Volume rendering (VR) (Figs. 7, 8) has replaced most earlier applications of surfacerendering with the notable exception of interior vessel analysis. VR is a technique that displays all of the 3D data at once. It works directly on the voxel intensities, and creates translucent renderings of the full volumetric data set. Objects with high signal intensity are opaque and objects with low signal intensity are transparent. As commonly implemented, VR generates an image in the following way. First, it computes color and partial opacity for each voxel. Then, it blends contributions made by voxels projecting to the same pixel on the picture plane (along a line from the viewer's eye through the data set). In most cases, the user manually sets color, opacity and brightness corresponding to different voxel intensities. In volume rendering, the signal intensity values that make up the image are assigned to be either visible or invisible, to be displayed with varying colors, and often to be displayed with varying opacity levels (transparency). The assignment ofthese characteristics to the voxels can be defined for specific imaging protocols and organ systems such as standard arterial-phase CE MRA alone or display of arterial-phase and venous-phase data together in different colors. Some systems use standard preset settings . However, these settings are much more difficult to establish for MRA than for CTA images. The main advantage of this technique is the ability to generate images without explicitly defining surface geometry. It reveals internal structures that would nor mally be hidden or omitted when using surface rendering techniques. VR thus appears very interesting for the study of arterial plaques.
Vendors have developed independent workstations and software that can display these three-dimensional images quickly, and which generally have some flexibility in terms of thresholding parameters (i.e. which voxels become visible). However, color maps and transparency levels may be more difficult to control for individual cases. Frequently, 3D VRimage displays of vascular anatomy provides excellent anatomicinformation for surgical planning.
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