Ultrafast MRI is susceptible to main field distortions ΔB0 that affect these images’ quality and faithfulness –particularly along the low-bandwidth dimension. This work shows that these distortions can be mapped from the spatial images themselves, without requiring additional information. This information becomes available from a time-frequency analysis of the signals, after freeing them from phase-wrapping complications. This hypothesis is explained, and results are demonstrated using both simulations and single-shot human brain images collected by spatiotemporal encoding (SPEN) techniques. The method opens a route to enhancing SPEN’s and EPI’s robustness to field inhomogeneities.