The In vivo neutron activation analysis (IVNAA) technique was first applied in human subjects to measure the total body contents of sodium, chlorine and calcium by Anderson and coworkers in 1964. IVNAA is an analytical tool based on nuclear rather than chemical or biochemical reactions. The prospective success of any IVNAA technique is a function of the product of the abundance of the element of interest in the body and its neutron cross-section. The major source of imprecision and inaccuracy of the delayed gamma technique is the variability of the fast and thermal neutron flux within the patient and this is particularly true of total body techniques. In photon scattering methodologies, a γ-ray or X-ray source is used to direct a beam of photons into the bone. In Compton scattering a photon interacts with an individual atomic electron, with some of the energy of the photon being taken up by the recoil of the electron.