Computer-assisted diagnosis (CAD) - the development of algorithms for systematic analysis of images was very hot at the RSNA meeting a few years ago - and I suspect that it will be for years to come. This field will incorporate a lot of image registration work, as researchers and doctors will be needing to bring together information from multiple imaging modalities and use that for a more personalized, or patient-specific approach to medicine.
GPU-based Monte Carlo simulations have also been hot over the last few years. We use these a lot in radiation oncology.
Modelling of cancer progression and response to treatment is also a big field, although the computational side of things isn't really the bottleneck right now (it's more the availability of quality biological data).