Analysis of temporal patient-specific MR distortions:A per-organ assessment
Nejad-Davarani S, Weiss S, Sevak P, Renisch S, and Glide-Hurst C. Analysis of temporal patient-specific MR distortions:A per-organ assessment. Med Phys 2017; 44(6):3137.
Purpose: As we move toward MR-only patient models for treatment planning, a need exists to fully quantify distortions. However, the stability of patient-level distortion corrections is not currently known for changing anatomy (e.g., bladder/rectal filling), particularly over long MRI acquisitions. This work quantifies organ-specific distortions and explores the need for temporal corrections in pelvis. Methods: Nine healthy male volunteers were scanned for 3-6 timepoints/subject (34 timepoints) at 1T, 1.5T and 3T following a fixed bladder filling protocol. At each timepoint, T2-weighted images (delineation) and dual-echo gradient-recalled echo images (B0-mapping) were acquired. Distortion maps were generated from B0 field maps based on bandwidth and T2-weighted acquisition pixel size. Prostate, seminal vesicles (SVs), bladder, rectum, and penile bulb were delineated by a physician to assess distortions on an organ-by-organ basis. To quantify temporal local distortion changes due to filling, distortion difference maps were generated and segmented by organ. Magnet stability was assessed. Results: Baseline organ-specific distortions [P5, P95] in full bladder conditions were as follows: [-0.61, 0.4] mm for prostate, [-0.38, 0.37] mm for bladder, [-0.66, 0.65] mm for rectum, and [-0.16, 0.33] mm for seminal vesicles. To evaluate local target regions, CTV-PTV ring distortions were [-0.61, 0.52] mm. Assessment of temporal displacement maps (full to empty bladder) revealed that patient-specific distortions changed < ± 0.5 mm for 99% of voxels in bladder, prostate, and SVs. In rectum, ∼5% of voxels displaced >0.5 mm. In the CTV-PTV ring, only 1% of voxels displaced >0.5 mm. Magnets were stable over the imaging sessions (-0.10 to 0.16 mm). Conclusion: Temporal patient-specific distortion changes for pelvic organs were <1 mm for >99% of all voxels. Distortions were larger near air-filled organs, suggesting that bladder/bowel preparation is warranted for MR-only pelvis planning. Results suggest the magnitude may be larger in anatomies with higher tissue/air ratios (e.g., head and neck).