Reducing distortions in echo‐planar breast imaging at ultrahigh field with high‐resolution off‐resonance maps

PURPOSE: DWI is a promising modality in breast MRI, but its clinical acceptance is slow. Analysis of DWI is hampered by geometric distortion artifacts, which are caused by off‐resonant spins in combination with the low phase‐encoding bandwidth of the EPI sequence used. Existing correction methods assume smooth off‐resonance fields, which we show to be invalid in the human breast, where high discontinuities arise at tissue interfaces. METHODS: We developed a distortion correction method that incorporates high‐resolution off‐resonance maps to better solve for severe distortions at tissue interfaces. The method was evaluated quantitatively both ex vivo in a porcine tissue phantom and in vivo in 5 healthy volunteers. The added value of high‐resolution off‐resonance maps was tested using a Wilcoxon signed rank test comparing the quantitative results obtained with a low‐resolution off‐resonance map with those obtained with a high‐resolution map. RESULTS: Distortion correction using low‐resolution off‐resonance maps corrected most of the distortions, as expected. Still, all quantitative comparison metrics showed increased conformity between the corrected EPI images and a high‐bandwidth reference scan for both the ex vivo and in vivo experiments. All metrics showed a significant improvement when a high‐resolution off‐resonance map was used (P < 0.05), in particular at tissue boundaries. CONCLUSION: The use of off‐resonance maps of a resolution higher than EPI scans significantly improves upon existing distortion correction techniques, specifically by superior correction at glandular tissue boundaries.