Per-organ assessment of subject-induced susceptibility distortion for MR-only male pelvis treatment planning

Background: Patient-specific distortions, particularly near tissue/air interfaces, require assessment for magnetic resonance (MR) only radiation treatment planning (RTP). However, patients are dynamic due to changes in physiological status during imaging sessions. This work investigated changes in subject-induced susceptibility distortions to pelvic organs at different bladder states to support pelvis MR-only RTP. Methods: Pelvises of 9 healthy male volunteers were imaged at 1.0 Tesla (T),1.5 T, and 3.0 T. Subject-induced susceptibility distortion field maps were generated using a dual-echo gradient-recalled echo (GRE) sequence with B-0 field maps obtained from the phase difference between the two echoes acquired at several bladder volume states (3-4/subject, 32 overall). T2 turbo spin echo images were also acquired at each bladder state for organ delineation. Magnet central frequency was tracked over time. Distortion map differences and boxplots were computed to characterize changes within the clinical target volume (CTV), bladder, seminal vesicles, and prostate volumes. Results: The time between the initial and final BO maps was 42.6 +/- 13.9 (range: 13.2-62.1) minutes with minimal change in magnet central frequency (0.02 +/- 0.05 mm (range: - 0.06 - 0.12 mm)). Subject-induced susceptibility distortion across all bladder states, field strengths, and subjects was relatively small (1.4-1.9% of all voxels in the prostate and seminal vesicles were distorted > 0.5 mm). In the bladder, no voxels exhibited distortions > 1 mm. An extreme case acquired at 3.0 T with a large volume of rectal air yielded 27.4-34.6% of voxels within the CTVs had susceptibility-induced distortions > 0.5 mm across all time points. Conclusions: Our work suggests that subject-induced susceptibility distortions caused by bladder/rectal conditions are generally small and subject-dependent Local changes may be non-negligible within the CTV, thus proper management of filling status is warranted. Future work evaluating the impact of multiple models to accommodate for extreme status changes may be advantageous.