Inherent Geometry Correction for Diffusion EPI Using the Reference Echoes as Navigators
Figure 5
3T diffusion EPI abdominal images at the lung-liver interface to show the effect of local frequency offsets opposite to the average phase as probed by the navigator: (a) phase shift direction P, product reconstruction, (b) phase shift direction P, phase-corrected reconstruction, (c) phase shift direction A, product reconstruction, and (d) T2W TSE. Lines are provided as guide for the eye to compare geometric distortions for the phase encoding directions P (dashed) and A (dotted) and for the T2W TSE scan (solid). The corrected image improves overall geometric fidelity, while the internal structures are severely distorted by the local susceptibility differences at the lung-liver interface, and the correction increases the geometric error for the position of the liver, which can be understood from the simulations in Figure 1 if the local frequency offset has opposite sign to the average frequency offset.