. Manual of operative surgery. Shift method with Mechanical Triangulation (U. S. Army). This methodwas first advanced by MacKenzie Da\-idson. In its simplest form, it is carried FLUOROSCOPIC LOCALIZATION 1273 out as follows. With the tube underneath the table and a fluoroscopic screenfirmly supported above the patient in a horizontal position and at a knowndistance from the tube, find the shadow of the foreign body. Close the dia-phragm (which must be centered with the target of the tube) to the smallestaperture that will show the shadow of the foreign body. Mark on the screenth
. Manual of operative surgery. Shift method with Mechanical Triangulation (U. S. Army). This methodwas first advanced by MacKenzie Da\-idson. In its simplest form, it is carried FLUOROSCOPIC LOCALIZATION 1273 out as follows. With the tube underneath the table and a fluoroscopic screenfirmly supported above the patient in a horizontal position and at a knowndistance from the tube, find the shadow of the foreign body. Close the dia-phragm (which must be centered with the target of the tube) to the smallestaperture that will show the shadow of the foreign body. Mark on the screenthe position of some definite point on the contour of the latter. For the sake ofaccuracy, this marking is best done by means of sharp-pointed metal indicatorswhich may be fastened to the frame of the screen. Move the tube in the horizontal plane a known distance and mark the newposition of the chosen point of the shadow contour. Make a chart as shownbelow (Fig. 1624) indicating graphically the two positions of the anti-cathode,. Fig. 1624.—Determination of the depth of a foreign body by radioscopic triangulation. Pb is the distance from level of tube to level of screen (or plate, if radiographic method is used). Prepresents first position of anticathode and b represents first foreign-body shadow. P represents secondposition of anticathode and b the foreign-body shadow produced in this position. If then the lines Phand Pb are drawn, their intersection at the point b must indicate the locus of the foreign body. Thelength of the line bh therefore indicates the actual distance of the foreign body from the screen or plate. the plane of the screen and the two positions of the shadow—all in theiractual measurements. Indicate by two straight lines the axial ray from each position of the anti-cathode to the corresponding shadow. The intersection of these two lines marksthe site of the selected point on the foreign body. The actual distance fromthe screen to this point is then determi
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