Radiation safety and radiobiology: decay calculation for Tc-99m: 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: decay calculation for Tc-99m

Sample item

A syringe holds 600 MBq of Tc-99m at 8 am. The half-life is 6 hours. What activity remains at 8 pm the same day?

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

The answer and the reasoning

A. 150 MBq

Twelve hours is two half-lives. The activity falls to half after 6 hours (300 MBq) and half again after 12 hours (150 MBq).

Option by option

  • A. 150 MBq (correct)

    Correct. Two half-lives leave one quarter of the starting activity.

  • B. 200 MBq

    200 MBq would result from dividing by three, which is not how exponential decay works.

  • C. 75 MBq

    75 MBq would need three half-lives, or 18 hours.

  • D. 300 MBq

    300 MBq is the activity after one half-life only.

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 A, 150 MBq. Twelve hours is two half-lives.

Why is "200 MBq" not the answer?

200 MBq would result from dividing by three, which is not how exponential decay works.

Why is "75 MBq" not the answer?

75 MBq would need three half-lives, or 18 hours.

Why is "300 MBq" not the answer?

300 MBq is the activity after one half-life only.

What concept does this question test?

It tests Radiation safety and radiobiology: decay calculation for Tc-99m in Radiology. The activity falls to half after 6 hours (300 MBq) and half again after 12 hours (150 MBq).

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: decay calculation for Tc-99m?

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.