Simple diffusion

Simple diffusion describes a mechanism whereby a radiotracer diffuses across cell membranes and then redistributes itself elsewhere in the body. The perfect example is the ability of Xe-133 gas to diffuse across membranes in the lungs and to circulate in the blood stream. Exchange diffusion involves the diffusion of a radiotracer into a cell where a chemical exchange takes place. For example, one of the earliest bone imaging agents, the F-18 fluoride ion (F-), was capable of exchanging with the hydroxide ion (OH-) on the hydroxyapatite structure of bone tissue to form F-18 fluorapatite, a very stable molecule. This permitted external visualization by collecting the 511 keV annihilation photons produced during the decay by positron emission of this isotope.