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Q. Zhang et al.

Fig. 13.9 DVR images of CT pulmonary data showing illumination (right image) compared with no illumination (left image)

pipeline, in which inner structural relationships were defined by a region connection calculus and were represented by a relation graph interface. In addition, Rautek et al. [68] gave a comprehensive review on illustrative visualization and envisioned their potential medical applications. Such illustrative techniques are sometimes integrated within commercial systems; however, there have been very few systematic studies done to verify the perceptual enhancement, nor to validate the clinical benefit [69].

13.4 Software-Based Raycasting

Raycasting [14] is a popular technique used to display a 3D dataset in two dimensions, in which the basic idea is to cast a ray from each pixel in the viewing plane into the volume, sampling the ray with a predetermined step in a front-to-back or back-to-front order using trilinear interpolation. In this process, a TF is used to map the scalar value to RGB color and opacity, which can be performed on every voxel in the volume before the sampling step (preclassification), or on the sampled scalar values along the casting ray after sampling, where post-, preintegrated, or postcolor attenuated classification can be used. Finally, the acquired optical values at these sampling points along the casting ray are composited using (13.5) or (13.6) to approximately compute the DVR integral (13.2), obtaining the corresponding final pixel color on the output image. Figure 13.10 illustrates the raycasting pipeline, and Fig. 13.11demonstrates this on four medical images.

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

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Fig. 13.10 Raycasting pipeline: sampling, optical mapping, and compositing

Fig. 13.11 Medical images rendered with software-based raycasting: (a) CT skull; (b) MR brain; (c) CT jaw, and (d) MR cerebral blood vessel

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