Tuned HTS SQUID-Detected Low Field MRI Using a Permanent Magnet for Pre-polarization With Automatic Transportation

We utilize a permanent magnet (PM) pair for sample prepolarization to improve the signal-to-noise ratio of low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements with a tuned high- T_c rf superconducting quantum interference device as a signal detector. The sample is first prepolarized in the gap of the PM pair and then automatically transported to the measuring position underneath the tuned superconducting quantum interference device in a magnetically shielded room. The transportation can be repeated in order to perform signal averaging. Because of the strong magnetic polarization field of about 1 T and the highly sensitive detector with a noise floor of about 7 fT/√Hz, the nuclear magnetic resonance spectrum of 0.5 ml fluorobenzene reaches a signal-to-noise ratio of about 13 in a single measurement, and 55 with 30 times averaging. The MRI images are acquired by using filtered back projection reconstruction. Twenty-four projections are obtained by recording free induction decay or spin echo signals with a gradient field applied at angular steps of 7.5^° each. Two-dimensional low-field MRI images of pepper and carrot pieces were acquired with different numbers of averages. Here, a spatial resolution of about 0.3 mm × 0.3 mm is achieved.