title: Assessment of background hydrogen by the Monte Carlo computer code MCNP-4A during measurements of total body nitrogen.
abstract: The use of a hydrogen internal standard to enable the estimation of absolute mass during measurement of total body nitrogen by in vivo neutron activation is an established technique. Central to the technique is a determination of the H prompt gamma ray counts arising from the subject. In practice, interference counts from other sources--e.g., neutron shielding--are included. This study reports use of the Monte Carlo computer code, MCNP-4A, to investigate the interference counts arising from shielding both with and without a phantom containing a urea solution. Over a range of phantom size (depth 5 to 30 cm, width 20 to 40 cm), the counts arising from shielding increased by between 4% and 32% compared with the counts without a phantom. For any given depth, the counts increased approximately linearly with width. For any given width, there was little increase for depths exceeding 15 centimeters. The shielding counts comprised between 15% and 26% of those arising from the urea phantom. These results, although specific to the Swansea apparatus, suggest that extraneous hydrogen counts can be considerable and depend strongly on the subject's size.
authors.affiliation: Department of Medical Physics and Clinical Engineering, Singleton Hospital, Swansea, United Kingdom. simon.ryde@swansea-tr.wales.nhs.uk
authors.forename: S J
authors.initials: SJ
authors.lastname: Ryde
authors.forename: F A
authors.initials: FA
authors.lastname: al-Agel
authors.forename: C J
authors.initials: CJ
authors.lastname: Evans
authors.forename: D A
authors.initials: DA
authors.lastname: Hancock
mesh_terms: D001823:Body Composition; D003198:Computer Simulation; D006801:Humans; D006859:Hydrogen; D009010:Monte Carlo Method; D009501:Neutron Activation Analysis / Q000379:methods; D009584:Nitrogen / Q000032:analysis*; D012984:Software
chemical_list: D006859:Hydrogen; D009584:Nitrogen