PET Detector
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.
© 2006 by the Rector & Visitors of the University of Virginia