Radiographic Parallax — ORE Part 1 MCQ

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HardRadiology & ImagingRadiographic ParallaxORE Part 1

Two periapical radiographs of an upper premolar are taken using horizontal parallax. For the second radiograph, the X-ray tube is shifted mesially. Relative to the palatal root image, one root image moves mesially. Which description correctly identifies that root?

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Reveal the answer and explanation

Correct answer: B — It is the palatal root

Explanation lettering: C = shown as A · A = shown as B · D = shown as C · B = shown as D

A is correct. With horizontal parallax, the image of a lingual/palatal object moves in the same direction as the X-ray tube, while the image of a buccal object moves in the opposite direction (the SLOB rule: same lingual, opposite buccal). Because the tube shift and the root-image shift are both mesial, the root is palatal. B reverses the buccal-object rule. C is incorrect because the receptor is placed palatally in this maxillary periapical projection, so the palatal root is nearer the receptor but is identified here by same-direction movement, not proximity alone. D concerns projection magnification and is not a reliable localization criterion. E is wrong because the stated tube shift and relative movement are sufficient for localization.

Reference: Faculty of Dental Surgery, Royal College of Surgeons of England. Guidelines for periradicular surgery, Imaging—Periapical radiography (2020): https://www.rcseng.ac.uk/-/media/Files/RCS/FDS/Publications/Periradicular_Surgery_Guidelines_2020.pdf