Distance between the x-ray tube and the patient on a mobile x-ray unit affects the need for additional filtration. Which statement is correct?

Distance between the x-ray tube and the patient on a mobile x-ray unit affects the need for additional filtration. Which statement is correct?

• A. Distance between the tube and patient has no effect on filtration needs
• B. Increased distance reduces the need for filtration
• C. Increased distance increases the need for filtration
• D. Increased distance reduces the need for additional filtration

Answer: (D)

Increasing the distance between the x-ray tube and the patient changes the beam spectrum that reaches the patient. As photons travel through air, low-energy photons are attenuated more than high-energy ones, so the beam becomes somewhat harder with greater distance. A harder beam has fewer soft photons that would otherwise require removal by filtration, so the need for additional filtration decreases. Of course, you still must ensure adequate image receptor exposure and manage overall dose, but the extra filtration is less necessary as distance increases.

Increasing the distance between the x-ray tube and the patient changes the beam spectrum that reaches the patient. As photons travel through air, low-energy photons are attenuated more than high-energy ones, so the beam becomes somewhat harder with greater distance. A harder beam has fewer soft photons that would otherwise require removal by filtration, so the need for additional filtration decreases. Of course, you still must ensure adequate image receptor exposure and manage overall dose, but the extra filtration is less necessary as distance increases.