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Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

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FIGURE 6-30. Air gap geometry canreduce the amount of scatter radiationreaching the detector system. When thedistance between the patient and thedetector is increi3sed (8), fewer x-rayphotons reach the detector system comparedwith the positioning in (A), owingto simple geometric considerations.tional object magnification and the reduced field of view that is imaged by a detectorof fixed dimensions. <strong>The</strong> additional object magnification often causes a loss ofspatial resolution, unless a very small focal spot is used. In general, air gaps have notenjoyed much use in general radiography <strong>for</strong> scatter cleanup, outside of chest radiography.In chest radiography, the larger SID allows a small air gap to be used (e.g.,20 cm) without imposing too much magnification. Grids are still routinely used inall of radiography other than extremity studies, some pediatric studies, and magnificationmammography views.Haus AG. <strong>The</strong> AAPM/RSNA physics tutorial <strong>for</strong> residents: Measures of screen-film per<strong>for</strong>mance.Radiographies 1996;16:1165-1181.

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