Medical application of neutron capture γ-ray spectroscopy: measurement of cadmium and nitrogen in living human subjects

In-vivo measurement of small quantities of Cd is possible due to the high radiative neutron-capture cross-section of 113Cd (12.3%, 20000 b). Under slow neutron capture in 113Cd, the excited 114Cd decays by prompt emission of cascade of gamma-rays of which the most intense is the 559 keV transition from the first excited state to the ground state. For a total kidney or liver dose of 670 mrem, the detection limits are 2.5 mg or 1.5 μg/g respectively. A table shows the results of a study on normal subjects with smoking and non-smoking history. The study indicates higher cadmium levels in the group of smokers. The method of measuring body N utilizes the 14N(n,γ)15N reaction. The total energy available on slow neutron capture is 10.83 MeV and approximately 15% of the de-excitations take place directly to the ground state of 15N. The irradiation facility is basically the same as that described for measurement of Cd. The Cd collimator, however is replaced by a second collimator designed to provide a wide beam 13 x 60 cm at the level of the bed. During the irradiation the subject lies on a motorized bed which moves across the neutron beam. The precision or reproducibility of the measurements was performed using an Alderson phantom. For a standard 70 kg man having 2000 g of N, the accuracy of the measurement is +-2% with an error of 1.3% for reproducibility, based on several measurements over a 6-month period. The total radiation dose for a bilateral irradiation is 45 mrem. Initial clinical studies will concentrate on sequential measurements of body N