The chest PA examination is one of the single most performed studies in radiology today. It can provide a wealth of information in a single examination. As in many other areas of radiology there is a conflict between high contrast, which enables subtle structures to be visualized, and wide latitude, which allows all areas of interest in the chest to be displayed in a single image. In order to optimize the design of receptor systems it is useful to establish and understand the latitude required for thoracic imaging. We have measured the distributions of x-ray transmittance within the lungs, heart, and abdomen for a population of 868 out-patients. The measurements were made with a resolution element approximately 2 X 2 cm, at a single x-ray beam quality, and with a low-scatter slot-beam geometry. Under these conditions, the required receptor latitude for capturing each area of interest in the thorax is derived as a function of body habitus. To capture all three regions the required receptor latitude for the PA examination varies from 11:1 to 81:1 with increasing patient size. The implications of these results for thoracic image-receptor design is discussed.
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