PET Scanning | CAIE 9702 Paper 4 Notes

Paper 4

Physics9702

The tracer marks where a process is active

Definition

1 mark

What is meant by a tracer?

Model answer: A material introduced into the body whose position can be detected, or which is absorbed by the tissue being studied.

A clean educational illustration of a patient lying on a bed inside a PET scanner. A detector ring surrounds the body and a coloured region inside the torso represents tracer concentration. — The radioactive tracer is inside the patient. The detector ring records gamma photons from annihilation events; it does not receive the positron.

A PET tracer contains a nuclide that decays by beta-plus emission. The emitted positron travels a short distance before meeting an electron.

{}^{18}_{9}\mathrm{F}\rightarrow{}^{18}_{8}\mathrm{O}+{}^{0}_{+1}\mathrm{e}

Common mistake

The detector does not receive the positron. The positron first annihilates with an electron inside the body.

One annihilation produces an opposite photon pair

Swipe left or right to see the whole diagram →

Two separate gamma photons leave one annihilation point in opposite directions. A coincident pair identifies one line of response; many such lines are processed to map tracer concentration.

The electron and positron annihilate. Their mass-energy becomes the energy of gamma photons (9702/42/O/N/23 Q9(b)(i)).
If their initial kinetic energy is negligible, each photon has an energy equal to the rest energy of one electron.
Two photons travel in opposite directions so that total momentum is conserved (9702/41/M/J/25 Q8(d)(iii)).

E_\gamma=m_ec^2=0.511\ \mathrm{MeV}

Your turnmass-energy check

Explain why an electron-positron pair initially at rest cannot annihilate to produce only one gamma photon.

Show worked answer

The initial total momentum is zero. One photon always has momentum, so two photons moving in opposite directions are needed to give zero total momentum.

Coincidence timing builds a tracer map

Detectors around the patient record gamma photons.
Two photons arriving at nearly the same time on opposite sides are treated as one annihilation event.
The event lies somewhere on the line joining those detectors.
Arrival-time data from many events are processed to build an image of tracer concentration in the tissue (9702/42/O/N/23 Q9(b)(ii)).

Method	Signal measured	What the image maps
ultrasound	returning sound echoes	boundaries and depth
CT	transmitted X-ray intensity	attenuation in thin sections
PET	opposite gamma-photon pairs	tracer concentration

Your turn9702/42/O/N/23 Q9(b)

Explain how detected gamma photons are used to produce a PET image. [2]

Show worked answer

The arrival times of the photons are processed, and the data build an image of tracer concentration in the tissue.

PET is a map of tracer concentration. It is not a photograph of the positron path and it is not an X-ray attenuation image.

The tracer marks where a process is active

Definition

1 mark

What is meant by a tracer?

Model answer: A material introduced into the body whose position can be detected, or which is absorbed by the tissue being studied.

A PET tracer contains a nuclide that decays by beta-plus emission. The emitted positron travels a short distance before meeting an electron.

{}^{18}_{9}\mathrm{F}\rightarrow{}^{18}_{8}\mathrm{O}+{}^{0}_{+1}\mathrm{e}

Common mistake

The detector does not receive the positron. The positron first annihilates with an electron inside the body.

One annihilation produces an opposite photon pair

Swipe left or right to see the whole diagram →

Two separate gamma photons leave one annihilation point in opposite directions. A coincident pair identifies one line of response; many such lines are processed to map tracer concentration.

The electron and positron annihilate. Their mass-energy becomes the energy of gamma photons (9702/42/O/N/23 Q9(b)(i)).
If their initial kinetic energy is negligible, each photon has an energy equal to the rest energy of one electron.
Two photons travel in opposite directions so that total momentum is conserved (9702/41/M/J/25 Q8(d)(iii)).

E_\gamma=m_ec^2=0.511\ \mathrm{MeV}

Your turnmass-energy check

Explain why an electron-positron pair initially at rest cannot annihilate to produce only one gamma photon.

Show worked answer

The initial total momentum is zero. One photon always has momentum, so two photons moving in opposite directions are needed to give zero total momentum.

Coincidence timing builds a tracer map

Detectors around the patient record gamma photons.
Two photons arriving at nearly the same time on opposite sides are treated as one annihilation event.
The event lies somewhere on the line joining those detectors.
Arrival-time data from many events are processed to build an image of tracer concentration in the tissue (9702/42/O/N/23 Q9(b)(ii)).

Method	Signal measured	What the image maps
ultrasound	returning sound echoes	boundaries and depth
CT	transmitted X-ray intensity	attenuation in thin sections
PET	opposite gamma-photon pairs	tracer concentration

Your turn9702/42/O/N/23 Q9(b)

Explain how detected gamma photons are used to produce a PET image. [2]

Show worked answer

The arrival times of the photons are processed, and the data build an image of tracer concentration in the tissue.

PET is a map of tracer concentration. It is not a photograph of the positron path and it is not an X-ray attenuation image.