f. Measuring the Conefield. The letter marker provides an essential piece of
identifying information. You should place the LM within the restricted conefield. If it is
placed far outside the area of exposure, it is likely not to be picked up by the ionizing
radiation (figure 4-37). (Scatter radiation may or may not pick it up). If, on the other
hand, it is placed too far in on the film, you may end up obscuring the very anatomical
structure that needs to be visualized (figure 4-38). Therefore, you must know where on
the film to place the letter marker. Proper placement of the letter marker becomes
especially critical when preparing film for a restricted conefield (CF) radiograph. When
placing the letter marker for a restricted conefield radiograph, some simple measuring
techniques will ensure that the letter marker for a restricted conefield is appropriately
placed. For a restricted six-inch conefield, position the blank film cassette. Then, turn
the collimator on. Cone down to the desired six inches. Locate the center of the
conefield (the spot where the crisscross shadow falls). Measure out three inches from
the center (figure 4-39). Place the letter marker just within the 3-inch margin.
Remember that placing it beyond the 3-inch mark will place it outside the area of
exposure. Placing R well within the 3-inch margin means you risk obscuring the body
part to be demonstrated. For an X-ray with an eight-inch restricted conefield, measure
out four inches from the center (figure 4-40) and place the letter marker just within the 4-
inch marker.
Figure 4-37. If the LM is outside the area on a restricted CF, the LM may not be
exposed the film.
Figure 4-38. Letter markers placed within area of exposure, but without obscuring
anatomical structure.
MD0961
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