You can create a key image within an image stack by checking the Marked Slices Indicator, which appears at the bottom right of image plane views of selected datasets. Functions implemented for marked slices include slice interpolation and removal of unrepresentative slices in an image stack (see Interpolating and Removing Image Slices), as well as extracting a new dataset from marked slices (see Extracting New Datasets from Marked Slices).
If the Image Plane view is not available in the scene, right-click a view and then choose Image Plane in the pop-up menu.
Make sure that the Marked Slices Indicator is visible in the display.
