Measurement of total-body oxygen, nitrogen, and carbon in vivo by photon activation analysis.

A method has been developed to measure total-body oxygen, nitrogen, and carbon in vivo using the x-ray beam of a 45-MV betatron and a whole-body counter. Following x-ray irradiation of living tissue, the positron emitting activation products 15O, 11C, and 13N are produced. The decay of these radionuclides has been measured in both phantoms and animals, and a computer curve-fitting algorithm used to resolve the decay curve into separate contributions from 15O, 11C, and 13N. The decay curve was corrected for interfering activity from 30P, 38K, and 34mCl, and in the case of live animals, also corrected for a substantial fraction of 11C lost through exhalation. Activation uniformity profiles have been measured for phantoms up to 30 cm in thickness. With a radiation dose of 20 cGy, total-body O, N, and C were measured in dead rats with estimated accuracies of +/- 1.4%, +/- 4.5%, and +/- 1.5% [1 standard deviation (SD)], respectively. With a radiation dose of 40 cGy, total-body O, N, and C were measured in living rats with estimated accuracies of +/- 1.4%, +/- 6.9%, and +/- 1.5% (1 SD), respectively. It is anticipated that total-body O, N, and C similarly could be measured in human subjects with a radiation dose of 1-2 cGy and with accuracies comparable to those obtained in rats. Although most of the measurements were made using a beam energy of 45 MV, we have shown that useful results may be achievable with a beam energy as low as 25 MV. This accurate, convenient, and safe technique for total-body O, C, and N measurement should have applications in the study of nutritional status in health and disease, both in human subjects and in animals.