Hydrogen background in total body nitrogen estimations

Abstract

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.

Evaluation of Methods to Assess Reduced Body Fat in Patients with Anorexia Nervosa

“…In those over 8 kg, precision (CV%) was <3% and in those <8 kg precision was 5%-7%. Accuracy in 10-to 27-kg phantoms was to within 3% of chemical measurements [21].…”

“…Accuracy was to within 1% for LTM using pediatric software and for 5 different phantoms with a fat content from 5.5% to 46% the coefficient of variation (CV) for LTM varied from 0.2% to 1.3%. For fat the paediatric software on DXA values was within 1% accuracy error but the CV ranged from 0.3% to 8.1%, the latter in the high-fat-content phantoms [21].…”

Validation of Estimates of Lean Tissue Mass Made by Dual‐Energy X‐ray Absorptiometry

“…Soft lean tissue mass (LTM) in patients is estimated by dual‐energy X‐ray absorptiometry (DXA). Our center uses DXA, combined with other tools for measuring body composition such as body protein measurements 1 and anthropometry, to develop multicompartment body composition models appropriate for children. DXA may be the most appropriate measurement of fat tissue mass in patients who are unable to have other measures performed‐for example, patients with burn injury.…”

Assessment of Background Hydrogen by the Monte Carlo Computer Code MCNP‐4A during Measurements of Total Body Nitrogen

“…Several authors have assessed the hydrogen background and report a considerable variation. Vartsky et al , 2 Baur et al , 3 Stamatelatos et al , 4 and Humphries et al 5 measured the background using phantoms containing D 2 O. Vartsky used phantoms of different shape and thickness and established the background to be independent of the presence of the phantom, contributing about 20% of the hydrogen counts. Baur measured a 1% to 4% background contribution in a facility designed for pediatric use.…”