PET brain "Ridge Segmentation" function:   finalized interface and functionality for 2D and 3D cases.  Simplistic skeletonization algorithm that works well in 2D, for some reason is not performing well in 3D and would require more time to integrate more precise 3D skeletonization.
Therefore, due to a limited time available,  "Process slices individually" option is always on in 3D case for the time being.