3DTM to display 3D ultrasound for cardiac ablation guidance, and Sharp et al. [100] employed this technique to visualize the light diffusion in 3D inhomogeneous tissue to provide visual information relating to structures located beneath the skin surface. Lopera et al. [101] used this DVR approach to view a stenosis in arbitrary image planes, and employed the rendering results to demonstrate the subsequent arterial bypass.

The third area of application is in dynamic imaging, deformation, and 4D data display. Levin et al. [102] developed a software platform based on TM, which was used to render multimodality 4D cardiac datasets interactively. A similar technique was used by Lehmann et al. [103] to visualize the beating heart in real time, in which a hierarchical memory structure was employed to improve bandwidth efficiency. In addition, Yuan et al. [104] designed a TF for nonlinear mapping density values in the dynamic range medical volumes, while Correa and his colleagues [105] presented a 3DTM algorithm that sampled the deformed space instead of the regular grid points, simulating volume deformations caused by clinical manipulations, such as cuts and dissections.

13.7.3 Improvements

13.7.3.1 Shading Inclusion

In 1996, shading was first included into TM approaches by Van Gelder et al. [23]. Both diffuse and specular shading models were added to texture-based DVR by Rezk-Salama et al. [92] with the use of register combiners and paletted texture. In addition, Kniss et al. [61] proposed a simple shading model that captured volumetric light attenuation to produce volumetric shadows and translucency. The shading model was also used with depth-based volume clipping by Weiskopf and his colleagues [62] in a 3DTM pipeline. To decrease shading artifacts, Lum et al. [66] introduced preintegrated lighting into the TM-based DVR, resulting in decreased lighting artifacts. Recently, Abellan and Tost [106] defined three types of shadings for the TM-accelerated volume rendering of dual-modality dataset, that is emission plus absorption, surface shading, and the mixture of both shadings, with userselected choice of shading model for a specific imaging modality.

13.7.3.2 Empty Space Skipping

As mentioned earlier, the DVR speed can often be improved if the empty spaces can be skipped during rendering procedure, for example, Li et al. [107–109] computed texture hulls of all connected nonempty regions, and they later improved the hull technique with “growing boxes” and an orthogonal binary space partitioning tree. Bethune and Stewart [110] proposed an adaptive slice DVR algorithm based