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Figure 10a. (a) Two-dimensional histogram based on intensities (x axis) and gradient magnitudes (y axis) obtained from CT angiographic data. This structure clearly demonstrates tissue boundaries as parabolic arcs. Voxels lying close to tissue boundaries or inside uniform tissue produce histogram "hits" along the upper or lower regions of the parabolas. Open arrow = air, open arrowhead = soft tissue, solid arrowhead = vessels, solid arrow = osseous tissue. (b) Voxels corresponding to osseous tissue (arrow) and vessels enhanced with contrast medium (arrowhead) are easily identifiable in the 2D transfer function editor. Predefined tissue boundary templates can be interactively placed and adjusted over the corresponding 2D histogram with immediate feedback on volume-rendered images.
