What is the collimator composed of?

• A. Aluminum
• B. Copper
• C. Lead or other high-atomic-number materials
• D. Glass

Answer: (C)

Collimators shape and limit the X-ray beam by absorbing photons outside the intended field. To do this effectively, the absorbing material must be very dense and have a high atomic number, so it attenuates X-rays strongly with a relatively thin layer. Lead fits this need perfectly, and other high-atomic-number materials serve the same purpose. That’s why the parts of the collimator that block off-axis radiation are made from lead or similar high-Z alloys. The outer housing may be aluminum for lightweight structure, but the shielding elements themselves rely on lead or comparable materials to define the beam. Materials like aluminum, copper, or glass don’t provide sufficient attenuation or practicality for this function.