Pergamon 0969-8043(94)00146-4 Prompt-gamma Neutron Activation Facility for Measuring Body Nitrogen In Vivo in Small Animals I. E. STAMATELATOS ~ and S. YASUMURA 2 ~Department of Medical Physics, Saint-Savas Hospital, 171. Alexandras Avenue. Athens 115 22, Greece 2Medical Department. Brookhaven National Laboratory, Upton, NY 11973, U.S.A. (Received 20 June 1994; in revised form 4 November 1994) We have designed a prompt-gamma neutron activation facility for in vivo measurements of total body N in small animals. The facility incorporates a 252C neutron source with an emission rate of 3.2 x 108ns -I (140/~g), and a heavy- water neutron colfimator-moderator assembly, together providing an optimum thermal flux-to-dose ratio for a 0.254).5 kg animal. Using two NaI(T1) detectors we can attain a fractional statistical error of measurement of 3.5% for a dose equivalent of approx 50 mSv, sufficiently low that serial measurements can be made on the same animal. 1. Introduction Measuring total body nitrogen (TBN) m vivo is essential in many studies of body composition where the assessment of total body protein and lean body mass are the major end-points (Cohn et al., 1980). Nitrogen has been measured by two neutron activation techniques. The first, the delayed technique, uses the fast neutron reaction, laN(n, 2n)13N, and measures the annihil- ation quanta from 13N positron decay. The second, the prompt technique, uses the thermal neutron reaction, 14N(n,'~)ISN. and measures the prompt-gamma yield (E~- 10.83 MeV) from 14N neutron capture. The latter has been the preferred method to assay TBN in vivo (Chettle and Fremlin, 1984). Several reports have described facilities for measuring TBN in vivo m humans by both techniques (Ellis et al.. 1987). Less attention has been given to measuring TBN in vivo m small animals. However. using animal models is the only practical way to have a sufficiently uniform popu- lation to be able to study one variable at a time without the confounding problems posed by multiple variables that are inherent in almost all clinical investigations The feasibility of measuring total body N in mice by the delayed technique was demonstrated by Nagai et aL 11969) and Biggin and Morgan (1971). Boddy et al. (1976), applied the delayed technique to study body composition changes, including N, in rats on a calcium-deficient diet. McNeill et al. (1988) successfully applied the prompt technique to measure N in rabbits (2-3 kg), using Pu/Be sources after modifying the Canadian human facility in Toronto. Ellis et al. (1992) modified the Am/Be-based facility for measuring human TBN in Houston, Texas, to measure N in vivo in genetically obese and lean pigs. However, there are no prompt-gamma facilities currently in operation to measure TBN in smaller animals (< 1 kg). The lack is because TBN has proved difficult to measure precisely enough in vh,o in such small Appl. Radiat. lsot. VOI. 46, No. 4, pp. 269-272, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/** $6.00 + 0.00 animals using a radiation dose that does not induce any changes in TBN. nor in body weight. In the present work. we describe a small animal prompt- gamma neutron activation (SAPGNA) facility that was designed to measure body nitrogen in vivo. primarily in rats. The SAPGNA is a new facility that adds to the capability of the Medical Department at Brookhaven National Laboratory to measure several body elements by in vivo techniques. 2. Description of the Facility 2.1. General Figures l a and b are schematic diagrams of the SAPGNA facility. Its main elements are a 252Cf neutron source, a neutron collimator assembly, a storage position for the source, an A1 transport tube to deliver the source from its storage position to the collimator, an animal holder, two NaI(TI) detectors, shielding and associated electronics. Heavy water is the principal neutron-moderating material. while concrete, polyethylene and Pb are used as shielding materials. 2.2. Procedure for measurements The measurement procedure has been designed in order to comply with the guidelines of the Institutional Animal Care and Use Committee (IACUC} The rat is placed in an A1 cylinder above the opening of the collimator and the detectors are positioned at right angles from the animal and at the top of the collimator. The source is moved from the storage to the activation position after the animal is in place. After the activation period, the source is returned to its storage position before the animal is removed. This pro- cedure reduces the dose to the operator while transferring the animals to and from the system, in keeping with the ALARA principle. 2.3. The neutron source The facility uses a 252Cf radionuclide source with neutron emission rate of 3.2 × 108 n/s (140/lg). The neutron radi- ation of 2s2Cf consists principally of neutrons from spon- taneous fission with an effective half-life of 2.65yr. Californium was chosen because it has a higher thermal fluence-to-dose ratio at the depth of peak thermal fluence in comparison to (c~,n) type radionuclide neutron sources detector animal cylinder / / lead concrete polyethylene ___\ ...... ,_'~/_'~,_ _ _ _.~ / • ,r 73, , [ ...... ~zZ_-S / heavy water 1 : Sources at activation position Scale : 20 cm 2. Sources at storage position 3: Animal position during measurement Fig. la. Small animal prompt 7 neutron activation (SAPGNA) facility at BNL using 252Cf sources (side view). 269