When performing a double contrast BE, air is likely to be found in which section of the bowel in the PA projection?

• A. Descending colon
• B. Ascending colon
• C. Transverse colon
• D. Rectum

Answer: (C)

In a double contrast barium enema (BE), air is utilized along with barium to enhance the visibility of the gastrointestinal tract. When obtaining a PA projection, the position of the patient plays a critical role in determining where the air will accumulate within the bowel.

In the PA view, the descending colon is located more posteriorly and the ascending colon more anteriorly. As a result, when the patient is in the supine position for the PA projection, air tends to rise and fill the more superiorly positioned segments of the colon due to its buoyancy. The transverse colon is centrally located and most often appears filled with air in this specific projection because it forms a bridge between the ascending and descending colons. This distribution allows for easier visibility of the structure during radiographic evaluation.

Understanding the dynamics of air and barium distribution in the bowel is key in radiology, as it aids in proper diagnosis and assessment of pathological conditions. This is why the transverse colon is typically where air is seen when performing a double contrast BE in the PA projection.