638 ADOLF F. VOIGT 



One of the principal difficulties is the correlation of the radio- 

 autograph with the microscopic structure of the material. It is 

 obviously necessary that correlation be made if the method is to be of 

 value. The use of fiducial marks on the tissue section and the photo- 

 graph is one way of obtaining such correlation, but with anything but 

 low magnification, this method is not capable of giving good register. 

 It should be borne in mind that the tissue itself usualh' must be 

 stained before histological study can be made. Several techniques 

 have been experimented with in which the tissue slice and photo- 

 graphic emulsion are not separated after exposure of the latter. In 

 one method the tissue slice that contains tracer is mounted on the 

 photographic plate or film which is then set aside for the proper ex- 

 posure period, the radioautograph is developed and fixed and the tis- 

 sue is treated with the proper histological stain {56). In another 

 technique the section is mounted on a microscope slide and emulsion 

 in liquid form is poured over it. Exposure and processing of the 

 photograph are done in the usual manner. Staining in this case is 

 more difficult since the section is covered by the emulsion, but staining 

 before the exposure is not a sure procedure since the radioactive atoms 

 would be likelj^ to move diu'ing the staining procedure. 



To go into the subject beyond this brief statement is bej-ond the 

 scope of this chapter but the following references to excellent articles 

 will help the beginner {3, p. 122; o4a,o4h,o7-59). 



G. MONITORING RADIATION FOR HE,4LTH AND SAFETY 



Closely related to the problem of measurement is that of monitor- 

 ing or personnel protection. In order that the neophyte in radio- 

 tracer research can have an appreciation of the subject and an under- 

 standing of the terms used in its discussion, a brief portion of this 

 chapter is devoted to the subject. 



1. Calculations 



The accepted unit of radiation dose in tissue is the roentgen or 

 r. unit (see chapters by Gow-en and by Gray) and others are derived 

 from it (17). It corresponds to an energy dissipation in air of 84 

 ergs per gram. Hence the other commonly used units for dosage 

 measurements, the roentgen equivalent physical (r.e.p.) or equivalent 

 roentgen (e.r.), are defined as the quantity of ionizing radiation of any 

 type that may be absorbed at the rate of 84 ergs per gram. Though 



