Imaging modalities and physics: CT ring artefact: practice question

A Radiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Radiology · Imaging modalities and physics: CT ring artefact

Sample item

On axial CT slices, a concentric circular band of altered density centred on the rotation axis appears on every image of both a patient and a water phantom. Which fault is the most likely cause?

AA miscalibrated or malfunctioning detector elementBPatient movement during the scanCTruncation of the scan field by a large patientDA dense metallic implant in the field of view

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. A miscalibrated or malfunctioning detector element

In a third-generation CT scanner each detector element traces a circle during rotation, so a single faulty or miscalibrated element produces a ring centred on the isocentre. The artefact is reproducible and is corrected by recalibration or detector service. Motion, metal and truncation produce blur, streaks and edge distortion instead.

Option by option

A. A miscalibrated or malfunctioning detector element (correct)
Correct. One bad detector channel is back-projected as a full ring.
B. Patient movement during the scan
Motion causes blurring, double contours or streaks and does not give a perfect ring.
C. Truncation of the scan field by a large patient
Truncation causes bright edge artefacts at the periphery, not a central ring.
D. A dense metallic implant in the field of view
Metal causes dark and bright streaks radiating from the implant.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Radiology question

What is the answer to this Radiology question?

The best answer is A, A miscalibrated or malfunctioning detector element. In a third-generation CT scanner each detector element traces a circle during rotation, so a single faulty or miscalibrated element produces a ring centred on the isocentre.

Why is "Patient movement during the scan" not the answer?

Motion causes blurring, double contours or streaks and does not give a perfect ring.

Why is "Truncation of the scan field by a large patient" not the answer?

Truncation causes bright edge artefacts at the periphery, not a central ring.

Why is "A dense metallic implant in the field of view" not the answer?

Metal causes dark and bright streaks radiating from the implant.

What concept does this question test?

It tests Imaging modalities and physics: CT ring artefact in Radiology. The artefact is reproducible and is corrected by recalibration or detector service. Motion, metal and truncation produce blur, streaks and edge distortion instead.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Imaging modalities and physics: CT ring artefact?

Learn signs with the image, and the disease they point to. Then use the flashcards and the other Radiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.