<p>A method and apparatus are provided for reconstructing list mode data acquired during a positron emission tomography scan of an object, the data including information indicative of a plurality of detected positron annihilation events. Detected events occurring in a region of interest are identified; the identified events are reconstructed using an iterative reconstruction technique which includes a ray tracing operation to generate volumetric data indicative of the region of interest, wherein the ray tracing operation traces only image matrix elements located in the region of interest; and a human readable image indicative of the volumetric data is generated. In another aspect an image mask and a projection mask are defined correlating to the region of interest; image matrix elements located in the region of interest are determined by applying the image mask; and detected events occurring in a region of interest are identified by applying the projection mask.</p>
