Receptor-targeted nuclear imaging is a method used to look at particular tissue, without biopsy. For example, in tissues like breast—which express estrogen receptors—estrogen, the ligand, can be tagged with an isotope of fluorine, F18. When the complex is injected into the blood stream, the F18-Fluoroestradiol accumulates in cells expressing the estrogen receptor.
To see where the radioligand accumulates, a PET-CT scan is used. PET, or positron emission tomography, uses gamma detectors to see the fluorine isotope.
As F18 decays, with a half-life of 110 minutes, it emits a positron—a positively charged subatomic particle. When the positron hits an electron, its negatively charged twin, they annihilate each other, producing a pair of photons or gamma particles.
These particles move in opposite directions and are picked up by the detectors. When two detectors identify—at a similar time—an annihilation event, the location of the event can be identified and mapped. Tumors will have multiple annihilation events captured by the PET scan.
To precisely localize the tumor, a CT, or computed tomography, scan is used, and the PET scan is overlaid onto the CT for viewing.