Pet apparatus

A PET apparatus comprising:

• a detector ring provided with a plurality of radiation detectors arranged around a measurement space in which a subject into which a drug labeled with a positron emitting radioisotope is injected is placed;
• a position adjustment unit configured to adjust a position of the detector ring by moving the detector ring relative to the subject in a body axis direction;
• a processing unit configured to repeatedly obtain and output a position or a position change amount of the subject relative to the detector ring in the body axis direction based on a coincidence detection event in which any two radiation detectors out of the plurality of radiation detectors detect a pair of gamma-rays generated by a positron emitted from the positron emitting radioisotope in the subject;
• and
• a control unit configured to instruct the position adjustment unit to adjust the position of the detector ring such that the subject is positioned within a predetermined range in the body axis direction in the measurement space based on the position or the position change amount of the subject output from the processing unit.