Nuclear imaging: PET principle: 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 · Nuclear imaging: PET principle

Sample item

In PET imaging with F-18 FDG, which physical event is detected by the scanner?

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

The answer and the reasoning

D. Two 511 keV photons emitted in opposite directions after positron annihilation

A positron emitted by F-18 travels a short distance, meets an electron and annihilates into two 511 keV photons at about 180 degrees. Coincidence detection by a ring of detectors localises the event. A 140 keV photon is the emission of Tc-99m used in SPECT.

Option by option

  • A. Radiofrequency echoes from protons

    Radiofrequency echoes are the MRI signal.

  • B. A single 140 keV gamma photon

    140 keV photons are characteristic of Tc-99m gamma imaging.

  • C. Characteristic X-rays from tungsten

    Tungsten X-rays are used in diagnostic radiography.

  • D. Two 511 keV photons emitted in opposite directions after positron annihilation (correct)

    Correct. Coincident 511 keV photon pairs form the PET signal.

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.

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Questions about this Radiology question

What is the answer to this Radiology question?

The best answer is D, Two 511 keV photons emitted in opposite directions after positron annihilation. A positron emitted by F-18 travels a short distance, meets an electron and annihilates into two 511 keV photons at about 180 degrees.

Why is "Radiofrequency echoes from protons" not the answer?

Radiofrequency echoes are the MRI signal.

Why is "A single 140 keV gamma photon" not the answer?

140 keV photons are characteristic of Tc-99m gamma imaging.

Why is "Characteristic X-rays from tungsten" not the answer?

Tungsten X-rays are used in diagnostic radiography.

What concept does this question test?

It tests Nuclear imaging: PET principle in Radiology. Coincidence detection by a ring of detectors localises the event. A 140 keV photon is the emission of Tc-99m used in SPECT.

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 Nuclear imaging: PET principle?

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.