How does Be window thickness influence beam filtration and photon yield?

• A. Thicker Be window adds inherent filtration, reducing low-energy photons and beam intensity; excessive thickness lowers photon yield.
• B. Thicker Be window increases photon yield.
• C. Be window thickness has no effect on filtration.
• D. Thicker Be window reduces tube voltage.

Answer: (A)

Be window thickness controls inherent filtration in an X-ray tube. The window is the barrier that photons must pass through to exit the tube, and its material (beryllium) absorbs photons, especially the low-energy ones. When the window is thicker, more of these low-energy photons are filtered out before they can emerge, which both reduces the number of photons that reach the patient side (the beam intensity) and shifts the remaining spectrum toward higher energies. This is why the photon yield decreases as thickness increases—the window absorbs part of the beam. The tube voltage setting isn’t altered by the window thickness; filtration changes the beam’s quality and quantity, not the applied voltage.