PURPOSE: To determine the radio isotope distribution in a desired section without use of any mechanical collimator accurately by analyzing detection position and energy of γ rays with detectors arranged in a ring at outer and inner rings.

CONSTITUTION: A gantry 10 has an inner ring 11 and an outer ring 12, which have detectors (in) and (In) arranged in a ring respectively. The inner ring 11 is variable in the radius while made rotatable relative to the outer ring 12. Outputs of the detectors (in) and (In) are sent to analysis means 13 and 14 and then, to a selector and sample holders 18 and 19. The outputs thereof are used to send ennergy information and information on detection position for indicating energy lost due to scattering sampled and held beforehand and the position of the detectors (I) and (i) respectively based on a pulse from a single channel analyzer 21 to determine a tomographic image from these outputs.