Figure 6b. Drawings illustrate the steps required to create an SSD image. (a) Step 1: Choose a viewing point. Step 2: Select a threshold value (in Hounsfield units) for the tissue of interest (eg, bone >150 HU). (b) Step 3: Cast rays from the viewing point to each point in the volume. Step 4: Identify the first point along each ray that is at the selected threshold value. Step 5: Measure the distance from the viewing point to the threshold point and place a gray pixel in the final image; discard all other points along the ray. Short distances produce bright pixels and longer distances produce dark pixels. This coding provides depth information by means of shading.