. Dental and oral radiography : a textbook for students and practitioners of dentistry . Fig. 37.—The patient can hold the film in position against the upper teeth by exerting slight pressure with the thumb. other apparatus before described) is moved to a positionwhere the rays coming from the tube, through the com-pression diaphragm and cylinder can be made to passthrough the desired areas and cast their shadows uponthe small him within the mouth (Fig. 37). In using this method upon the upper teeth, the great-est care must be exercised if the shadows produced arefree from distortion, for the
. Dental and oral radiography : a textbook for students and practitioners of dentistry . Fig. 37.—The patient can hold the film in position against the upper teeth by exerting slight pressure with the thumb. other apparatus before described) is moved to a positionwhere the rays coming from the tube, through the com-pression diaphragm and cylinder can be made to passthrough the desired areas and cast their shadows uponthe small him within the mouth (Fig. 37). In using this method upon the upper teeth, the great-est care must be exercised if the shadows produced arefree from distortion, for the film must be held within theupper arch against the lingual side of the teeth and thepalate, and must occupy a position which is in a different DENTAL AND ORAL RADIOGRAPHY TECH1STIC 83 plane from that occupied by the roots of the teeth. When-ever it is necessary to direct the rays upon structures thatlie at an angle with the plate or film, correct shadowsmay be obtained by adhering to the following rule: Bi- m+p-^. Fig. 38.—Correct and incorrect technic. sect the angle made by the plane of the object, and theplane of the film, and direct the rays so that they willfall perpendicular to this bisected plane/ Failure to adhere strictly to this rule is one of the 84 DENTAL AND ORAL RADIOGRAPHY most common causes of partial or complete failure inproducing true shadowgraphic representations of thedenial structures. For instance, if the rays are directedProm too low a source, the shadows will be lengthened,or if they be directed from too high a source, the shad-ows will be foreshortened, the amount of elongation orforeshortening being in direct proportion to the amountof deviation from the proper focal point. The importance of adhering strictly to this rule isgraphically shown in Fig. 38,* where an upper centralincisor and the adjacent teeth are radiographed. In theupper picture (A) the rays are passing in from too lowa source with the result that the image imposed upon thefilm is l
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