Sci-Fri AM: YIS-01: Comprehensive MR distortion correction: Phantom validation and in-vivo application

MR images provide excellent diagnostic information; however, their treatment planning utility is limited due to geometric uncertainties from both system and patient related sources. Despite this concern, interest in developing MR-based treatment planning protocols is on the rise because of the ease with which clinically relevant structures can be identified in MR. Here we present our systematic approach to quantifying both machine (gradient non-linearity and B₀ inhomogeneity) and patient (susceptibility and chemical shift) distortions. Gradient non-linearities were previously measured using a 3D grid phantom while the remaining types of distortion were measured using a double gradient echo scan to obtain a B₀ distortion map specific to each object/patient. Distortion measurement and correction were validated on phantoms and then implemented on a volunteer. B₀ inhomogeneity and susceptibility distortions were simulated by offsetting the x² -y² shims; maximum absolute distortion was reduced from 5.4 mm to 1.0 mm and mean (± standard deviation) was reduced from 1.7 ± 1.4 mm to 0.4 ± 0.2 mm. Chemical shift distortion was qualitatively evaluated using a phantom containing fat and water inserts; displacement of the fat signal was much improved following distortion correction. Intensity correction was validated using a uniformity phantom and undistorted image profiles were compared to distorted image profiles and to profiles corrected for geometric and geometric/intensity distortion; the need for intensity correction was clearly demonstrated. Once all types of distortion correction were validated on phantoms, the technique was implemented on a volunteer brain image. Both GE and multi-shot EPI images were corrected.