Why is a PA chest radiograph preferred over an AP projection in adult patients?

• A. It reduces radiation dose
• B. It makes the heart appear larger
• C. It minimizes heart magnification and yields a more accurate cardiothoracic ratio
• D. It is faster to perform

Answer: (C)

The important idea is how projection affects the apparent size of the heart. The X-ray beam diverges, so the distance between the heart and the image receptor changes how large the heart looks on film. In AP chest radiographs, the heart is farther from the receptor and closer to the X-ray source, which magnifies the cardiac silhouette and can falsely increase the cardiothoracic ratio. In PA chest radiographs, the heart is closer to the detector and farther from the source, reducing this magnification and giving a more accurate representation of heart size. That’s why PA is preferred in adults when feasible—to avoid overestimating heart size and to obtain a true cardiothoracic ratio. If AP must be used, be aware that heart size may appear larger due to magnification.