MEDICO-BIOLOGIC RESEARCH 



temporary histology was developed due to 

 the introduction of coloring (or staining). 

 The same possibility exists in microradiog- 

 raphy. 



Until now the object of microradiography 

 made with white radiation was approached 

 mainly from a morphological point of view. 

 An analytical physico-chemical approach to 

 such a microradiograph is also possible to 

 some extent. This idea was suggested by 

 Lamar que (81) and worked out by Eng- 

 strom (53), who proposed to radiograph 

 simultaneously the specimen and, as the 

 author calls it, the reference system. This 

 consists of foils of nitrated cellulose having 

 different thicknesses. The composition of 

 cellulose is as follows: C— 43%, 0—41%, 

 N — 7 % and H — 5 %, that is, it is quite 

 close to the composition of protein. Fig. 4 

 shows how microradiography proceeds in 

 this case. It is obvious that each thickness 

 of foil produces a ciuite definite contrast 

 which may be compared with those of tissue 

 components. Consequently, according to this 

 author, the mass (weight) of biological tis- 

 sue may be determined. This interesting idea 

 is connected, however, with some practical 

 difficulties, limitations and possible errors, 

 which are discussed in detail in the original 

 works on this subject (see especially Bratt- 

 gardt and Hyden) (36). 



The coefficient of scattering in biological 

 tissue seems to be quite considerable (see 

 equation 3). It increases proportionally to 

 the wavelength of x-rays and to the percent- 

 age of water in radiographed tissue. How- 

 ever, since specimens are mostly dehydrated, 

 cut in thin sections and are in close contact 

 with the emulsion during radiography, the 

 influence of scattering on the formation of 

 penumbras may be neglected. 



The thickness of the object and its in- 

 fluence on image formation will next be dis- 

 cussed. 



X-ray Image Factor. A microradio- 

 graphical image is the result of complicated 

 interrelations among the qualitative resolv- 



PHOTOGRAPHIC EMULSION 



DARKENING OF 

 EMULSION BY X-RAY 



STANDARDS -VARYING 

 KNOWN MASS 



ALUMINUM FILTER 



Fig. 4. Scheme of Engstrom's reference system 

 radiographed with specimen. Standards produce 

 image of various contrasts depending upon known 

 mass of foil. Identical image contrasts of reference 

 system and specimen have evidently the identical 

 mass (from Fitzgerald) (63). 



ing power of x-rays, absorption of x-rays by 

 varied object components, geometrical reso- 

 lution by x-ray beam, and the graininess of 

 the emulsion. The last two items have a 

 direct relation to the image formation and 

 therefore will be discussed in this paragraph. 



Geometrical Characteristics of the X-ray 

 Beam. Let us imagine that the immobile 

 specimen is radiographed with divergent 

 x-rays emitted from the point-like focal spot, 

 on a practically grainless emulsion of negli- 

 gible thickness and with no interference of 

 secondary radiation. Then the unsharpness 

 will be directly proportional: (a) to the size 

 of the focal spot discussed above, (b) to the 

 thickness of the specimen and, connected 

 with it, the superimposition of images of 

 microstructures, and (c) the distance be- 

 tween specimen and emulsion (Fig. 5). 



It is obvious that the influence of the sec- 

 ond factor may be diminished by making 

 the object as thin as possible; the best 

 remedy for the third is the closest contact 

 between specimen and emulsion. 



There are many superimpositions of im- 

 ages of microstructures lying at different 

 levels of a three-dimensional specimen as 

 seen in a two-dimensional microradiograph. 

 The thicker the specimen the greater is the 

 number of these superimpositions and the 

 less is the chance to obtain an image of 

 microstructure free of them. It is observed 

 that the unsharpness caused by superimposi- 



597 



