Rectangular Collimation Dose — ORE Part 1 MCQ
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Correct answer: A — Approximately 50–60% dose reduction
Explanation lettering: E = shown as B · D = shown as C · B = shown as D · C = shown as E
Rectangular collimation reduces the radiation dose for periapical radiographs by approximately 50–60% compared to round collimation. This is supported by multiple peer-reviewed studies and a 2019 systematic review, which found dose reductions ranging from 40–92% depending on measurement method and receptor type. The 50–60% figure reflects both absorbed dose to the target region and effective dose in clinical practice. Option D (90%) incorrectly conflates off-target organ dose reduction with primary beam dose. Options B, C and E significantly underestimate the benefit. Rectangular collimation is a fundamental ALARA (as low as reasonably achievable) strategy endorsed by radiation protection bodies worldwide and reduces both scatter and primary beam exposure outside the diagnostic area.
Reference: Shetty A, et al. Evidence on radiation dose reduction using rectangular collimation: a systematic review. Int Dent J. 2019;69(2):84–97. https://pubmed.ncbi.nlm.nih.gov/29959778/