Figure 2.

EPI distortions stem from inhomogeneity of the main magnetic field and are most pronounced at tissue/air and tissue/bone interfaces due to the large local field inhomogeneity caused by susceptibility variations. MR image formation assumes the linear field gradient used for spatial encoding is achieved exactly as planned. However, due to the main field inhomogeneity, the net field deviates from the desired linear change, leading to incorrect mapping of voxels. The result is image distortion (eg, compression in the frontal lobe, as shown in the figure). As the encoding field for EPI phase encode is relatively weak compared to local field changes, distortions are severe along this direction. Encoding field for EPI readout is much stronger than local field changes and the image distortion along this direction is negligible.