824 HANDBOOK OF PHOTOGRAPHY 



determined at will. The voltage between the filament and target (sometimes called 

 the "cathode" and "anode," respectively) may be as high as 1,000,000 volts, although 

 ordinarily voltages of from 30,000 to 200,000 volts (30 to 200 kilovolts) are used. The 

 current through the tube is commonly between about 10 and 100 ma. (0.010 to 0.100 

 amp.). 



The point at which the beam of electrons emitted from the filament strikes the 

 target is known as the focal spot. This spot should be as small as possible in order 

 that the tube may simulate as much as possible a point source, and thereby produce 

 radiographs which are as sharp as possible. If this focal point is unduly large, the 

 resulting radiographs will be less sharply defined. 



Factors in Making Radiographs. — In addition to as small a focal spot as is possible, 

 there are other conditions which contribute to correct radiographic images. The 

 distance between the anode and the object being radiographed should be as great 

 as is practical. While a distance of 25 in. from anode to film maj^ be satisfactory for 

 radiography of the extremities, with thicker parts it is usually necessary to increase this 

 distance. 



The film should be as close to the object as possible. Distance between object 

 and film allows the rays to spread to such an extent before reaching the film that hazi- 

 ness occurs. 



The film should not deviate too far from perpendicularity to the line of the prin- 

 cipal rays; otherwise serious distortion will result. 



In the radiography of thick objects, scattered rays form the greater part of the 

 radiation passing to the film. Any effective method of preventing this scattered 

 radiation from reaching the film produces a marked improvement in the quality of 

 the image. 



In the medical field the most effective way to reduce scattered rays from the object 

 is through the use of a Potter-Bucky diaphragm. This apparatus is a moving grid 

 interposed between the part to be taken and the film. The grid is composed of a series 

 of lead strips held in place by interv^ening wood strips. The lead strips are so tilted 

 that the plane of each is in line with the tube focal spot; these strips have the function 

 of absorbing the scattered rays which come from the part radiographed, so the larger 

 image portion is formed hj primary raj^s from the tube focal spot. 



The grid can be flat or curved, but, to eliminate direct shadows of the lead strips, it 

 must be attached to some mechanism for moving it between object and film while an 

 exposure is being made; such a device is generallj^ incorporated as part of the 

 diaphragm. 



For correct use of the Bucky diaphragm, the anode-film distance must be at least 

 25 in. or more, dependent upon its design, and it must be placed so the primary X-ray 

 beam will pass directly through the spaces between the lead strips. Longitudinal 

 shifts are permissible, but lateral shifts should not be extended more than 1 or 2 in. 

 from the grid center. 



Medical Radiography. — It is almost impossible to condense into table form all the 

 variable factors which affect the taking of a clinical radiograph. Added to the large 

 number of X-ray tubes with different ratings, there is the variation in calibration of 

 machines and difference in size of patients, as well as the preference of the attending 

 physician in matters of film density, contrast, etc. Nor is it practical for a technician 

 to learn the correct position for the patient in various pictures except through actual 

 observation. As an example of the variation in X-ray tube ratings and their effect 

 on technique, an instance may be cited. The General Electric X-ray Co. recommends 

 the following factors for radiography of the kidney with two of their tubes: With one 

 tube the exposure is given as 100 ma. at 50 kilovolts for 2 sec. at a distance of 36 in., 

 while with another the technique is 60 ma. at 56 kilovolts for 2 sec. at 30 in. Also 



