Some Applications of Measurements of Whole-body Radioactivity in Clinical Medicine.

Interest in the radioactivity of the human body stemmed initially from the use of radium salts medicinally and the ingestion of radium, with tragic consequences, by the early luminizers. The first measurements of body radioactivity were reported by Schlundt, Barker and Flinn (1929) who could estimate in vivo body burdens of radium in the region of 5 to 100 lig. Instruments with a sensitivity sufficient to detect the natural gamma-ray activity of the body were first developed by Sievert (I95i) and Burch and Spiers (I953). The gamma radiation from the body normally arises from the radioactive isotope, 40K, which is present in all natural potassium with a constant abundance of o.oi i8%. These researchers used high-pressure ionization chambers and, with an observation time of two hours, could estimate body potassium with a standard error of 30 to 50%. Stimulated by the needs of radiological protection with the increasing use of atomic energy and radioactive isotopes, the newly developed scintillation detectors were employed in the later I950'S to construct yet higher sensitivity counters which had the further advantage of enabling gamma-rays of differing energies to be distinguished by pulse height selection techniques. These developments resulted in an increase in sensitivity of 50,000 times that of Schlundt's original apparatus and today body potassium can be measured with a standard error of i or 2% in an observation time of I5 minutes (Spiers, I962). Clearly the determination of body potassium with this order of ease and accuracy is of great clinical value and practical usefulness. Furthermore, with this extreme sensitivity, tracer studies with radioisotopes can now be conducted over extended periods of time with very low levels of administered activity. In Leeds, two whole-body counters are in operation which largely complement one another. In the first, shown in Fig. i, three plastic scintillator detectors (20 in. x io in. x 61 in.) are grouped around a seated subject to give a system of high sensitivity and moderate energy resolution, well suited to body potassium measurements (Burch, Hughes, Iinuma, Overton and Appleby, I962). In the other, shown in Fig. 2, four sodium iodide crystals (6 in. x 4 in.) are arranged two above and two below a supine subject. It is also possible to locate individual detectors near specific WHOLE BODY COUNTER IN STEEL ROOM