Define a radioactive tracer .

A radioactive tracer is a substance containing radioactive nuclei that is introduced to the body and can be absorbed by tissue, in order to study the structure and function of organs.

Give three properties that make isotopes such as technetium-99m and fluorine-18 suitable for use as radioactive tracers.

They bind to organic molecules commonly found in the body (such as glucose or water) They emit gamma radiation They decay into a stable isotope

Fluorine-18 has an even shorter half-life than technetium-99m , so the patient is exposed to radiation for a .......... time.

Fluorine-18 has an even shorter half-life than technetium-99m , so the patient is exposed to radiation for a shorter time.

True or False? Radioactive tracers used in PET scanning are chosen to have a long half-life, to allow plenty of time for the scan.

False. Tracers are chosen to have a short half-life , so the patient is exposed to radiation for as short a time as possible.

Define positron emission tomography (PET) .

PET is a type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues.

What is fluorodeoxyglucose, and how is it used in PET scanning?

Fluorodeoxyglucose is a glucose molecule with radioactive fluorine attached. It is injected or swallowed into the patient; tissues and organs that absorb it appear as bright areas on the scan, allowing diagnosis.

What type of radioactive decay does fluorine-18 undergo, and what particle is emitted?

Fluorine-18 undergoes β + decay , emitting a positron .

State examples of conditions that can be diagnosed using tracers in PET scanning.

Cancer Heart conditions and areas of decreased blood flow Brain injuries , including Alzheimer's and dementia

Define annihilation .

Annihilation occurs when a particle meets its equivalent antiparticle: both are destroyed, and their mass is converted into energy.

Describe what happens when a positron and an electron annihilate, from positron emission to the production of gamma rays.

The positron travels less than a millimetre before colliding with an electron The positron and electron annihilate , converting their mass into energy Two gamma-ray photons are produced The photons move apart in opposite directions

PET tracers have a .......... half-life, so they begin emitting positrons almost immediately and expose the patient to radiation for a short time.

PET tracers have a short half-life, so they begin emitting positrons almost immediately and expose the patient to radiation for a short time.

State one advantage and one disadvantage of a PET tracer having a short half-life.

Advantage: allows for a short exposure time to radiation Disadvantage: the patient must be scanned quickly, and not all hospitals have access to expensive PET scanners

True or False? Annihilation can only occur between an electron and a positron.

False. Annihilation can occur between any particle and its antiparticle counterpart — electron-positron annihilation is simply the example relevant to PET scanning.

Define the line of response .

The line of response is the straight line joining the two detectors that register a pair of gamma-ray photons from a single annihilation event.

Describe the role of the crystal scintillator in a PET detector.

An incident gamma photon excites an electron in the crystal to a higher energy state As the excited electron travels through the crystal, it excites further electrons When the excited electrons fall back to their original state, the energy released is emitted as visible light photons

Describe the role of the photomultiplier in a PET detector.

It amplifies the faint light photons produced by the scintillator and converts them into an electrical signal .

The more gamma-ray photons detected at a particular point, the more .......... is present in that tissue, appearing as a bright point on the image.

The more gamma-ray photons detected at a particular point, the more tracer is present in that tissue, appearing as a bright point on the image.

True or False? The two gamma-ray photons produced by annihilation travel off in random directions.

False. Due to conservation of momentum , the two photons travel in straight lines in exactly opposite directions .

A LevelPhysicsCambridge (CIE)Flashcards24. Medical PhysicsPET Scanning

PET Scanning (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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FrontRadioactive Tracers
Define a radioactive tracer.
FrontRadioactive Tracers
Give three properties that make isotopes such as technetium-99m and fluorine-18 suitable for use as radioactive tracers.
BackRadioactive Tracers
They bind to organic molecules commonly found in the body (such as glucose or water)
They emit gamma radiation
They decay into a stable isotope

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Cards in this collection (21)

Define a radioactive tracer.
A radioactive tracer is a substance containing radioactive nuclei that is introduced to the body and can be absorbed by tissue, in order to study the structure and function of organs.
Give three properties that make isotopes such as technetium-99m and fluorine-18 suitable for use as radioactive tracers.
- They bind to organic molecules commonly found in the body (such as glucose or water)
- They emit gamma radiation
- They decay into a stable isotope
Fluorine-18 has an even shorter half-life than technetium-99m, so the patient is exposed to radiation for a .......... time.
Fluorine-18 has an even shorter half-life than technetium-99m, so the patient is exposed to radiation for a shorter time.
True or False?
Radioactive tracers used in PET scanning are chosen to have a long half-life, to allow plenty of time for the scan.
False.
Tracers are chosen to have a short half-life, so the patient is exposed to radiation for as short a time as possible.
Define positron emission tomography (PET).
PET is a type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues.
What is fluorodeoxyglucose, and how is it used in PET scanning?
Fluorodeoxyglucose is a glucose molecule with radioactive fluorine attached. It is injected or swallowed into the patient; tissues and organs that absorb it appear as bright areas on the scan, allowing diagnosis.
What type of radioactive decay does fluorine-18 undergo, and what particle is emitted?
Fluorine-18 undergoes β⁺ decay, emitting a positron.
State examples of conditions that can be diagnosed using tracers in PET scanning.
- Cancer
- Heart conditions and areas of decreased blood flow
- Brain injuries, including Alzheimer's and dementia
Define annihilation.
Annihilation occurs when a particle meets its equivalent antiparticle: both are destroyed, and their mass is converted into energy.
What quantities are conserved during an annihilation event?
- Mass
- Energy
- Momentum
Describe what happens when a positron and an electron annihilate, from positron emission to the production of gamma rays.
- The positron travels less than a millimetre before colliding with an electron
- The positron and electron annihilate, converting their mass into energy
- Two gamma-ray photons are produced
- The photons move apart in opposite directions
PET tracers have a .......... half-life, so they begin emitting positrons almost immediately and expose the patient to radiation for a short time.
PET tracers have a short half-life, so they begin emitting positrons almost immediately and expose the patient to radiation for a short time.
State one advantage and one disadvantage of a PET tracer having a short half-life.
- Advantage: allows for a short exposure time to radiation
- Disadvantage: the patient must be scanned quickly, and not all hospitals have access to expensive PET scanners
True or False?
Annihilation can only occur between an electron and a positron.
False.
Annihilation can occur between any particle and its antiparticle counterpart — electron-positron annihilation is simply the example relevant to PET scanning.
Define the line of response.
The line of response is the straight line joining the two detectors that register a pair of gamma-ray photons from a single annihilation event.
State the equation used to calculate the energy of a gamma-ray photon produced by annihilation, and give the meaning of each symbol.
$E = h f = m_{e} c^{2}$
- h = Planck's constant
- f = frequency of the photon
- m_e = mass of the electron or positron
- c = speed of light in a vacuum
State the equation for the momentum of a gamma-ray photon.
$p = \frac{E}{c}$
where E = photon energy and c = speed of light in a vacuum
Describe the role of the crystal scintillator in a PET detector.
- An incident gamma photon excites an electron in the crystal to a higher energy state
- As the excited electron travels through the crystal, it excites further electrons
- When the excited electrons fall back to their original state, the energy released is emitted as visible light photons
Describe the role of the photomultiplier in a PET detector.
It amplifies the faint light photons produced by the scintillator and converts them into an electrical signal.
The more gamma-ray photons detected at a particular point, the more .......... is present in that tissue, appearing as a bright point on the image.
The more gamma-ray photons detected at a particular point, the more tracer is present in that tissue, appearing as a bright point on the image.
True or False?
The two gamma-ray photons produced by annihilation travel off in random directions.
False.
Due to conservation of momentum, the two photons travel in straight lines in exactly opposite directions.

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