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 su btracted before calculating TEN from the ratio of nitrogen counts (at 10.8 MeV) to hydrogen counts (at 2.2 MeV). The hydrogen acts as an int ernal standard. The background under the hydrogen peak is usually igno red. The current study was undertaken to measure the variation of the hydrogen background (HB) in patients of different sizes and to determi ne whether the accuracy of TBN measurements is improved when an HB cor rection is incorporated. Heavy-water (D2O) phantoms were used to quant ify patient HB. D2O simulates patient neutron scattering without contr ibuting to the hydrogen peak. Equations were developed to predict HB i n patient measurements. HB was found to vary from 9% of total counts f or 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 anthropom etric phantoms of known composition when an HB correction is incorpora ted.