Magnetic resonance imaging based radiotherapy treatment planning: problems, solutions, and applications

Lesley Baldwin
2010
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 inhouse software. Using multiple
more » ... 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-topeak 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%. friend in Edmonton; Josh, Pat, Dave and Bridget, Stefan and Sanja: you helped me do things I didn't know were possible. You are strong and determined and you make me want to be the same. Most of all, you are ridiculous amounts of fun; Gavin and Nickie, Suz and Trav, Rahul and Erin, Robin and Erin, and Sarah, who make winter something to look forward to; Charmaine and Sim,
doi:10.7939/r3b91f fatcat:gmvij3dl6nezra2fwpud76m5ze