Figure 2.

Experimental design to compare geometric distortion correction using voxel shift maps (VSMs) computed from conventional static ΔB₀ field measurements from a multi-gradient-echo (MGRE) scan, and from dynamic FIDnav- and DOCMA-based field offsets. Static ΔB₀ field maps were calculated by taking the Hermitian inner product (HIP) over channels of the phase difference between multi echo images and scaling to obtain ΔB_0,0(r). Coefficients describing the change in B₀ were estimated from FIDnavs and used to compute maps of δB_0,n(r). These were superposed with the static ΔB₀ map to compute ΔB_0,n(r) at each time point n. For the DOCMA approach, maps of ΔB_0,n(r’) were computed in distorted image space from the HIP of multi-echo EPI images. VSMs were computed from each ΔB₀ measurement and used to unwarp the first-echo EPI image.