1. A medical imaging apparatus comprising: an X-ray imaging apparatus including an X-ray tube configured to emit X-rays for generating an X-ray CT image; a nuclear medicine imaging apparatus including a detector configured to detect radiation for generating a nuclear medicine image; a changing unit configured to change at least one of an emission direction and an emission width of the X-rays so that the X-rays are incident on the detector; and a determining unit configured to determine whether the detector detects the X-rays, thereby determining whether there is a failure in the detector.