MEDICAL USES OF THE CYCLOTRON — SPEAR 143 



some cases a piece can be taken from the animal under anesthesia 

 without killing it. 



(b) Autophotography. — The tracer substance is administered and 

 after the predetermined time the animal is killed or a portion of an 

 organ removed (biopsy) under anesthesia. The tissue is sectioned as 

 for microscopic purposes and some of the sections are stained with 

 colored dyes and others placed upon photographic plates. The areas 

 of fogging, which correspond to radioactive deposits, are compared 

 with the almost identical stained preparations and the distribution 

 of the tracer substance studied qualitatively. In this method the 

 sections used for fogging plates are impregnated in the final stage of 

 preparation with collodion in place of the paraffin customarily used 

 in histological work. For large organs, such as one of the long bones, 

 the autophotograph can be compared with an X-ray picture of the 

 same part (e. g., see pi. 3, fig. 2). Very thin substances — the leaf 

 of a plant, for example — do not need to be sectioned on a microtome 

 but can be placed direct upon the photographic plate (pi. 2, fig 1). 

 The fogging of the plate is due to the emission of the beta rays from 

 the tracer substance, and the best photographic results are obtained 

 when something like 2 X 10 6 beta particles per sq. cm. strike the film 

 during the exposure period. 



(c) Geiger counter. — A method involving the sacrifice of the subject 

 under examination is obviously not applicable to hospital practice ! 



For the detection of tracer substances in human subjects a prepara- 

 tion is chosen which emits gamma radiation (e. g., radio iodine). 

 After administration, a Geiger counter, placed on the surface of the 

 body (16, 20) over the organ to be investigated, records the radio- 

 activity due to tracer present (pi. 2, fig. 2). Kadio iodine is made by 

 the deuteron bombardment of metallic tellurium, and has a half -life 

 of 8 days. It emits both beta and gamma rays. 



(d) Double tag. — An ingenious modification of the autophoto- 

 graphic method is occasionally used. By using the unstable H 3 in 

 the preparation of the dyes f@r staining it is possible to combine 

 color change and radioactivity in the same preparation. 



By such methods it has been possible to show the selective absorp- 

 tion of various activated substances in particular tissues ; a variation 

 in absorption in different states of activity of living tissues due either 

 to normal circumstances in health or abnormal conditions imposed 

 by disease ; and even a variation in different parts of the same cell (21) 

 which can be examined after disintegration of a mass of cells by the 

 ultracentrif uge which separates the central and heavier nuclear mate- 

 rial from the surrounding and lighter cytoplasm. 



The amount of tracer substance taken up depends on the total con- 

 tent of the same (inactive or natural) elements in the tissue at the time 



