The invention relates to a positron emission tomography (PET) device, and more particularly, to a PET device that involves changing the diameter of a detection means according to the size of a test subject. The PET device includes a body having an opening therein to receive a subject and a detection means surrounding the received subject in the circumferential direction of the opening. The detection means may detect radiation emitted by the subject. Additionally, the PET device includes a driving means driving the detection means. The detection means includes detection heads arranged in a first group in the circumferential direction of the opening, and detection heads disposed in a second group between the detection heads in the first group. The detection heads in the first group are moveable in a radial direction of the detection means by means of the driving means.
