Improved clinical facility for in vivo nitrogen measurement

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.