What happens when an object is in front of the focal trough?

• A. Appears shortened
• B. Anterior teeth appear elongated
• C. No distortion
• D. Image appears doubled

Answer: (B)

In panoramic imaging, the focal trough is the curved zone where structures are in sharp focus. Objects outside that zone are distorted due to the geometry of the rotating x-ray beam and the fixed detector.

If something sits in front of the focal trough (toward the lips), it is effectively closer to the x-ray source than the focal plane. This position causes magnification of the image along the vertical dimension, so the anterior teeth appear longer than they actually are. That’s why anterior teeth on the image look elongated. If the object were behind the trough, it would tend to be foreshortened, which is the opposite distortion.