OSPRay cannot do “cinematic rendering” (Siemens’ term for photorealistic volumetric rendering of medical images) yet - see raytracing in vtk. As far as I understand, OSPray is also primarily runs on the CPU, so most likely it won’t be able to provide near-interactive frame rates.
NVidia OptiX (via VisRTX) is more promising as it can take advantage of modern GPUs, but it does not yet support volume rendering at all.
I guess we need to wait until VTK’s OSPRay and/or OptiX volume rendering capabilities improve. Probably things would happen faster by providing funding to Kitware. We would not like to be the only group to fund this but if others are interested in this, too, then we would be happy to contribute.