An array of detectors is arranged in a specially shielded room to capture gamma rays emitted by the body. The shielding screens the detectors from outside radioactive interference, making it possible to detect very low gamma signals.

Historically, the first whole body counters were built to test people (for example, nuclear industry workers) for contamination by man-made radioactivity. Because accidents and contamination were rare, other uses were sought for these devices.

Certain compounds with radiotracers (small amounts of radioactive elements) were seen to concentrate in select organs. For example, iodine is taken up by the thyroid. This led, in part, to the development of nuclear medicine.

A naturally occurring radioisotope is ⁴⁰K, an isotope of potassium (K). It is evenly distributed throughout the body and gives off a continuous gamma signal. Potassium is an indicator of fat-free mass.

The average potassium content of an adult male is approximately 140 grams, or about 15 milligrams of 40K. This translates to a natural signal given off by the body of 30,000 gammas/minute. In adult females, the level is about 20,000 gammas/minute. Smaller subject size means lower amounts of 40K and therefore a weaker signal.

The body contains several natural and man-made radioactive isotopes, for example radium, cesium, and thorium.

Accurate whole body counting requires efficient detectors, effective shielding, and sophisticated computer equipment. The system must be able to identify the ⁴⁰K signal and account for interference from other isotopes.

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