitant gradient artifacts
is described. Noise propagation is analyzed experimentally, and the main source
of correlated noise is identified. Accelerated imaging based on one-dimensional
undersampling and 1D SENSE (sensitivity encoding) image reconstruction is
studied in the case of the 2D phantom. Actual 3-fold imaging acceleration in
comparison to single-average fully encoded Fourier imaging is demonstrated.
These results show that parallel imaging methods are efficient in ULF MRI, and
that imaging performance of SQUID-based instruments improves substantially as
the number of channels is increased.
