Despite their unmatched soft-tissue contrast, Magnetic Resonance (MR) images suffer from wide-ranging image distortions; this has raised questions about their suitability as an imaging modality upon which to base conformal radiation therapy treatment plans. This thesis addresses image distortion as it relates to the implementation of MR-based radiation therapy treatment planning (MR-RTP). A grid phantom was imaged at 3T to determine the 3D distortion field using in-house software. Using multiple images, both machine- and object-related sources of distortion were separated such that individual evaluation of distortion sources is possible. Over the imaging volume, nonlinearities in the gradients led to peak-to-peak image distortions of up to 11 mm. For in-vivo distortion quantification, the method was augmented with a modified gradient echo sequence which measures the phase evolution due to underlying field inhomogeneities. The amount of distortion measured using this technique is dependent upon both patient anatomy and sequence parameters, but was found to contribute 5.7 mm at maximum. The methods presented can be combined to provide comprehensive distortion rectification such that mean residual image distortion is reduced to well below the pixel resolution. Finally, distortion quantification and correction methods were applied to a clinical MR-RTP study of prostate patients. The dosimetric consequences of distortion correction were investigated by comparing 3D conformal and intensity modulated radiation therapy plans developed based on
both uncorrected and corrected MRI data sets. Total image distortions
and those directly affecting the prostate and organs at risk (OARs) were
assessed and target doses, OAR doses, and dose volume histograms were
compared. Maximum distortion (from all sources) was 7.8 mm. With the exception of two patients, changes in plan dosimetry were insignificant (<2% / <1Gy). Two patients who were poorly position suffered larger distortions in the target region which led to dosimetric differences of up to 4.2%. / Medical Physics
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:AEU.10048/1308 |
| Date | 11 1900 |
| Creators | Baldwin, Lesley |
| Contributors | Fallone, Gino (Oncology), Sloboda, Ron (Oncology), Rathee, Satyapal (Oncology), Robinson, Don (Oncology), Wilman, Alan (Biomedical Engineering), Ten Haken, Randall (Radiation Oncology, University of Michigan) |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 11398139 bytes, application/pdf |
| Relation | L. N. Baldwin, K. Wachowicz, S. D. Thomas, R. Rivest, and B. G. Fallone, "Characterization, predication, and correction of geometric distortion in 3T MR images," Med Phys 34, 388-399 (2007), L. N. Baldwin, K. Wachowicz, and B. G. Fallone, "A two-step scheme for distortion rectification of magnetic resonance images," Med Phys 36, 3917-26 (2009)., L. N. Baldwin, K. Wachowicz, and B. G. Fallone, "Discontinuities in the distortion field: correction of the fat-shift artifact," in Proc Intl Soc Magn Reson Med, Stockholm, 2010, p. 3091. |
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