Figure 1.

Principles of PET. A biologically active molecule is labeled with a positron emitting radioisotope as in the example FDG. FDG is injected intravenously, distributes throughout the body via bloodstream, and enters into organs, where it traces transport and phosphorylation of glucose. Positrons emitted from the nucleus of F-18 are antielectrons that travel a short distance and combine with an electron, and annihilation occurs with their masses converted into their energy equivalent (E = mc²) through emission of two 511-keV photons 180° apart. The two 511-keV photons are electronically detected as a coincidence event when they strike opposing detectors simultaneously. The figure illustrates one line of coincidence detection, but in an actual tomograph, 6–70 million detector pair combinations record events from many different angles around subject simultaneously. After correction for photon attenuation, tomographic images of tissue concentration are reconstructed. “Blocks” of detectors are arranged around the circumference, with each containing 32–64 detector elements, for a total of tens of thousands of elements. PET scanners provide hundreds of tomographic image planes of either selected organ or entire body. A single 6-mm-thick longitudinal section is shown from a woman with metastasis bilaterally to lung (arrow) from previously treated ovarian cancer. Black is highest metabolic rate in image. Human PET scanner resolution is about 5–6 mm in all three dimensions. Reprinted with permission from ref. 31.