1. A method of determining the depth of a target organ within a patient, comprising the following steps: acquiring a first nuclear medicine image of the target organ using a focussing collimator at a first predetermined height; acquiring a second nuclear medicine image of the target organ using the same collimator at a second predetermined height; computer-identifying an anatomical landmark associated with the target organ as said landmark appears in the first nuclear medicine image; computer-identifying the same anatomical landmark associated with the target organ as said landmark appears in the second nuclear medicine image; and determining, from differences in size between said identified landmarks and said heights, the depth of the target organ within the patient.