Imaging modalities and physics: CT noise and dose: 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 noise and dose

Sample item

A radiologist wants to halve the quantum noise in a CT image while keeping all other settings the same. By roughly what factor must the dose (mAs) be raised?

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

The answer and the reasoning

D. Four times

Quantum noise is inversely proportional to the square root of the number of detected photons. To halve noise, the photon count and hence the dose must be increased fourfold, which is why dose reduction costs image quality.

Option by option

  • A. Eight times

    Eight times the dose would cut noise to about a third, which is more than needed.

  • B. Two times

    Doubling the dose reduces the noise by only about 30 percent.

  • C. One and a half times

    A 50 percent increase in dose reduces noise by roughly 18 percent only.

  • D. Four times (correct)

    Correct. Noise falls with the square root of dose, so halving it needs four times the dose.

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.

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Questions about this Radiology question

What is the answer to this Radiology question?

The best answer is D, Four times. Quantum noise is inversely proportional to the square root of the number of detected photons.

Why is "Eight times" not the answer?

Eight times the dose would cut noise to about a third, which is more than needed.

Why is "Two times" not the answer?

Doubling the dose reduces the noise by only about 30 percent.

Why is "One and a half times" not the answer?

A 50 percent increase in dose reduces noise by roughly 18 percent only.

What concept does this question test?

It tests Imaging modalities and physics: CT noise and dose in Radiology. To halve noise, the photon count and hence the dose must be increased fourfold, which is why dose reduction costs image quality.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 noise and dose?

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.