Hydrogen background in total body nitrogen estimations.

Total body nitrogen (TBN) is measured by in vivo prompt gamma neutron capture analysis. Usually the background under the nitrogen peak is subtracted before calculating TBN from the ratio of nitrogen counts (at 10.8 MeV) to hydrogen counts (at 2.2 MeV). The hydrogen acts as an internal standard. The background under the hydrogen peak is usually ignored. The current study was undertaken to measure the variation of the hydrogen background (HB) in patients of different sizes and to determine whether the accuracy of TBN measurements is improved when an HB correction is incorporated. Heavy-water (D2O) phantoms were used to quantify patient HB. D2O simulates patient neutron scattering without contributing to the hydrogen peak. Equations were developed to predict HB in patient measurements. HB was found to vary from 9% of total counts for a 99 kg subject to 19% for a 7 kg infant. It was demonstrated that the accuracy of TBN measurements is improved for child-sized anthropometric phantoms of known composition when an HB correction is incorporated.