PET/CT

PET/CT > Physics > PET Detector

PET Detector

PET imaging utilizes a dedicated PET camera system which includes multiple rings of detectors. Similar to gamma cameras, the PET detectors consist of scintillation crystals coupled with photomultiplier tubes. The scintillation crystals used in clinical PET imaging are either bismuth germanium oxide (BGO), gadolinium oxyorthosilicate\ (GSO), or lutetium oxyorthosilicate (LSO).

The ring design utilizes the concept that two photons detected in close temporal proximity (on the order of 6 to 12 nanoseconds) by two opposed detectors in the ring are likely to have originated from a single annihilation event in the body, somewhere along a line between the two detectors. Such a simultaneous detection is termed a “coincidence”. All of the coincidence events detected during an imaging period are recorded by the PET computer system as a raw data set. As in single photon emission computed tomography (SPECT) examinations in nuclear medicine, the coincidence data in PET is reconstructed by a computer to produce cross-sectional images in the axial, sagittal, and coronal planes.

Diagram of a PET detector ring showing two photons from a single annihilation event in the body striking opposite crystals.

Because of photon attenuation and absorption in tissue, many annihilation events result in only one of the two photons reaching the detector, a so called “single event”. These single events are discarded by the PET processing computer. Even though a very large number of the overall photons incident upon the detectors (single events) must be discarded, the principle of coincidence detection provides a so-called “electronic collimation”. Because of this electronic collimation PET scanners are inherently much more efficient than gamma cameras with markedly improved count statistics (better signal to noise ratios) and thus have much better spatial resolution compared to gamma cameras.

State of the art PET scanners are full-ring systems that completely surround the patient. The cameras have multiple adjacent detector rings that allow for a relatively large field of view at each table position. For a fixed total scan time and standardized radiopharmaceutical dose, the large filed of view provided by multiple detector rings allows more time at each table position compared to smaller field of view systems and thus allows more total counts to be detected during an examination with resultant improved sensitivity and resolution.