What is the recommended SID for the parietoacanthial projection (open-mouth Waters method) of the sinuses, performed with a horizontal beam?

• A. 40 inches
• B. 50 inches
• C. 30 inches
• D. 20 inches

Answer: (A)

For this sinus view, the distance from the X-ray source to the image receptor is chosen to balance sharpness and dose. A 40-inch SID is standard for parietoacanthial open-mouth Waters images of the sinuses, especially when using a horizontal beam. This distance minimizes magnification of the sinus structures while still presenting an adequate field for the sphenoid and maxillary sinuses and keeping the exposure practical for the patient. Using a shorter distance would increase magnification and degrade detail; a longer distance wouldn’t significantly improve quality but would increase exposure geometry issues and patient dose. Therefore, 40 inches is the best choice.