Due to field inhomogeneities, MR-images suffer from image distortions in frequency-encoding direction. Using MRI as a tool for dose planning in MR-guided radiation therapy, image distortions play an important role in defining safety margins and the planning target volume (PTV). To investigate the influence of susceptibility-related field inhomogeneities on the distortion in prostate and cervix, distortions maps based on phase images were calculated for data acquired in a MR-guidance study. It was found that maximum distortions in prostate and rectum can be as large as 0.6 mm or 1.0 mm respectively, which should be considered when defining PTVs.
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