Low-field MRI measurements using a tuned HTS SQUID as detector and permanent magnet pre-polarization field

In order to improve the signal-to-noise ratio (SNR) of low-field (LF) magnetic resonance imaging (MRI) measurements with a tuned high-Tc rf superconducting quantum interference device (SQUID) as a signal detector, we utilize a permanent magnet (PM) pair for sample pre-polarization. MRI images are acquired by using filtered back projection reconstruction. The projections are obtained by recording free induction decay or spin echo signals with the gradient field applied at different angles. For every projection, the sample is first pre-polarized in the gap of the PM pair and then mechanically transported to the measuring position underneath the tuned SQUID. 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−1/2, two-dimensional LF-MRI images of water phantoms are obtained with sufficient SNR even without averaging. These images demonstrate the feasibility of LF-MRI based on a tuned SQUID detector, permanent magnet pre-polarization and a moving sample. (paper)