Which SID is commonly used to reduce heart magnification in chest radiography?

• A. 40 inches
• B. 72 inches
• C. 30 inches
• D. 20 inches

Answer: (B)

Longer source-to-image distance reduces the apparent size of the heart on a chest radiograph. When the X-ray beam travels from a farther source to the image receptor, the geometric projection causes less magnification of structures between them, so the heart appears closer to its true size and the mediastinal contours are more accurate. In chest radiography, a 72-inch SID is standard because it minimizes heart magnification while providing practical room setup and good image sharpness. Remember that increasing SID lowers the exposure reaching the receptor, so exposure factors must be adjusted (usually by increasing mAs) to keep receptor exposure adequate. Shorter distances would magnify the heart more and distort its size, whereas the longer distance helps achieve a truer representation of thoracic anatomy.