Which statement correctly describes the left lateral decubitus abdominal radiograph?

• A. It is used to visualize the colon only.
• B. It is used to detect free intraperitoneal air under the right hemidiaphragm.
• C. It is used to evaluate diaphragmatic motion.
• D. It is used to confirm fluid levels in the pelvis.

Answer: (B)

A left lateral decubitus abdominal radiograph is done with the patient lying on the left side and the beam horizontal to look for free air in the abdomen. The key idea is that gas will rise to the highest point of the cavity, so when the patient is on the left side, any free intraperitoneal air collects under the right hemidiaphragm. That makes small amounts of air visible beneath the right diaphragm, which is exactly how this view helps detect perforation or free air.

This view isn’t about visualizing the colon specifically, nor is it the best choice for assessing diaphragmatic motion (that’s typically done with fluoroscopy or other dynamic studies). While decubitus views can show fluid levels, using the left lateral decubitus to confirm pelvic fluid levels isn’t the primary purpose of this projection.