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SU‐G‐JeP2‐10: On the Need for a Dynamic Model for Patient‐Specific Distortion Corrections for MR‐Only Pelvis Treatment Planning
Author(s) -
GlideHurst C,
Zheng W,
Stehning C,
Weiss S,
Renisch S
Publication year - 2016
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4957030
Subject(s) - distortion (music) , nuclear medicine , pelvis , medicine , expiration , prostate , displacement (psychology) , radiology , physics , respiratory system , anatomy , psychology , amplifier , optoelectronics , cmos , cancer , psychotherapist
Purpose: Patient‐specific distortions, particularly near tissue/air interfaces, require assessment and possible corrections for MRI‐only radiation treatment planning (RTP). However, patients are dynamic due to changes in physiological status and motion during imaging sessions. This work investigated the need for dynamic patient‐specific distortion corrections to support pelvis MR‐only RTP. Methods: The pelvises of healthy volunteers were imaged at 1.0T, 1.5T, and 3.0T. Patient‐specific distortion field maps were generated using a dual‐echo gradient‐recalled echo (GRE) sequence with B0 field maps obtained from the phase difference between the two echoes acquired at two timepoints: empty and full bladders. To quantify changes arising from respiratory state, end‐inhalation and end‐expiration data were acquired. Distortion map differences were computed between the empty/full bladder and inhalation/expiration to characterize local changes. The normalized frequency distortion distributions in T2‐weighted TSE images were characterized, particularly for simulated prostate planning target volumes (PTVs). Results: Changes in rectal and bowel air location were observed, likely due to changes in bladder filling. Within the PTVs, displacement differences (mean ± stdev, range) were −0.02 ± 0.02 mm (−0.13 to 0.07 mm) for 1.0T, −0.1 ± 0.2 mm (−0.92 to 0.74 mm) for 1.5T, and −0.20 ± 0.03 mm (−0.61 to 0.38 mm) for 3.0T. Local changes of ∼1 mm at the prostate‐rectal interface were observed for an extreme case at 1.5T. For end‐inhale and end‐exhale scans at 3.0T, 99% of the voxels had Δx differences within ±0.25mm, thus the displacement differences due to respiratory state appear negligible in the pelvis. Conclusion: Our work suggests that transient bowel/rectal gas due to bladder filling may yield non‐negligible patient‐specific distortion differences near the prostate/rectal interface, whereas respiration had minimal effect. A temporal patient model for patient‐specific distortion corrections may be advantageous for MR‐only RTP, although further investigations in larger cohorts are needed to fully characterize distortion magnitude. The submitting institution has research agreements with Philips Healthcare. Research sponsored by a Henry Ford Health System Internal Mentored Grant.