RADIOAUTOGRAPHY 



298 



RADIOAUTOGRAPHY 



silver bromide granules makes it pos- 

 sible to localize the radioactivity to 

 a comparablj'^ small area in the tissue. 



It IS, therefore, possible by this 

 method to virtually "see" a "labeled" 

 element in an animal tissue. Further- 

 more, the fate of that element in the 

 animal body may be detected by follow- 

 ing the element through from structure 

 to structure. The metabolism of any 

 labeled physiological or pharmacologi- 

 cal compound can thus theoretically be 

 traced throughout the animal body 

 using the radioautographic method, 

 providing the labeled material, be it 

 a product of synthesis or degradation, 

 is retained in its original site in a tissue 

 during processing for radioautography. 

 The advantage of this method for the 

 progress of histology is apparent, since 

 the significance of known structures will 

 often be revealed by the substance 

 which they incorporate. From the 

 medical point of view the sites of ac- 

 cumulation of the isotopes as well as 

 their length of stay in the body provide 

 useful information as to the radio- 

 toxicity of known amounts of these iso- 

 topes. The applications of radioautog- 

 raphy to biology and medicine have 

 been fully reviewed most recently bji- 

 Gross, J., Bogoroch, R., Nadler, N. J., 

 and Leblond, C. P., Am. J. Roentg. 

 and Radium Ther., 1951, 65, 420-458. 

 The use of a-ray-producing isotopes was 

 examined by Yagoda, H., Radioactive 

 Measurements with Nuclear Emulsions. 

 New York: John Wiley and Sons, Inc., 

 1949. 



As with other methods involving the 

 use of radioisotopes, it is assumed that 

 the chemical behavior of a labeled sub- 

 stance is identical with that of its stable 

 counterpart. This will also be true of 

 its biological behavior providing that 



1) the amount of radioactivity admin- 

 istered be small enough not to have a 

 significant radio-chemical effect, and 



2) the amount or weight of material in- 

 jected be sufficiently small not to pro- 

 duce a significant increase in the amovmt 

 of this substance present in the circula- 

 tion. The labeled substance will then 

 truly act as a "tracer" of the normal 

 metabolism. 



The success of radioautography de- 

 pends on 1) the geometrical relationship 

 between specimen and emulsion, the 

 best conditions being a minimal dis- 

 tance between source and emulsion, a 

 thin section (5ai or less), and a thin 

 emulsion coating (10;u or less) ; 2) the 

 features of the emulsion — the silver 

 bromide grains should be uniform (for 

 contrast), of a small size, and in high 

 concentration; and 3) the characteristics 



of radiations (i.e., energy and intensity) 

 — particles of low energy such as soft 

 /3-rays give a better resolution. For 

 a more detailed discussion of each of 

 these factors see Nadler, N. J., Cana- 

 dian J. Med. Sci., 1951, 29, 182-194; Gross 

 et al., referred to above; Doniach, 

 I. and Pelc, S. R., Brit. J. Radiol., 

 1950, 23, 184-192. 



Exposure time for a radioautograph 

 is influenced by these three groups of 

 factors: 1) Geometrical relations. Thus, 

 minimizing the distance between emul- 

 sion and specimen will tend to reduce 

 exposure time, while decreasing the 

 thickness of section and emulsion will 

 tend to prolong it. 2) Properties of 

 photographic emulsions. Emulsions vary 

 in sensitivity to /3-rays. Also, the more 

 contrasty the emulsion is, the narrower 

 is the range of satisfactory exposure 

 time. 3) Character of the radiation. 

 The radiation influences the exposure 

 time not only by its concentration per 

 unit area, but also according to the 

 half life and energy of the isotopes used. 

 With soft radiation, the absorption in 

 the tissue will also increase the exposure 

 time. 



It is important to keep the exposure 

 to a minimum, as overexposure results 

 in a more diffuse image with correspond- 

 ing loss in resolution. Unstained 

 "test" slides developed periodically 

 will indicate the optimum exposure 

 time. 



Preparation of Tissues. 



In the case of soft tissues, paraffin 

 or celloidin sections are prepared in 

 the routine histological manner and de- 

 hydrated with or without staining. 

 For practical purposes, 5^t paraffin sec- 

 tions and 10/i celloidin sections seem 

 to be most convenient. Carbowax 

 (Blank, H., McCarthy, P. L., and De- 

 Lamater, E. D., Stain Techn., 1951, 

 26, 193-197) and frozen dried (Holt, 

 M. W., Cowing, R. F., and Warren, S., 

 Science, 1949, 110, 328-330) sections 

 have also been used. 



Bones and teeth may be embedded 

 and cut according to Bloom's method 

 (see Leblond, C. P., Wilkinson, G. W., 

 B(51anger, L. F., and Robichon, J., Am. 

 J. Anat., 1950, 86, 289-341) for bones of 

 young animals or according to Arnold, 

 J. (Science, 1951, 114, 178-180) for hard 

 bones of older animals. Methods em- 

 ploying grinding machines have to be 

 used for teeth as described by Hoecker, 

 F. E. and Roofe, P. G. (Radiology, 

 1949, 52, 856-865), Laude, P. P., Janes, 

 R. G. and Boyd, J. D. (Anat. Rec, 

 1949, 104, 11-15), Sognnaes, R. F., Shaw, 

 J. H., Solomon, A. K. and Harvold, E. 

 (Anat. Rec, 1949, 104, 319-330). 



