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 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.

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.