Low Field MRI Detection With Tuned HTS SQUID Magnetometer

We set up a low field (LF) magnetic resonance imaging (MRI) system with a tuned high-T_c (HTS) superconducting quantum interference device (SQUID) consisting of a SQUID magnetometer and a liquid-nitrogen-cooled LC resonant pickup circuit, which are inductively coupled. Around 9 kHz, the configuration enhances the sensitivity of the system to 6-7 fT/√Hz. Meanwhile, the relatively large diameter of the coil L compensates the magnetometer´s drawback of a small pickup area. We performed our experiments with a prepolarization field of 10 mT in a magnetically shielded room. The measurement field was 212.2 μT, corresponding to the Larmor frequency of 9.038 kHz. The compromise between the signal-to-noise ratio and the spatial resolution for our system was studied by recording one-dimensional MRI images of carrots under different gradient fields. For the sample of two carrot slices with center distance of 2.8 cm, a gradient field of 20 Hz/cm was suitable. Two-dimensional images were finally acquired. A HTS system for biological LF MRI detection by a tuned SQUID is demonstrated to be feasible.