title: Parallel MRI at microtesla fields.
abstract: 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 mu T 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 threefold 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.
authors.affiliation: Los Alamos National Laboratory, Group of Applied Modern Physics, MS D454, Los Alamos, NM 87545, USA. vzotev@lanl.gov <vzotev@lanl.gov>
authors.forename: Vadim S
authors.initials: VS
authors.lastname: Zotev
authors.forename: Petr L
authors.initials: PL
authors.lastname: Volegov
authors.forename: Andrei N
authors.initials: AN
authors.lastname: Matlashov
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Espy
authors.forename: John C
authors.initials: JC
authors.lastname: Mosher
authors.forename: Robert H
authors.initials: RH
authors.lastname: Kraus
mesh_terms: D016477:Artifacts; D004867:Equipment Design; D005583:Fourier Analysis; D006225:Hand / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging