Abstract
The purpose of this study was to investigate performance of the couch and coil mounts designed for MR‐simulation prostate scanning using data from ten volunteers. Volunteers were scanned using the standard MR scanning protocol with the MR coil directly strapped on the external body and the volunteer lying on the original scanner table. They also were scanned using a MR‐simulation table top and pelvic coil mounts. MR images from both setups were compared in terms of body contour variation and image quality effects within particular organs of interest. Six‐field conformal plans were generated on the two images with assigned bulk density for dose calculation. With the MR‐simulation devices, the anterior skin deformation was reduced by up to 1.7 cm. The hard tabletop minimizes the posterior body deformation which can be up to 2.3 cm on the standard table, depending on the weight of volunteer. The image signal‐to‐noise ratio reduced by 14% and 25% on large field of view (FOV) and small FOV images, respectively, after using the coil mount; the prostate volume contoured on two images showed difference of 1.05±0.66 cm3. The external body deformation caused a mean dose reduction of 0.6±0.3 Gy, while the coverage reduced by 22%±13% and 27%±6% in V98 and V100, respectively. A dedicated MR simulation setup for prostate radiotherapy is essential to ensure the agreement between planning anatomy and treatment anatomy. The image signal was reduced after applying the coil mount, but no significant effect was found on prostate contouring.PACS numbers: 87.55.D‐, 87.61.‐c, 87.57.C‐
Highlights
MR images reflect the proton density of tissues and they are able to better distinguish different soft tissues
In order to reduce the multimodality registration error and simplify treatment workflows, there has been considerable recent interest in the introduction of MRI simulation for prostate radiation therapy where the treatment plan is generated using MRI data alone.[12,13,14,15] there are three significant challenges which need to be addressed before MRI can be used as the sole imaging modality for treatment planning: 1) geometric distortion of the MR image; 2) lack of electron density information for dose calculation; and 3) MR patient positioning system differences from the treatment machine
Because this study concentrates on clinical effects, the images were postprocessed by the clinical protocols, including using the vendor-provided Prescan Normalize (Siemens) filter to correct the intensity inhomogeneity across the image, and the geometric distortion of the image was corrected by the vendor-provided 3D distortion correction algorithm
Summary
MR images reflect the proton density of tissues and they are able to better distinguish different soft tissues. The superior soft tissue contrast of MR imaging provides a lower level of interobserver variation[7] and results in more consistent treatment plans with smaller volume margins.[8,9,10] MR images are typically manually registered with a patient’s planning CT to enable prostate contouring. This registration of MR to CT for treatment planning can introduce geometric errors into treatment planning. In order to reduce the multimodality registration error and simplify treatment workflows, there has been considerable recent interest in the introduction of MRI simulation for prostate radiation therapy where the treatment plan is generated using MRI data alone.[12,13,14,15] there are three significant challenges which need to be addressed before MRI can be used as the sole imaging modality for treatment planning: 1) geometric distortion of the MR image; 2) lack of electron density information for dose calculation; and 3) MR patient positioning system differences from the treatment machine
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