Imaging modalities and physics: indirect versus direct flat panel: 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: indirect versus direct flat panel

Sample item

A flat-panel digital radiography detector converts X-rays first into light with a caesium iodide layer, and then into charge using a photodiode array. How is this type classified?

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

The answer and the reasoning

C. Indirect conversion detector

Indirect systems use a scintillator that turns X-rays into light, which a photodiode and thin-film transistor array converts to charge. Direct systems use amorphous selenium to convert X-rays straight to charge, which gives less light spread and a sharper image.

Option by option

  • A. Storage phosphor detector

    Storage phosphor plates in computed radiography store energy and are read by a laser outside the cassette.

  • B. Photon counting detector

    Photon counting detectors register each photon and its energy individually.

  • C. Indirect conversion detector (correct)

    Correct. A scintillator and photodiode in sequence define indirect conversion.

  • D. Direct conversion detector

    Direct conversion uses amorphous selenium, with no intermediate light step.

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 C, Indirect conversion detector. Indirect systems use a scintillator that turns X-rays into light, which a photodiode and thin-film transistor array converts to charge.

Why is "Storage phosphor detector" not the answer?

Storage phosphor plates in computed radiography store energy and are read by a laser outside the cassette.

Why is "Photon counting detector" not the answer?

Photon counting detectors register each photon and its energy individually.

Why is "Direct conversion detector" not the answer?

Direct conversion uses amorphous selenium, with no intermediate light step.

What concept does this question test?

It tests Imaging modalities and physics: indirect versus direct flat panel in Radiology. Direct systems use amorphous selenium to convert X-rays straight to charge, which gives less light spread and a sharper image.

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: indirect versus direct flat panel?

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.