Imaging modalities and physics: pulse height analyser: 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: pulse height analyser

Sample item

A gamma camera accepts only counts within a narrow energy window around 140 keV for Tc-99m. What is the purpose of this window?

ATo shorten the acquisition timeBTo correct for decay of the radionuclideCTo reject scattered photons that have lost energyDTo increase the sensitivity of the collimator

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

The answer and the reasoning

C. To reject scattered photons that have lost energy

Photons that scatter in the patient lose energy and give wrong position information. The pulse-height analyser accepts only events near the photopeak, which improves contrast and resolution at the cost of a few counts.

Option by option

A. To shorten the acquisition time
A narrower window accepts fewer counts and does not shorten acquisition.
B. To correct for decay of the radionuclide
Decay correction is done in software using the half-life and does not need an energy window.
C. To reject scattered photons that have lost energy (correct)
Correct. Scattered photons have lower energy and are discarded by the window.
D. To increase the sensitivity of the collimator
Collimator sensitivity depends on hole size and length, not on the energy window.

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 C, To reject scattered photons that have lost energy. Photons that scatter in the patient lose energy and give wrong position information.

Why is "To shorten the acquisition time" not the answer?

A narrower window accepts fewer counts and does not shorten acquisition.

Why is "To correct for decay of the radionuclide" not the answer?

Decay correction is done in software using the half-life and does not need an energy window.

Why is "To increase the sensitivity of the collimator" not the answer?

Collimator sensitivity depends on hole size and length, not on the energy window.

What concept does this question test?

It tests Imaging modalities and physics: pulse height analyser in Radiology. The pulse-height analyser accepts only events near the photopeak, which improves contrast and resolution at the cost of a few counts.

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 modalities and physics: pulse height analyser?

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.