. The principles and practice of roentgenological technique . Fig. 193.—The shadow of the sphere cast by thedivergent ray is larger than the shadow of asimilar sphere outHned by the central the target plate distance diminishesthe distortion. (Grashey.) Corollary: With a part at a distance fromthe recording surface, the distortion is mini-mized by increasing the tube object distance. Practical Afflicalioii: The tube distanceshould be such as to give a minimum distor-tion. Twenty inches (50 target-skindistance has been adopted for general pur-poses. When, because of plaste
. The principles and practice of roentgenological technique . Fig. 193.—The shadow of the sphere cast by thedivergent ray is larger than the shadow of asimilar sphere outHned by the central the target plate distance diminishesthe distortion. (Grashey.) Corollary: With a part at a distance fromthe recording surface, the distortion is mini-mized by increasing the tube object distance. Practical Afflicalioii: The tube distanceshould be such as to give a minimum distor-tion. Twenty inches (50 target-skindistance has been adopted for general pur-poses. When, because of plaster cast or ban-daging it is impossible to bring the part inclose apposition to the plate, the tube platedistance is increased. When it becomes neces-. FlG. 194.—Schematic representation of the shadowof a series of oral objects, like the lumbar ver-tebra. The shaded parts represent the shadowsof the lower surface, that nearest to the record-ing surface, hence sharper and smaller; thelighter parts, the shadows of the upper surfacesfurthest from the recording surface, hence hazyand larger. (Grashey.) sary to estimate the diameters of a structure—the distortion is reduced to a minimum by along tube-plate distance. This is usually twometers. (Teleroentgenography). Note: The intensity of the illuminationdiminishes in inverse ratio to the square ofthe target-plate distance. Corollarv \arying the target recording 144 RADIOLOGICAL AXIOMS surface distance will cause a deviation of theshadow of an object. The deviation will begreater the further away the object lies fromthe recording surface (Fig. 192). Relationship of Object to Central Ray. 3. The nearer the object lies to the centralray. the truer and sharper its shadow (). The gr
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