A lateral lumbar spine image demonstrates adequate receptor exposure and acceptable contrast using a fixed technique. How will these image factors change if the same technique is used to acquire a collimated view of the L5-S1 articulation?

• A. decreased receptor exposure and increased contrast
• B. increased receptor exposure and increased contrast
• C. increased receptor exposure and decreased contrast
• D. decreased receptor exposure and decreased contrast

Answer: (A)

Collimating narrows the beam to the anatomy of interest, so even with the same technique (the same mA, time, and kVp), less area is irradiated and a smaller portion of the beam reaches the image receptor. That means the total exposure the receptor receives drops.

At the same time, a smaller field means less tissue is irradiated and less scatter is produced. Scatter is a major factor that degrades image contrast, so reducing it improves contrast. The result is lower receptor exposure on the image but better (higher) contrast for the focused L5–S1 view.