CHAPTER XXX 

 RADIOGRAPHY 



By Robert C. Woods 



Introduction. — The shadow picture resulting when X raj's pass through an object 

 and fall upon a sensitive photographic film is known as a "radiograph." The process 

 of taking such a picture is called "radiography," and the apparatus used is a "radio- 

 graphic machine." While these terms are most commonly used, others may be 

 correctly substituted, such as skiagraph for radiograph or roentgen raj^ for X ray. 



Radiography may be roughly divided into three classifications: (1) clinical, (2) 

 industrial, and (3) diffraction. The purpose of clinical radiography is to study the 

 outline of the human frame with all its surrounding tissues so as to locate accurately 

 the site of possible disease, injurj^, or foreign body. Industrial radiography provides a 

 nondestructive test for studying the internal structure of industrial niaterials with 

 the purpose of discovering subsurface defects. While diffraction methods are not 

 generally classed under radiography, a literal interpretation of the word "radiograph" 

 requires the inclusion of diffraction patterns, since they are also shadowgraphs pro- 

 duced by X rays. Diffraction pictures are used as a method to disclose the crystalline, 

 molecular, or atomic structure of almost any material and are invaluable in manj^ ways 

 to industry, medicine, and pure research. 



Characteristics of X Rays. — The principal characteristics of X rays may be sum- 

 marized as follows: X rays are produced in special types of vacuum tubes by the impact 

 of electrons on a target of hard metal, the rays being emitted in straight lines from 

 the target where they originate. The number of rays produced is proportional 

 to the current passing through the tube, whereas their penetrating power depends upon 

 the voltage at which the tube is operated. X rays are a form of electromagnetic 

 energy and travel with a velocity approaching 186,000 miles per second. X rays 

 behave sometimes as waves and sometimes as discrete particles, in common with other 

 forms of electromagnetic radiations. The X-ray spectrum has a mean wavelength 

 which depends upon the voltage at which this tube is operated. The rays are incap- 

 able of producing visual sensation unaided, but their presence may be indicated visu- 

 ally through the use of fluorescent screens which emit visible light when acted upon 

 by X rays. The rays are capable of penetrating substances which are opaque to 

 visible light but are attenuated more or less in passing through solid materials, the 

 amount of attenuation depending upon the thickness of the object and its atomic 

 structure. 



X-ray Photography. — It has been found that X rays have the very important 

 property of producing a latent image on photographically sensitive materials, 

 and this property is made use of in radiography. It enables the X-raj^ worker to 

 record on film facts about the internal condition of closed structures which would 

 normally be invisible. This effect depends on the fact that wherever X rays strike a 

 gelatin emulsion of silver bromide, the silver salt is reduced to a black metallic condi- 

 tion. For all practical purposes this reduction is proportional to the intensity of the 

 incident radiation. The photographic effect is also approximately inversely propor- 

 tional to the square of the distance between the target of the X-ray tube and the 

 sensitive film. 



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