Demonstration of Gamma Radiation from Living Patient Following Thorotrast Injection

Abstract

SINCE the introduction of thorotrast several years ago, it has been well received by the profession for the purpose of intravenous injection as a contrast medium to delineate the liver and spleen on the radiograph, but as thorium is one of the more radio-active elements, the question naturally arose as to the probability of its radiation doing damage to the reticulo-endothelial system after many years of bombardment. One writer has dismissed the matter with the statement that the radiation is negligible. Others have reached that conclusion by placing the material on a photographic film for a few hours, and, when no blackening occurred, assuming that the human body could not be damaged. I believe that I have definitely shown that the radio-activity from this preparation is well within the limits of danger. The Geiger Counter (1) has opened great possibilities in the measurement of small quantities of radiation. Perhaps it is no exaggeration to say that it does the same thing for the radiologist that the microscope does for the biologist. Briefly, it consists of a glass tube which is partly evacuated and has a cathode charged to a fairly high potential. The anode is connected to a system of amplifying tubes so that the feeble impulse of a photo-electron entering the tube becomes sufficiently powerful to operate a loud speaker, signal light, or mechanical counting device. The delicacy of this instrument is shown by the fact that it responds well to a 10 mg. needle of radium at a distance of more than one hundred feet. When a sealed ampule of thorotrast is brought near this instrument, the response is immediate and violent. Realizing that the accurate determination of the gamma-ray activity of a dose of thorotrast would be of value, I started work on the problem. A small standard of radium was obtained, not anything comparable to radium needles such as are used medically, but a crude radium salt containing three micrograms of radium element. To make this amount intelligible to the radiologist who is in the habit of dealing with milligrams, it is well to state that this standard represents an investment of about twenty-four cents. This standard was compared with the three ampules of thorotrast as follows: in a room where there was no known radio-activity and at a time when no x-ray machines were in operation, the Geiger Counter was allowed to make a base count which, of course, included cosmic radiation and any small amount of radiation which may have emanated from the soil or building. This count varies at different locations but, once established, is used as a correction factor on the instrument. Next, the thorotrast is placed on a wooden frame along with the counting tube, the distance being an arbitrary one, but one which can be exactly duplicated. Counts are made over a long period of time so that the average per minute can be determined.