Magnetic resonance imaging to improve structural localisation in radiotherapy planning

The purpose of this thesis is to develop the role of magnetic resonance imaging (MRI) in the radiotherapy (RT) planning process. This began by assessing a prototype inline three-dimensional distortion correction algorithm. A number of quality assurance tests were conducted using different test objects and the 3D distortion correction algorithm was compared with the standard two-dimensional version available for clinical use on the MRI system. Scanning patients using MRI in the RT position within an immobilisation mask can be problematic, since the multi-channel head coils typically used in diagnostic imaging, are not compatible with the immobilisation mask. To assess the image quality which can be obtained with MR imaging in the RT position, various MRI quality assurance phantoms were positioned within an immobilisation mask and a series of image quality tests were performed on four imaging coils compatible with the immobilisation mask. It was shown that only the 4-channel cardiac coil delivered comparable image quality to a multi-channel head coil. An investigation was performed to demonstrate how MRI patient position protocols influence registration quality in patients with prostate cancer undergoing radical RT. The consequences for target volume definition and dose coverage with RT planning were also assessed. Twenty patients with prostate cancer underwent a computed tomography (CT) scan in the RT position, a diagnostic MRI scan and an MRI scan in the RT position. The CT datasets were independently registered with the two MRI set-ups and the quality of registration was compared. This study demonstrated that registering CT and MR images in the RT position provides a statistically significant improvement in registration quality, target definition and target volume dose coverage for patients with prostate cancer. A similar study was performed on twenty-two patients with oropharyngeal cancer undergoing radical RT. It was shown that when patients with oropharyngeal cancer undergo an MRI in the RT position there are significant improvements in CT-MR image registration, target definition and target volume dose coverage

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Variability in prostate and seminal vesicle delineations defined on magnetic resonance images, a multi-observer, -center and -sequence study

Background: The use of magnetic resonance (MR) imaging as a part of preparation for radiotherapy is increasing. For delineation of the prostate several publications have shown decreased delineation variability using MR compared to computed tomography (CT). The purpose of the present work was to investigate the intra- and inter-physician delineation variability for prostate and seminal vesicles, and to investigate the influence of different MR sequence settings used clinically at the five centers participating in the study. Methods: MR series from five centers, each providing five patients, were used. Two physicians from each center delineated the prostate and the seminal vesicles on each of the 25 image sets. The variability between the delineations was analyzed with respect to overall, intra-and inter-physician variability, and dependence between variability and origin of the MR images, i.e. the MR sequence used to acquire the data. Results: The intra-physician variability in different directions was between 1.3 - 1.9 mm and 3 - 4 mm for the prostate and seminal vesicles respectively (1 std). The inter-physician variability for different directions were between 0.7 - 1.7 mm and approximately equal for the prostate and seminal vesicles. Large differences in variability were observed for individual patients, and also for individual imaging sequences used at the different centers. There was however no indication of decreased variability with higher field strength. Conclusion: The overall delineation variability is larger for the seminal vesicles compared to the prostate, due to a larger intra-physician variability. The imaging sequence appears to have a large influence on the variability, even for different variants of the T2-weighted spin-echo based sequences, which were used by all centers in the study