Whole body counting

In health physics, whole-body counting refers to the measurement of radioactivity within the human body. The technique is primarily applicable to radioactive material that emits gamma rays. Alpha particle decays can also be detected indirectly by their coincident gamma radiation. In certain circumstances, beta emitters can be measured, but with degraded sensitivity.

This must not be confused with a "whole body monitor" used for personnel exit monitoring, which is the term used in radiation protection for checking of external contamination of a whole body leaving a radioactive contamination controlled area.

If a gamma ray is emitted from a radioactive element within the human body due to radioactive decay, and its energy is sufficient to escape then it can be detected. This would be by means of either a scintillation detector or a semiconductor detector placed in close proximity to the body. Radioactive decay may give rise to gamma radiation which cannot escape the body due to being absorbed or other interaction whereby it can lose energy; so account must be taken of this in any measurement analysis.

There are many ways a person can be positioned for this measurement: sitting, lying, standing. The detectors can be single or multiple and can either be stationary or moving.

The advantages of whole-body counting are that it measures body contents directly, does not rely on indirect methods (such as urinalysis) and that it can measure insoluble radionuclides in the lungs.

On the other hand, disadvantages of whole-body counting are that it can only be used for gamma emitters, except in special circumstances, and it can misinterpret external contamination as an internal contamination. On its own, it may be unable to distinguish between radioisotopes that have similar gamma energies. Alpha and beta radiation is largely shielded by the body and will not be detected externally, but the coincident gamma from alpha decay may be detected, as well as radiation from the parent or daughter nuclides.

A stand-up whole-body counter with phantom (mannequin) for calibration.

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