Contrast agents: effect of tube voltage on iodine contrast: 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.

Try it

Radiology · Contrast agents: effect of tube voltage on iodine contrast

Sample item

A radiologist lowers the tube voltage for a CT angiogram in a slim patient and finds that vessels enhance more brightly at the same iodine dose. What physical principle explains this?

ALower photon energy lies closer to the K-edge of iodine, so photoelectric absorption by iodine risesBLower kV reduces beam hardening in the vesselCIodine emits fluorescence that adds signalDIodine attenuates by Compton scatter, which increases at low kV

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

The answer and the reasoning

A. Lower photon energy lies closer to the K-edge of iodine, so photoelectric absorption by iodine rises

Iodine has a K-edge near 33 keV, and photoelectric absorption jumps just above it. Lower kV shifts the mean photon energy towards the K-edge, so iodinated vessels appear brighter or less contrast is needed. The trade-off is more noise in large patients.

Option by option

A. Lower photon energy lies closer to the K-edge of iodine, so photoelectric absorption by iodine rises (correct)
Correct. Photoelectric absorption by iodine increases near its K-edge.
B. Lower kV reduces beam hardening in the vessel
Beam hardening is not the reason for brighter vessels.
C. Iodine emits fluorescence that adds signal
Fluorescence does not contribute to CT signal.
D. Iodine attenuates by Compton scatter, which increases at low kV
Compton scatter is largely independent of atomic number and falls with energy.

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, Lower photon energy lies closer to the K-edge of iodine, so photoelectric absorption by iodine rises. Iodine has a K-edge near 33 keV, and photoelectric absorption jumps just above it.

Why is "Lower kV reduces beam hardening in the vessel" not the answer?

Beam hardening is not the reason for brighter vessels.

Why is "Iodine emits fluorescence that adds signal" not the answer?

Fluorescence does not contribute to CT signal.

Why is "Iodine attenuates by Compton scatter, which increases at low kV" not the answer?

Compton scatter is largely independent of atomic number and falls with energy.

What concept does this question test?

It tests Contrast agents: effect of tube voltage on iodine contrast in Radiology. Lower kV shifts the mean photon energy towards the K-edge, so iodinated vessels appear brighter or less contrast is needed. The trade-off is more noise in large patients.

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 Contrast agents: effect of tube voltage on iodine contrast?

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.