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Fig. 3 —Diagram of hypothetical 22.5-cm-patient setup. Measurements were obtained at isocenter, and source-to-isocenter distance was known. We used inverse-square correction factors to determine anteroposterior (0°) and posteroanterior (180°) entrance exposures for a hypothetical 22.5-cm adult patient and a hypothetical 14-cm pediatric patient, to compare our results to typical adult chest radiograph entrance exposure (16 mR or 4.1 x 10^-6 C/kg).