In-depth understanding of the multimodal deformable image registration tool of a 0.35 T MR-linac

Linear accelerators (linac) with embedded magnetic resonance (MR) imaging involves a specific workflow that includes systematically a deformable image registration (DIR) with computed tomography images for the management of electron densities. Poor quality registration outputs could cause suboptimal dose calculations and planning treatments. The treatment planning system of the 0.35 T MR-linac MRIdian (ViewRay Inc., Oakwood Village) includes such a multimodal DIR tool with several adjustable parameters. To assist Viewray's users, this work investigates the influence of each parameter from the DIR tool. The default set of DIR parameters has been questioned through body locations (Thorax, Abdomen, Pelvis) and two quantitative metrics (the mutual information and mean absolute error). While the iterations number could be higher defined by default, the pyramidal number parameters (or control grid with a low impact) should be more carefully escalated. The default regularization parameters are consistent; both ways ensure optimal results, and the stiffness default value should be considered as a safeguard against unrealistic deformations. Depending on the location considered, the same variation in a specific parameter can have different effects on the registration results. Although the quantitative metrics are not directly correlated with the quality of the registrations, this work should help users manage and optimize the DIR step.