Estimation of total body potassium in the presence of interfering radio isotopes

A whole body counter employing a 32 4'' x 4'' x 16'' single-crystal NaI(T1) detectors has been assembled and used to monitor total body potassium-40 (TBK-40) emissions from subjects participating in mineral bioavailability studies. The system calibration has been performed daily by counting 4 Na-22 sources and having a computer-controlled power supply set the voltage on each photomultiplier tube so that the centroid of the 1.275 MeV photopeak, observed by each detector, occurred at a designated channel in the computer-based multichannel pulse height analyzer. The total net Na-22 counts from the 32 detectors has been recorded as a stability check, and when the 1n of the daily total count was plotted as a function of time, a straight line was observed (r = 0.998) with a slope of -0.00072 d-1 +/- 0.000004 d-1. This system has been designed to estimate corrected gamma ray activity, independent of radionuclide distribution and body size, and to use matrix inversion to estimate activities from a number of isotopes having overlapping gamma ray spectra