Ultrasound physics: coupling gel: 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: coupling gel

Sample item

A trainee scans the abdomen without any gel on the probe and gets a poor image. What is the physical reason that coupling gel is needed?

AIt amplifies the ultrasound frequency before entry into tissueBIt absorbs the scattered echoes to reduce noiseCIt increases the speed of sound in the skinDIt removes the air layer, which has a huge acoustic impedance mismatch and reflects almost all the beam

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

The answer and the reasoning

D. It removes the air layer, which has a huge acoustic impedance mismatch and reflects almost all the beam

Reflection at an interface depends on the difference in acoustic impedance. Air against skin reflects nearly all of the sound, so gel is used to exclude air and let the beam enter the body.

Option by option

A. It amplifies the ultrasound frequency before entry into tissue
Frequency is set by the transducer and cannot be raised by gel.
B. It absorbs the scattered echoes to reduce noise
Gel is a coupling medium and does not absorb echoes.
C. It increases the speed of sound in the skin
Gel does not change the speed of sound in tissue in any useful way.
D. It removes the air layer, which has a huge acoustic impedance mismatch and reflects almost all the beam (correct)
Correct. Gel removes air and so reduces reflection at the probe-skin interface.

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, It removes the air layer, which has a huge acoustic impedance mismatch and reflects almost all the beam. Reflection at an interface depends on the difference in acoustic impedance.

Why is "It amplifies the ultrasound frequency before entry into tissue" not the answer?

Frequency is set by the transducer and cannot be raised by gel.

Why is "It absorbs the scattered echoes to reduce noise" not the answer?

Gel is a coupling medium and does not absorb echoes.

Why is "It increases the speed of sound in the skin" not the answer?

Gel does not change the speed of sound in tissue in any useful way.

What concept does this question test?

It tests Ultrasound physics: coupling gel in Radiology. Air against skin reflects nearly all of the sound, so gel is used to exclude air and let the beam enter the body.

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: coupling gel?

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.