The invention addresses the problem of reproducibly estimating statistics in structurally significant regions of interest of medical volumes where the data is acquired as projections of a Cartesian-space distribution. The regions of interest (ROI) are specified to a high resolution using a spatial reference dataset, and these are mapped to the spatial frame of reference of the apparatus on which the data was acquired. This allows the regions to be projected on to the detector loci of the apparatus and ROI activity is estimated directly from measurement projections. The invention also addresses problems associated with poor delineation of ROIs.