To develop a method for slice‐wise dynamic distortion correction for EPI using rapid spatiotemporal B0 field measurements from FID navigators (FIDnavs) and to evaluate the efficacy of this new approach relative to an established data‐driven technique.
A low‐resolution reference image was used to create a forward model of FIDnav signal changes to enable estimation of spatiotemporal B0 inhomogeneity variations up to second order from measured FIDnavs. Five volunteers were scanned at 3 T using a 64‐channel coil with FID‐navigated EPI. The accuracy of voxel shift measurements and geometric distortion correction was assessed for experimentally induced magnetic field perturbations. The temporal SNR was evaluated in EPI time‐series acquired at rest and with a continuous nose‐touching action, before and after image realignment.
Field inhomogeneity coefficients and voxel shift maps measured using FIDnavs were in excellent agreement with multi‐echo EPI measurements. The FID‐navigated distortion correction accurately corrected image geometry in the presence of induced magnetic field perturbations, outperforming the data‐driven approach in regions with large field offsets. In functional MRI scans with nose touching, FIDnav‐based correction yielded temporal SNR gains of 30% in gray matter. Following image realignment, which accounted for global image shifts, temporal SNR gains of 3% were achieved.
Our proposed application of FIDnavs enables slice‐wise dynamic distortion correction with high temporal efficiency. We achieved improved signal stability by leveraging the encoding information from multichannel coils. This approach can be easily adapted to other EPI‐based sequences to improve temporal SNR for a variety of clinical and research applications.