Radiation safety and radiobiology: half value layer calculation: 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 · Radiation safety and radiobiology: half value layer calculation

Sample item

The half value layer of lead for a diagnostic beam is 0.5 mm. What fraction of the beam is transmitted through 1.5 mm of lead?

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

The answer and the reasoning

D. One eighth

1.5 mm is three half value layers. Each layer halves the intensity, so transmission is 1/2 x 1/2 x 1/2, which is one eighth (12.5 percent).

Option by option

  • A. One sixth

    One sixth does not follow from repeated halving.

  • B. One third

    One third does not follow from repeated halving.

  • C. One quarter

    One quarter would be the result of two HVLs, or 1.0 mm.

  • D. One eighth (correct)

    Correct. Three HVLs reduce the beam to one eighth.

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, One eighth. 1.

Why is "One sixth" not the answer?

One sixth does not follow from repeated halving.

Why is "One third" not the answer?

One third does not follow from repeated halving.

Why is "One quarter" not the answer?

One quarter would be the result of two HVLs, or 1.0 mm.

What concept does this question test?

It tests Radiation safety and radiobiology: half value layer calculation in Radiology. 5 mm is three half value layers. Each layer halves the intensity, so transmission is 1/2 x 1/2 x 1/2, which is one eighth (12.5 percent).

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 Radiation safety and radiobiology: half value layer calculation?

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.