If upright chest radiograph is not feasible to evaluate for free air, which radiographic projection is commonly used?

• A. Upright chest radiograph
• B. AP abdomen
• C. Supine abdomen radiograph
• D. Left lateral decubitus abdomen

Answer: (D)

When you can’t do an upright chest X-ray, you still want a view that makes free air visible. A left lateral decubitus abdominal radiograph does this effectively. In this position, the patient lies on their left side and the X-ray beam is horizontal. Any free intraperitoneal air will rise to the nondependent (right) side and collect beneath the right hemidiaphragm, where it creates a distinct radiolucent crescent against the liver and abdominal wall. This makes even small amounts of free air detectable on a single image, which is why this projection is commonly used when an upright study isn’t possible. The other views are less sensitive for detecting small amounts of pneumoperitoneum or require an upright posture to reveal the air.