Which statement about the focal trough width is true?

• A. The focal trough width is fixed for all patients.
• B. The focal trough width changes with patient age only.
• C. The focal trough width is determined solely by the receptor size.
• D. The focal trough width varies with the rotational arc and center of rotation path.

Answer: (D)

In panoramic imaging, the focal trough is the curved zone where anatomy is imaged with minimal distortion. Its width isn’t fixed for every patient; it’s defined by the imaging geometry—the rotational arc and the center of rotation path. If you change the arc the machine uses to rotate around the patient, or alter the center of rotation path, the region that falls inside the sharp zone changes size. That’s why the trough width can vary with how the machine moves around the patient, not with the patient’s age or with receptor size alone. The receptor sets how much area you can capture, but it doesn’t determine how wide the sharp zone is.