The use of technetium 99m as a clinical tracer element.

THE VARIETY of radioactive tracer chemicals currently available for medical use is almost embarrassingly large and is still increasing. The value of any proposed new isotopes should therefore be carefully considered. No apology however, need be made for the introduction of Technetium 99m by Scheer and Maier-Borst (1963) also by Harper, Beck, Charleston and Lathrop (1964), since this substance has manv advantages in its application to scanning and in the observation of transients. (Clark, Deegan, McKendrick, Herbert and Kulke, 1965). It has been evident for some time that many radioisotopes in common use in medicine are not best suited for scanning purposes. The ideal radioactive agent should usually conform to the following cri,teria:(a) The gamma ray energy should be such as to permit easy collimation without too much absorption in tissue and at the same time deliver the lowest radiation dose. The optimum energy is then within the range 80-200 Kev. (b) Beta emission and conversion electrons should preferably be absent as they increase the radiation dose without contributing to the scan. (c) The half life should be a few hours only, permitting biological equilibrium in the body to be obtained with the smallest radiation dose. At the same time this will allow the background to be sufficiently reduced for further tests to be carried out in a few days. (d) The radioisotope should be available carrier free or at least in high specific activity to avoid toxic or hypersensitivity reactions. A radioisotope with these properties may be administered in millicurie quantities with a hazard no greater than that of conventional X-ray investigations. Since Technetium 99m epitomises these criteria it seemed worthy of further investigation as a tracer isotope. It will be shown that multimillicurie doses may safely be used to obtain high counting rates which can give results of high statistical significance. Production and Physical Properties Tc99m is most conveniently produced carrier free as a pertechnetate (TcO4) in isotonic saline from a Mo99 generator which has a half-life of 67 hours. These generators in which the Mo99 is incorporated in an alumina column, are regularly available from the Radiochemical Centre, Amersham, England and other suppliers. Tc99m decays to Tc99 by isomeric transition with a half-life of 6.0 hours emitting a 0.140 MeV gamma ray which is 8-11% converted. (Strominger, Hollander and Seaborg, 1958). Tc99 itself decays by beta emission to stable Ru99, and has a half-life of 2.1 x 105 years so that only millimicrocurie quantities of this isotope are produced by a tracer dose. The Specific Gamma Ray Emission r is 0.61 r/mch at 1 cm, and the half-value layers in lead and water are 0.026 cms and 5.0 cms respectively. Since the gamma ray energy is near that which gives minimum r and there is no beta emission, adequate counting rates can be obtained with a comparatively small radiation dose. Conventional instruments give efficient collimation, but due to the low gamma ray energy, shielding and collimator length may be reduced to make a high sensitivity lightweight detector (Harper, Beck and others, 1964).