Figure 17. Data limitations of SSD. Surface data are segmented from other data by means of manual selection or an attenuation threshold. The graph in the lower part of the figure represents an attenuation threshold selected to include the brightly contrast-enhanced renal cortex and renal vessels during CT angiography. The "virtual spotlight" in the upper left corner represents the gray-scale shading process, which in reality is derived by means of a series of calculations. To illustrate the "hollow" data set that results from discarding all but the surface rendering data, the illustration was actually created by using a volume-rendered image of the kidney with a cut plane transecting the renal parenchyma. Subsequent editing was required to remove the internal features of the object while preserving the surface features of the original image. HU = Hounsfield units.