13
Positron Emission Tomography*
182
Positron emission tomography (PET) is based on the detection in coinci-
dence of the two 511-keV annihilation radiations that originate from b+-
emitting sources (e.g., the patient containing b+-emitting radioactivity).
Positrons are annihilated in body tissue and produce two 511-keV annihi-
lation photons that are emitted in opposite directions (180°). Two photons
are detected in an electronic time interval, called “coincidence time
window,” by two detectors connected in coincidence. Conversion of 511-
keV photons to light photons in the detector, formation of a pulse by the
PM tube, and pulse-height analysis follow the same principles as in con-
ventional gamma cameras. Detectors are arranged in the array of several
rings to have the organ of interest in the field of view. Data collected over
360° simultaneously around the body axis of the patient are used to recon-
struct the image of the activity distribution in the slice of interest. Because
the two opposite photons are detected in a straight line, no collimator is
needed to limit the field of view, and the technique is called the electronic
collimation.
Positron-Emitting Radionuclides
Various positron-emitting radionuclides are used in preparing radiophar-
maceuticals for PET studies, and the production and characteristics of the
most common ones are listed in Tables 5.1 and 13.1. Of these,^82 Rb-RbCl is
commonly used for myocardial perfusion imaging and^18 F-fluorodeoxyglu-
cose for metabolic imaging of the heart, brain, and various types of can-
cers. Readers are referred to reference books on the synthesis of PET
radiopharmaceuticals.
- This chapter is adapted from Saha (2005, Chapters 2–5).