Which of the following describes the primary benefit of collimating the exposure field size to only the required anatomy?

• A. A decrease in the patient's effective dose
• B. A decrease in the patient's skin dose
• C. An increase in spatial resolution
• D. An increase in receptor exposure

Answer: (A)

Limiting the exposure field to only the anatomy of interest directly reduces the amount of tissue that's irradiated. Because the effective dose is a risk-weighted measure of radiation absorbed by all tissues, irradiating fewer tissues lowers the overall radiation risk to the patient. So the primary benefit is a decrease in the patient’s effective dose.

Note that a smaller field also lowers the skin entrance dose and reduces scatter, which can improve image quality, but the central point is the broader reduction in radiation risk captured by effective dose. An increase in spatial resolution isn’t driven by collimation, and keeping the same technique with a smaller field typically reduces, not increases, receptor exposure.