The invention relates to a processing system and corresponding method for processing image data and for indexing regions of interest in an object of interest. The system comprises a registration unit for registering the image data, a sub-volume generator generating sub-volumes from the image data, a composite image generator for generating multiple sets of composite images from each sub-volume, each set of composite images representing a different projection. The system comprises an indexing unit for generating indexed regions of interest in each sub-volume by assessing each of the set of composite images in parallel and a combining unit for combining each set of composite images of each sub-volume into a scan level prioritization.