Tissue definition is available when viewing images in 3D or when using Vessel Analysis or Coronary Analysis. Tissue definition lets you define 3D tissues to better visualize particular regions or areas of the CT or MR volume. When you view images using one of these applications, you can choose to hide or show tissue definition to better visualize pathology.
1. Right-click the image, select View As, then select Vessel Analysis, Coronary Analysis, or 3D.
2. If required, you can change the type of image. Right-click the image, select Change Image, then select Angiography MIP or Volume Rendering.
3. From the 3D tab of the toolbar click Tissue Definition 
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The Tissue Definition toolbar appears. A default highlight appears on the image. The highlighted area has a center and width calculated according to the range of pixels that are colored.
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