Rapid measurement and correction of spatiotemporal B 0 field changes using FID navigators and a multi‐channel reference image

Purpose To measure spatiotemporal B 0 field changes in real time using FID navigators (FIDnavs) and to demonstrate the efficacy of retrospectively correcting high‐resolution T 2 * ‐weighted images using a novel FIDnav framework. Methods A forward model of the complex FIDnav signals was generated by simulating the effect of changes in the underlying B 0 inhomogeneity coefficients, with spatial encoding provided by a multi‐channel reference image. Experiments were performed at 3T to assess the accuracy of B 0 field estimates from FIDnavs acquired from a 64‐channel head coil under different shim settings and in 5 volunteers performing deep‐breathing and nose‐touching tasks designed to modulate the B 0 field. Second‐order, in‐plane spherical harmonic (SH) inhomogeneity coefficients estimated from FIDnavs were incorporated into an iterative reconstruction to retrospectively correct 2D gradient‐echo images acquired in both axial and sagittal planes. Results Spatiotemporal B 0 field changes measured from rapidly acquired FIDnavs were in good agreement with the results of second‐order SH fitting to the measured field maps. FIDnav field estimates accounted for a significant proportion of the ΔB 0 variance induced by deep breathing (64 ± 21%) and nose touching (67 ± 34%) across all volunteers. Ghosting, blurring, and intensity modulation artifacts in T 2 * ‐weighted images, induced by spatiotemporal field changes, were visibly reduced following retrospective correction with FIDnav inhomogeneity coefficients. Conclusions Spatially resolved B 0 inhomogeneity changes up to second order can be characterized in real time using the proposed approach. Retrospective FIDnav correction substantially improves T 2 * ‐weighted image quality in the presence of strong B 0 field modulations, with potential for real‐time shimming.