Static field homogeneity artifacts due to magnetic flux expulsion by HTS coils for high-resolution magnetic resonance imaging

High-Temperature Superconducting (HTS) radio frequency coils can greatly improve the sensitivity of Magnetic Resonance Imaging (MRI), with signal to noise ratios increased by up to 16 times. However, as homogeneity of the static field B0 is a major condition for MR acquisitions, their propensity to warp the magnetic field in their vicinity as a result of magnetic flux expulsion could cause severe image degradation. In this Letter, we report on the observation of artifacts generated by magnetic flux expulsion from a HTS coil. To that end, we acquired MR images of a water sample in the vicinity of a nonresonant HTS coil for different cooling conditions. These are zero field cooling and field cooling of the coil inside a clinical MRI bore magnet of 1.5 T, for a range of working temperatures of the HTS coil. After zero-field cooling, we observed significant artifacts in the MR images, increasing with lower temperature; images acquired after field cooling did not display these alterations. We propose a phenomenological model for the magnetic field disturbance induced by the HTS coil. This model reproduces, in good agreement with the experiments, the phase distortion observed in the MR images and allows one to extract the critical current density of the HTS coil, an important technological parameter of superconductors. Finally, we discuss the necessity of field cooling as a way to avoid these B0 inhomogeneity artifacts.