Imaging physics: photon interactions: 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 physics: photon interactions

Sample item

Which statement about the photoelectric effect in diagnostic radiology is correct?

AIt is the main cause of scatter fogging the imageBIt occurs only at megavoltage energiesCIts probability is independent of atomic numberDIts probability varies roughly with the cube of the atomic number and gives good contrast between bone and soft tissue

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

The answer and the reasoning

D. Its probability varies roughly with the cube of the atomic number and gives good contrast between bone and soft tissue

Photoelectric absorption is proportional to about Z cubed and inversely to the cube of energy. This is why bone and iodine or barium absorb more than soft tissue and give contrast. It also deposits the full photon energy and so contributes to patient dose.

Option by option

A. It is the main cause of scatter fogging the image
Scatter fog arises mainly from Compton scatter.
B. It occurs only at megavoltage energies
The photoelectric effect predominates at low diagnostic energies, not megavoltage.
C. Its probability is independent of atomic number
Compton scatter is the interaction that is nearly independent of Z, not the photoelectric effect.
D. Its probability varies roughly with the cube of the atomic number and gives good contrast between bone and soft tissue (correct)
Correct. The Z cubed dependence produces differential absorption and image contrast.

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, Its probability varies roughly with the cube of the atomic number and gives good contrast between bone and soft tissue. Photoelectric absorption is proportional to about Z cubed and inversely to the cube of energy.

Why is "It is the main cause of scatter fogging the image" not the answer?

Scatter fog arises mainly from Compton scatter.

Why is "It occurs only at megavoltage energies" not the answer?

The photoelectric effect predominates at low diagnostic energies, not megavoltage.

Why is "Its probability is independent of atomic number" not the answer?

Compton scatter is the interaction that is nearly independent of Z, not the photoelectric effect.

What concept does this question test?

It tests Imaging physics: photon interactions in Radiology. This is why bone and iodine or barium absorb more than soft tissue and give contrast. It also deposits the full photon energy and so contributes to patient dose.

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 Imaging physics: photon interactions?

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.