Imaging modalities and physics: PET detector coincidence: 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 modalities and physics: PET detector coincidence

Sample item

In a PET scanner, a single 511 keV photon detected on one side of the ring without a partner on the opposite side within the timing window is discarded. Why?

AA single photon indicates a detector faultBOnly coincident detection locates the annihilation along a line of responseCThey are always scattered and unusableDSingle photons carry too little energy

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

The answer and the reasoning

B. Only coincident detection locates the annihilation along a line of response

Positron annihilation produces two 511 keV photons travelling in opposite directions. Detecting both within a few nanoseconds defines the line along which the event happened, and this removes the need for a collimator.

Option by option

A. A single photon indicates a detector fault
A lone photon is common because the other one is absorbed or scattered, and it does not signal a fault.
B. Only coincident detection locates the annihilation along a line of response (correct)
Correct. Coincidence detection defines the line of response and replaces physical collimation.
C. They are always scattered and unusable
Not every single event is scattered, but without its pair it cannot be localised.
D. Single photons carry too little energy
A 511 keV photon has enough energy, and the problem is the missing partner.

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 B, Only coincident detection locates the annihilation along a line of response. Positron annihilation produces two 511 keV photons travelling in opposite directions.

Why is "A single photon indicates a detector fault" not the answer?

A lone photon is common because the other one is absorbed or scattered, and it does not signal a fault.

Why is "They are always scattered and unusable" not the answer?

Not every single event is scattered, but without its pair it cannot be localised.

Why is "Single photons carry too little energy" not the answer?

A 511 keV photon has enough energy, and the problem is the missing partner.

What concept does this question test?

It tests Imaging modalities and physics: PET detector coincidence in Radiology. Detecting both within a few nanoseconds defines the line along which the event happened, and this removes the need for a collimator.

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 Imaging modalities and physics: PET detector coincidence?

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.