Multichannel ULF-MRI Study in Magnetic Unshielded Urban Laboratory Environment

Ultralow field (ULF) magnetic resonance imaging (MRI), which obtains images in the static magnetic field typically on the orders of tens to hundreds of microteslas, exhibits some potential advantages. However, the ULF-MRI system faces the challenge of poor signal-to-noise ratio (SNR). In addition to the introduction of a prepolarization technique and the usage of extremely sensitive superconducting quantum interference devices (SQUIDs), the multisensor parallel imaging technique can also improve the SNR and allow accelerated image acquisition. In this paper, a four-channel ULF-MRI system was built with low-T_c SQUID-based second-order axial gradiometer in an urban laboratory environment without magnetic shield. To suppress the temporal field fluctuation and to balance the environment gradient field tensor, an active compensation and a gradient field shimming system were developed. Using the Fourier imaging method and weighted image superposition, the 2-D and 3-D four-channel MRI images of pepper pieces were obtained at a measurement field of 129 μT, i.e., a Larmor frequency of 5.5 kHz with an in-plane resolution of about 2 mm × 2 mm. These results demonstrated the feasibility of performing 2-D and 3-D multisensor parallel ULF MRI in the unshielded urban environment.