304	Q. Zhang et al.

13.4.1 Applications and Improvements

Because of its rendering speed, raycasting has been not often used in clinical applications until now. However, it was a significant 3D display method in some applications ever when graphics hardware accelerated DVR was not commonly available. For example, Sakas et al. [70] used raycasting to display 3D ultrasound data of a fetus, and Hohne [71] and Tiede et al. [72] employed this algorithm for anatomical visualization. Many of the approaches to improve raycasting techniques have focused on schemes to eliminate unnecessary voxels from the computation.

13.5 Splatting Algorithms

Splatting is a popular DVR algorithm which was first proposed by Westhover [73–75] and was improved in terms of quality and speed by the research community over the years [76, 77]. This technique was developed to accelerate the speed of DVR at the expense of lower accuracy, and calculates the influence of each voxel in the volume on multiple pixels in the output image. This algorithm represents the volume as an array of overlapping basis functions called reconstruction kernels, which are commonly rotationally symmetric Gaussian functions with amplitudes scaled by the voxel values. This process is described in Fig. 13.12, and Fig. 13.13 presents examples of images rendered with the splatting algorithm.

13.5.1 Performance Analysis

Splatting is efficient because it reorders the DVR integral, making the preintegration of reconstruction kernels possible, so that each voxel’s contribution to the integral

Fig. 13.12 Splatting pipeline: the optical model is evaluated for each voxel and projected onto the image plane, leaving a footprint (splat). Then these footprints are composited to create the final image

13 Medical Image Volumetric Visualization: Algorithms, Pipelines...

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Fig. 13.13 Medical images rendered with splatting: (a) CT skull with shading; (b) MR brain with shading [76]; (c) CT jaw, and (d) MR cerebral blood vessel [77]

can be viewed separately. Another major advantage is that only voxels relevant to the image are projected and rasterized, so empty (transparent) regions can easily be skipped. However, because all of the splats are composited back-to-front directly without considering the kernel overlaps, the basic splatting algorithm is plagued by artifacts known as “color bleeding,” where the colors of hidden objects or background appearing in the final image.

13.5.2 Applications and Improvements

Vega-Higuera et al. [78] exploited texture-accelerated splatting to visualize the neurovascular structures surrounded by osseous tissue in CTA data in real time.