Ultrasound: colour Doppler aliasing: 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 · Ultrasound: colour Doppler aliasing

Sample item

On colour Doppler of a vessel with high-velocity flow, the colour changes abruptly from red to blue within the lumen and a mosaic appears. Which setting change is most appropriate to correct the aliasing?

ADecrease the baseline angle to 90 degreesBIncrease the pulse repetition frequency (velocity scale)CIncrease the transmit frequencyDDecrease the pulse repetition frequency

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

The answer and the reasoning

B. Increase the pulse repetition frequency (velocity scale)

Aliasing occurs when the Doppler shift exceeds half the pulse repetition frequency (Nyquist limit). Raising the PRF or scale, lowering the transmit frequency or shifting the baseline can remove it. A true mosaic from turbulence can look similar and indicates stenosis.

Option by option

A. Decrease the baseline angle to 90 degrees
A 90 degree angle would remove the Doppler signal altogether.
B. Increase the pulse repetition frequency (velocity scale) (correct)
Correct. Raising PRF raises the Nyquist limit and removes aliasing.
C. Increase the transmit frequency
A higher transmit frequency increases Doppler shift and worsens aliasing.
D. Decrease the pulse repetition frequency
Lowering PRF makes aliasing worse.

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 B, Increase the pulse repetition frequency (velocity scale). Aliasing occurs when the Doppler shift exceeds half the pulse repetition frequency (Nyquist limit).

Why is "Decrease the baseline angle to 90 degrees" not the answer?

A 90 degree angle would remove the Doppler signal altogether.

Why is "Increase the transmit frequency" not the answer?

A higher transmit frequency increases Doppler shift and worsens aliasing.

Why is "Decrease the pulse repetition frequency" not the answer?

Lowering PRF makes aliasing worse.

What concept does this question test?

It tests Ultrasound: colour Doppler aliasing in Radiology. Raising the PRF or scale, lowering the transmit frequency or shifting the baseline can remove it. A true mosaic from turbulence can look similar and indicates stenosis.

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 Ultrasound: colour Doppler aliasing?

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.