How far anterior to the external auditory meatus (EAM) should the central ray be directed during a SMV projection of the sinuses, with a horizontal beam?

• A. .5 inch (1 cm)
• B. .75 inch (2 cm)
• C. 1 inch (2.5 cm)
• D. 1.5 inch (3 cm)

Answer: (B)

In the SMV projection, the central ray is kept horizontal and directed perpendicular to the infraorbitomeatal line so it passes through midline skull base structures, including the sella turcica and the sphenoid sinus. You place the entrance point about 2 cm (roughly three-quarters of an inch) anterior to the external auditory meatus. This slight forward positioning aligns the beam with the midline basilar structures and ensures the sphenoid and frontal area of the sinuses are included in the image.

If you aim too close to the EAM (too posterior), you risk missing part of the sinus area or miscentering the view; if you go too far anterior (too far in front), you can over-penetrate or shift the projection away from the midline. The chosen offset provides the correct alignment to visualize the sinuses while keeping the beam perpendicular to the IOML.