Imaging techniques (A-level only) Flashcards – AQA A-level Physics (7408)

What can detection outside the body reveal about an introduced gamma-emitting tracer?

Its distribution within the body.

How can a radioactive tracer be directed toward a particular organ?

Incorporate the radioisotope into a compound with an affinity for that organ.

What half-life is desirable for a radioactive tracer?

Long enough for preparation and imaging, but short enough to limit prolonged exposure.

Why must a tracer’s gamma photons have sufficient energy?

To escape the body and be detected.

What is the approximate half-life of technetium-99m?

About 6 hours.

What is the approximate energy of gamma photons emitted by technetium-99m?

$140\,\mathrm{keV}$

How can technetium-99m be used to target different organs?

It can be attached to different compounds.

What is the approximate half-life of iodine-131?

About 8 days.

What is the approximate energy of gamma photons emitted by iodine-131?

Approximately $364\,\mathrm{keV}$.

Which organ takes up radioactive iodide?

The thyroid.

Besides its use as radioactive iodide, how can iodine-131 be used as a tracer?

It can be used to label compounds.

What additional emission from iodine-131 contributes to radiation exposure in the body?

Beta particles.

What is the approximate half-life of indium-111?

About 2.8 days.

What are the two approximate gamma-photon energies emitted by indium-111?

$171\,\mathrm{keV}$ and $245\,\mathrm{keV}$.

How can indium-111 be used to target particular tissues?

It can label compounds that target those tissues.

What does a molybdenum-technetium generator provide, and what is it used for?

$\ce{^{99m}Tc}$ for use as a radioactive tracer.

What does molybdenum-99 decay into in a molybdenum-technetium generator?

Technetium-99m ($\ce{^{99m}Tc}$).

What is the approximate half-life of molybdenum-99?

About 66 hours.

What is elution in a molybdenum-technetium generator?

Washing technetium-99m out with saline as a solution containing pertechnetate ions, while the molybdenum remains on the column.

Why can a molybdenum-technetium generator be used again after elution?

More technetium-99m accumulates as molybdenum-99 continues to decay.

What practical advantage does the longer-lived parent provide in a molybdenum-technetium generator?

The generator can be transported and stored while the shorter-lived technetium-99m is obtained locally when needed.

What does a PET scan image?

The distribution of a positron-emitting radioactive tracer in the body.

How is a PET radionuclide targeted to the tissue or activity being investigated?

It is incorporated into a compound taken up by the tissue or involved in the biological activity.

What happens to an emitted positron before annihilation in PET?

It travels a short distance and annihilates with an electron.

What photons are produced by positron–electron annihilation in PET?

Two gamma photons, each with energy approximately $511\ \text{keV}$, travelling in approximately opposite directions.

What photon pairs do PET detectors around the body record?

Pairs arriving at nearly the same time.

What does a detected photon pair indicate about an annihilation in PET?

A line along which the annihilation occurred.

How is a PET image reconstructed?

Many detected photon pairs are used to reconstruct the tracer distribution.