Imaging modalities and physics: Mo-99 Tc-99m generator equilibrium: 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: Mo-99 Tc-99m generator equilibrium

Sample item

A nuclear medicine technologist elutes a Mo-99/Tc-99m generator each morning. Mo-99 has a half-life of 66 hours and Tc-99m 6 hours. What explains why Tc-99m activity is regenerated to a maximum roughly a day after elution?

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

The answer and the reasoning

D. Transient equilibrium

When the parent half-life is longer than the daughter's (by a factor of about ten), the daughter activity peaks and then decays with the parent's half-life. This is transient equilibrium, and it gives about 24 hours for Tc-99m ingrowth. Secular equilibrium requires a parent that is far longer lived.

Option by option

  • A. Isomeric transition of the parent

    Mo-99 decays by beta emission to Tc-99m, and the isomeric transition is the later gamma emission of Tc-99m.

  • B. No equilibrium

    No equilibrium applies only when the daughter is longer lived than the parent.

  • C. Secular equilibrium

    Secular equilibrium needs a parent with a half-life vastly longer than the daughter's, such as Sr-82 and Rb-82.

  • D. Transient equilibrium (correct)

    Correct. The parent is moderately longer lived, so the daughter peaks and then follows the parent's decay.

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, Transient equilibrium. When the parent half-life is longer than the daughter's (by a factor of about ten), the daughter activity peaks and then decays with the parent's half-life.

Why is "Isomeric transition of the parent" not the answer?

Mo-99 decays by beta emission to Tc-99m, and the isomeric transition is the later gamma emission of Tc-99m.

Why is "No equilibrium" not the answer?

No equilibrium applies only when the daughter is longer lived than the parent.

Why is "Secular equilibrium" not the answer?

Secular equilibrium needs a parent with a half-life vastly longer than the daughter's, such as Sr-82 and Rb-82.

What concept does this question test?

It tests Imaging modalities and physics: Mo-99 Tc-99m generator equilibrium in Radiology. This is transient equilibrium, and it gives about 24 hours for Tc-99m ingrowth. Secular equilibrium requires a parent that is far longer lived.

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: Mo-99 Tc-99m generator equilibrium?

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.