Smooth volume rendering of labeled medical data on consumer graphics hardware

One of the most important applications of direct volume rendering is the visualization of labeled medical data. Explicit segmentation of embedded subvolumes allows a clear separation of neighboring substructures in the same range of intensity values, which can then be used for implicit segmentation of fine structures using transfer functions. Nevertheless, the hard label boundaries of explicitly segmented structures lead to voxelization artifacts. Pixel-resolution linear filtering can not solve this problem effectively. In order to render soft label boundaries for explicitly segmented objects, we have successfully applied a smoothing algorithm based on gradients of the volumetric label data as a preprocessing step. A 3D-texture based rendering approach was implemented, where volume labels are interpolated independently of each other using the graphics hardware. Thereby, correct trilinear interpolation of four subvolumes is obtained. Per-label post-interpolative transfer functions together with inter-label interpolation are performed in the pixel shader stage in a single rendering pass, hence obtaining Z, high-quality rendering of labeled data on GPUs. The presented technique showed its high practical value for the 3D-visualization of tiny vessel and nerve structures in MR data in case of neurovascular compression syndromes.