How does rectangular collimation affect patient dose and image quality compared with round collimation?

• A. Round collimation reduces exposure more.
• B. Rectangular collimation reduces patient exposure significantly (roughly 60–70%) with minimal impact on diagnostic image quality when properly positioned.
• C. Rectangular collimation increases exposure
• D. It has no effect on dose or image quality

Answer: (B)

Rectangular collimation focuses the x-ray beam to the size of the image receptor, so only the tissue within that rectangle is exposed. A round beam that covers the same receptor area irradiates more tissue, leading to a larger irradiated volume and more scatter. By switching to a rectangular field, the patient’s exposure is significantly reduced—roughly 60–70% when the field is properly positioned. At the same time, image quality stays diagnostic because the entire area of interest is captured and the reduction in scatter can even improve contrast. The caveat is proper alignment: if the field is misaligned, you can clip anatomy or need retakes, which would increase exposure again.