A PET scanner utilizes a ring of detector modules to precisely map gamma ray coincidences. A standalone PET scan can often be unclear and can be difficult to distinguish precise location of anatomical structure so PET scanner are often paired with CT Scanners or MRI Scanner. In this video, we can see a PET system paired with a CT System to produce a PET/CT System. The upper portion of the ring of detector modules can be seen in the rear of the gantry and the CT system is in the forefront.
Related Questions: How does PET scanner work?