What affects the size and width of the focal trough?

• A. The receptor size alone.
• B. The rotational arc and the path of the center of rotation.
• C. The exposure time.
• D. The patient age.

Answer: (B)

The focal trough is the curved volume in which structures are sharply imaged as the X-ray source and detector rotate around the patient. Its size and width come from the geometry of that rotation, not from dose or receptor size.

A larger rotational arc sweeps a bigger portion of the head, widening the trough, while the path the center of rotation follows shapes where that trough sits and how wide it appears. In other words, how far and how smoothly the beam and detector move around the patient directly sets the trough’s dimensions.

Receptor size mainly affects how much of the trough you can capture at once and the image resolution, but it doesn’t determine how wide the trough itself. Exposure time influences image brightness and motion blur, not the geometry of the imaging volume. Age may change anatomy, but the fundamental determinant of trough width is the machine’s rotation geometry.