The diameter of the positioning indicating device (PID) at the beam exit should be no more than how many inches?

• A. 2 inches
• B. 2.75 inches
• C. 3 inches
• D. 4 inches

Answer: (B)

Controlling the size of the x-ray beam as it leaves the tube is all about beam collimation. The diameter of the PID at the beam exit determines how wide the beam can spread toward the tooth. Keeping that exit diameter to a maximum of 2.75 inches ensures the field stays compact and matches the typical receptor size used for common dental images, which helps minimize exposure to surrounding tissues and reduces scatter. If the diameter were larger, the beam would diverge more, increasing the irradiated area and potentially degrading image quality due to greater penumbra. A smaller diameter, like 2 inches, would still be within the limit, but the question asks for the upper bound, which is 2.75 inches. Diameters of 3 inches or 4 inches exceed the recommended maximum and would enlarge the field beyond what’s advised, increasing patient dose and reducing control over the image field.