Search

The design and construction of a hospital clinical facility for in vivo prompt gamma neutron activation analysis for total body nitrogen (TBN) measurement is described. The use of 252Cf neutron sources gives a better signal-to-background ratio compared with 238Pu-Be sources of equal strength, thus yielding better reproducibility of measurements. By measuring the hydrogen and nitrogen signals separately using appropriate gating circuits, signal-to-background ratio is further improved. Measurements using a urea phantom (5.63 kg nitrogen as urea in 34.53 kg of water) show that 2 x 6 micrograms 252Cf sources gives a nitrogen signal-to-background ratio of 5.6 (compared with 3.4 in the case of a 2 x 10 Ci 238Pu-Be source) and a reproducibility for nitrogen signal of +/- 1.1% (CV) and for hydrogen signal (internal standard) of +/- 2.33% (CV). Approximately 30 minutes of patient's time is required for each TBN measurement with an estimated reproducibility of +/- 3.8% (CV). The radiation dose to the patient is about 0.2 mSv (effective dose equivalent; QF = 10) per 20 min measurement. A report for the clinician is produced within a few minutes after the measurement by a dedicated IBM-PC computer. The entire facility is clean, comfortable and the electronics and computer processing are simple and economical.

A mehtod of measuring body nitrogen by neutron capture gamma analysis is described. The neutron flux is provided by four collimated 5 Ci Pu-Be sources, placed to give a bilateral irradiation of the subject. The 10-83 MeV thermal neutron capture gamma rays from 14N are detected by two heavily shielded 5 in X 4 in NaI(Tl) detectors. Results indicate a reproducibility +/-3% (1 SD) for a 10 min irradiation of the chest area, while giving a radiation dose equivalent of 50 mREM.

A major problem in the measurement of nitrogen in the body by in vivo neutron activation analysis is the size of the background. Investigations show that random summing of gamma rays in the range 4-7 MeV is a major contributor. By direct comparison, 252Cf is shown to be a better neutron source than Pu-Be in this regard. Data are presented on the contribution to the background of water and chloride in the body.

Measurement of nitrogen in the human body gives a measure of total protein in the body. Nitrogen (N) may be measured in vivo by a nuclear technique in which the characteristic nitrogen gamma rays emitted after capture of neutrons are detected and counted. The accuracy (as determined by comparison of the measurement of nitrogen in a pig by nuclear and chemical methods) is better than 10%. The reproducibility is less than 6%. The radiation dose is about 5 millirads. This method had been used with 43 patients and 23 normal volunteers and the results are reported. The data are presented in the from of total N content, percentage N by weight, and as a comparison of N and potassium (K) in the body. It is found that the mean percentage of N in the normal males is 2.5%, giving a value of 1.75 kg in the 70-kg standard man. The mean percentage values are not greatly different between normal volunteers and malnourished patients. Taking all subjects, N/K ratios ranged from 7.5 to 20 g/g. Thus K cannot be used as a predictor of N. Nitrogen content of normals correlates well (r = 0.68) with height squared; for malnourished patients there is again a good correlation (r = 0.81) but the regression line for patients is approximately 34% below that for the volunteers consistent with their depleted state.

Lean body mass by anthropometry (AMLBM), total body potassium (TBK), and total body nitrogen (TBN) by prompt gamma analysis, are reported in 38 malnourished patients during total parenteral nutrition. Over long periods (greater than 2 months) TBN increased 32% while AMLBM and TBK rose only by 9.2 and 9.5%, respectively. Changes in AMLBM and in K were significantly correlated, but changes in nitrogen were not correlated with either. From short-term studies, the same conclusions can be obtained; although there are changes in lean body mass and in potassium, there is no correlation with such changes in nitrogen. With repletion, changes in body protein are not necessarily related to changes in AMLBM or to TBK: the latter two reflecting total and intracellular water, respectively, but not protein content.