It is 30 years since the first measurements of the elemental composition of the human body were undertaken by Anderson et al. (1964) using the technique of in vivo neutron activation analysis (IVNAA). Their work followed a report (Hoffman & Hempelmann, 1957) of two nuclear reactor accidents in 1945–6 during which ten persons were exposed to bursts of radiation (fast neutrons and gamma-rays). A measure of the serum 24Na activity induced in the body was used to estimate the neutron intensity to which the subjects had been exposed. It was subsequently realised that a controlled irradiation with neutrons of known intensity could be used as an investigative tool.
The IVNAA technique involves the irradiation of the total or partial body by a beam of neutrons and the detection of characteristic gamma-rays arising from neutron interaction with the nuclei of the element or elements of interest. Although the principle of the technique is simple, the factors that determine the successful measurement of a particular element in vivo can be complex. Nevertheless, IVNAA has been successfully developed at several centres over the last three decades and gained increasing acceptance as a clinically useful, although specialised measurement technique. During this time considerable research has been undertaken to optimise the measurement of both bulk (e.g. Ca, C, Cl, H, N, Na, O and P) and trace (e.g. Al, Cd, Cu, Fe and Si) elements. However, the measurements of calcium, cadmium and nitrogen have been the most widely applied.