Comparison Of Viewray 0.35 T Mri And Conventional Ct Scans For Normal Structure Segmentation

ViewRay is a novel image-guided radiation therapy approach with an integrated MR unit and three Cobalt-60 heads. In order to take advantage of the high resolution and high soft tissue contrast available in MR images, we investigated how 3D images obtained with the low field 0.35 T MR in ViewRay perform as an alternative to CT scans for radiation therapy treatment planning. Though there is a lack of electron density information, a great deal of normal and target structure delineation can be visualized (in some cases possibly superior) with MR as well. A comparison of the two imaging modalities’ ability to produce comparable contours will be illustrated. CT scans following institutional protocols were performed on five patients with contours drawn on the resulting images. The same five patients then underwent a planning scan on the MRI-guided ViewRay TrueFISP-based pulse sequence. The contours of the same organs were repeated on the MR images and compared to those from the CT scan. Metrics used to compare the contours included physical volume as well as the Dice Similarity Coefficient (DSC). Sites included were: brain, brainstem, esophagus, heart, lungs, spinal cord, and the external body contour. Throughout the duration of the study, image quality stability was quantitatively ensured with regular performance of the ACR MRI phantom test protocol. The brain, lungs, and heart had the highest DSC (0.985, 0.863, and 0.843 respectively). Among those organs with lower DSC include the brainstem, spinal cord, and esophagus. Those targets best agreeing in volume were the heart (5% average difference) and the brain (2% average difference). Contour volume for the brainstem and lung varied the most (27% and 19%), though lung volume is expected to vary with changes in respiratory pattern. All ACR phantom tests were within acceptable limits throughout the study. The data suggests that ViewRay image quality is comparable to CT scans for the brain, heart, and lungs, given the agreement in volume and Dice coefficients. MR images from ViewRay have the potential to serve as treatment planning data for future MRI-guided radiation therapy, without drastically changing our geometrical understanding of normal structures. Future studies will lead to a more precise knowledge of organ image quality by increasing the number of subjects.