A method and device is described for correcting PET images for the attenuation of effects of the tissues and periodic movements of the patient. The device provides gamma-rays for image registration along with the CT scan. The gamma-rays are detected by the same detectors that form the emission PET scan. The device for generating the gamma-rays is a rod that contains an array of detectors, and a radioactive source that emits two rays in coincidence with each other. One of these rays is a gamma ray, and the other can be a beta ray, an alpha ray, or a low energy x-ray. The position of the rod, which is moveable within the ring of the PET scanner, is monitored so that its position can be coordinated with the signals generated from the ring detectors.
