354 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



sitometric measurement. Statistical analysis of these data indicated that 

 there was a correlation between radioassay and densitometric measure- 

 ment significant at the 1 per cent level. The agreement between bones 

 is such as to give greater confidence in densitometric comparison than in 

 radioassay. This results from the difficulty of sampling regions contain- 

 ing narrow lines of deposition. Similar studies have been reported by 

 Dudley and Dobyns (62), particularly in connection with the estimation 

 of radiation dosage to tissue. 



Track and Grain Counting. At high magnifications it becomes neces- 

 sary to count tracks or grains for estimation of photographic density. 

 This can be accomplished by the use of a micrometer placed in the ocular 

 to outline a given area. Some of the photographic and technical consid- 

 erations have been discussed in references (4 to 6, 8, 10, 11, 18). In gen- 

 eral, the stripping-film method is best for ciuantitative autoradiography, 

 since the emulsion thickness and the distance between specimen and emul- 

 sion are constant and reproducible. The coating method, however, has 

 also been used satisfactorily. Nuclear emulsions should be employed on 

 account of the regularity of the grains. If the number of grains is suffi- 

 ciently low to be counted, their number is proportional to the amount of 

 radioactivity in the specimen. Also, since grain counting is tedious at 

 best, exposure and development should be controlled so as to produce low 

 grain concentrations. Doniach and Pelc (18) have found a count of 10 

 grains/ 100 n'- above background to be statistically significant. This pro- 

 vides a real advantage in reducing the amount of radioactivity and/or 

 exposure time required, as compared with methods based on densitometric 

 or photometric measurements. 



Alpha particles give well-defined tracks in nuclear emulsions, in which 

 case each track may serve to identify a single disintegrating atom. 

 Miller and Hoecker (63) have described methods for quantitative estima- 

 tion of alpha emitters in bone by observation of the numbers of tracks 

 produced. Special emulsions are now available which can be used to show 

 tracks of beta particles, as illustrated by the studies of Boyd and Levi (64) 

 on C'' appearance in the liver of rats injected with C^^-labeled glycine. 

 By varying the focus of the microscope, it is possible to trace a track to 

 its origin at the surface of the emulsion. 



Perhaps of more importance than absolute quantitation is the fact that 

 grain- or track-counting procedures provide excellent resolution and per- 

 mit the objective estimation of fine differences in intensity between adja- 

 cent cells or parts of cells. Johnston (65) has described grain-counting 

 procedures used for localization of P^- in the intracellular zones of the 

 columnar absorbing cell of the rat. Histological and stripping-film tech- 

 niques were employed, as described earlier, with every effort to obtain 

 maximum resolution. Grain counts Avere made over a standard area of 



