Methods, systems and apparatuses for processing data associated with nuclear medical imaging techniques are provided. Data is ordered in LUT's and memory structures. Articles of manufacture are provided for causing computers to carry out aspects of the invention. Data elements are ordered into a plurality of ordered data groups according to a spatial index order, and fetched and processed in the spatial index order. The data elements include sensitivity matrix elements, PET annihilation event data, and system and image matrix elements, the data grouped in orders corresponding to their processing. In one aspect geometric symmetry of a PET scanner FOV is used in ordering the data and processing. In one aspect a system matrix LUT comprises total number of system matrix elements equal to a total number of image matrix elements divided by a total number of possible third index values.