Development of SQUID-Based Compact Low-Field MRI System

We have developed a compact low-field magnetic resonance imaging (MRI) system that can be integrated with a magnetoencephalography system to facilitate the simultaneous measurement of magnetoencephalography and MRI for small animals. The superconducting quantum interference device gradiometer that detects the nuclear magnetic resonance signal was operated by a band-pass type flux-locked loop, and the artifact produced by the decay of the polarizing field was improved. Our MRI system includes a desktop-sized planar coil set (350 × 350 × 188 mm
³
). It has five pairs of coils: a pair of cylindrical coils for the polarizing field and four pairs of shielded planar coils for the measurement field and the 3-D gradient fields. On account of the grace of the coil patterns that were created by a target field method, the homogeneities of the magnetic field were below ±0.2% over 40 mm diameter sphere volume (DSV). The pulse sequence to detect the spin-echo signal was designed for 2-D Fourier imaging. A 41-mm field-of-view with a pixel size of 1.3 mm was achieved. We carried out a 2-D imaging measurement using a phantom created by a 2 × 10
³
mm
³
NiCl
₂
aqueous solution and obtained a clear image.