Biodistribution is a method of tracking where compounds of interest travel in an experimental animal or human subject.
For example, in the development of new compounds for PET (positron emission tomography) scanning, a radioactive isotope is chemically joined with a peptide (subunit of a protein).
This particular class of isotopes emits positrons (which are antimatter particles, equal in mass to the electron, but with a positive charge).
When ejected from the nucleus, positrons encounter an electron, and undergo annihilation which produces two gamma rays travelling in opposite directions.
The organs of interest (usually: blood, liver, spleen, kidney, muscle, fat, adrenals, pancreas, brain, bone, stomach, small intestine, and upper and lower large intestine, a tumor if present) are placed in pre-weighed containers and weighed, then placed into a device that measures radioactivity (e.g. gamma radiation).
In addition, the procedure is in essence the same as for medical diagnostic imaging in the clinic with two main differences: (1) novel compounds under development may be injected into animals subject to scrutiny and approval of the detailed experimental plan while clinicians can only inject radiolabelled compounds that had been tested rigorously and approved for use in humans; (2) animals usually need to be anaesthetized for the duration of the scan (on the order of minutes) while humans are awake and simply need to stay still during the scan.