Multi-volume rendering using depth buffers for surgical planning in virtual reality.

Purpose: Planning highly complex surgeries in virtual reality (VR) provides a user-friendly and natural way to navigate volumetric medical data and can improve the sense of depth and scale. Using ray marching-based volume rendering to display the data has several benefits over traditional mesh-based rendering, such as offering a more accurate and detailed visualization without the need for prior segmentation and meshing. However, volume rendering can be difficult to extend to support multiple intersecting volumes in a scene while maintaining a high enough update rate for a comfortable user experience in VR.

Methods: Upon loading a volume, a rough ad hoc segmentation is performed using a motion-tracked controller. The segmentation is not used to extract a surface mesh and does not need to precisely define the exact surfaces to be rendered, as it only serves to separate the volume into individual sub-volumes, which are rendered in multiple, consecutive volume rendering passes. For each pass, the ray lengths are written into the camera depth buffer at early ray termination and read in subsequent passes to ensure correct occlusion between individual volumes.

Results: We evaluate the performance of the multi-volume renderer using three different use cases and corresponding datasets. We show that the presented approach can avoid dropped frames at the typical update rate of 90 frames per second of a desktop-based VR system and, therefore, provide a comfortable user experience even in the presence of more than twenty individual volumes.

Conclusion: Our proof-of-concept implementation shows the feasibility of VR-based surgical planning systems, which require dynamic and direct manipulation of the original volumetric data without sacrificing rendering performance and user experience.