Measuring in a first semiconductor crystal two anode channels and two cathode channels and measuring in a second semiconductor crystal one anode channel and one cathode channel; responsive to an energy of a sum of the two anode channels being within an energy window and an energy of the one anode channel being within the energy window: separating the two anode channels and the two cathode channels into combinations of anode-cathode channel pairs; for each of the anode-cathode channel pairs, determining a respective direction difference angle, each respective direction difference angle being determined via use of the one anode channel and one cathode channel; determining a determined one of the direction difference angles that has a smallest value; and setting as an initial interaction position of a photon a selected one of the anode-cathode channel pairs that corresponds to the determined direction difference angle. Additional embodiments are disclosed.