A prompt gamma in vivo neutron activation analysis facility for measurement of total body nitrogen in the critically ill

The quantitative assessment of the nutritionally-important components of body composition (protein, water and fat), provides a basic tool for research into the metabolic and nutritional problems of critically ill patients requiring intensive care. The construction and calibration of a facility that provides bilateral irradiation of patients with neutrons from two 281 GBq ²³⁸Pu-Be source is described. Patients are scanned over a 36 min period, and composite prompt gamma -spectra are collected from two 5 in*4 in NaI(Tl) detectors, placed on either side of the patient, using a conventional spectroscopy analysis system. A ratio of nitrogen to hydrogen counts corrected for body habitus and background is derived, from which protein can be estimated. This technique, combined with the measurement of water by tritium dilution, enables protein, water and fat to be estimated with precisions of 4.2%, 1.5% and 6.3% respectively, for a total dose equivalent (neutrons and tritium betas) of less than 0.5 mSv per examination.