Ultrasound physics: 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 physics

Sample item

A sonographer finds no Doppler signal from a vessel lying exactly perpendicular to the ultrasound beam, even though blood is flowing. Why?

AAliasing occurs only when the beam is perpendicular to flowBThe beam is reflected completely and cannot enter the vesselCRed cells move too slowly when flow is at right angles to the beamDThe Doppler shift depends on the cosine of the angle between beam and flow, which is zero at 90 degrees

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

The answer and the reasoning

D. The Doppler shift depends on the cosine of the angle between beam and flow, which is zero at 90 degrees

The Doppler frequency shift is proportional to the cosine of the angle of insonation. At 90 degrees the cosine is zero, so no shift is detected. Angles below about 60 degrees are preferred for accurate velocity measurement.

Option by option

A. Aliasing occurs only when the beam is perpendicular to flow
Aliasing arises when the velocity exceeds what the pulse repetition frequency can measure, whatever the angle.
B. The beam is reflected completely and cannot enter the vessel
The beam penetrates the vessel normally, and the problem is the lack of a frequency shift.
C. Red cells move too slowly when flow is at right angles to the beam
Red cell speed does not depend on the angle of the probe.
D. The Doppler shift depends on the cosine of the angle between beam and flow, which is zero at 90 degrees (correct)
Correct. At a right angle, none of the flow velocity is directed along the beam.

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 D, The Doppler shift depends on the cosine of the angle between beam and flow, which is zero at 90 degrees. The Doppler frequency shift is proportional to the cosine of the angle of insonation.

Why is "Aliasing occurs only when the beam is perpendicular to flow" not the answer?

Aliasing arises when the velocity exceeds what the pulse repetition frequency can measure, whatever the angle.

Why is "The beam is reflected completely and cannot enter the vessel" not the answer?

The beam penetrates the vessel normally, and the problem is the lack of a frequency shift.

Why is "Red cells move too slowly when flow is at right angles to the beam" not the answer?

Red cell speed does not depend on the angle of the probe.

What concept does this question test?

It tests Ultrasound physics in Radiology. At 90 degrees the cosine is zero, so no shift is detected. Angles below about 60 degrees are preferred for accurate velocity measurement.

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 Ultrasound physics?

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.