CAIE A-Level Physics 24.3 PET Scanning

Practise analysing PET tracers, radioactive decay, annihilation, opposing gamma detection, energy conservation and detector processing.

Exam points

explain the role of a tracer and ?? decay in PET scanning
explain particle-antiparticle annihilation using mass-energy and momentum conservation
explain how PET annihilation produces opposing gamma photons for detection
calculate gamma-ray energy from electron-positron annihilation using E = 2mc?
interpret detector timing and processing as an image of tracer concentration

Covered syllabus points

24.3.1—A tracer is a substance containing radioactive nuclei that can be introduced 24.3.2—That a tracer that decays by β+ decay is used in positron emission 24.3.3—Annihilation occurs when a particle interacts with its antiparticle and that 24.3.4—That, in PET scanning, positrons emitted by the decay of the tracer 24.3.5—The energy of the gamma-ray photons emitted during the annihilation of an 24.3.6—The gamma-ray photons from an annihilation event travel outside the body and

[Maximum number: 5]

Fluorine-18 $\left({ }_{9}^{18} \mathrm{~F}\right)$ is a radioactive nuclide that is used as a tracer in positron emission tomography (PET scanning). Fluorine-18 decays to a nuclide of oxygen ( O ) according to

{ }_{9}^{18} \mathrm{~F} \longrightarrow{ }_{P}^{Q} \mathrm{X}+{ }_{8}^{R} \mathrm{O} \text {. }

(a)

State what is meant by a tracer.

[ 1 ]

(b)

Explain how the radioactive decay of fluorine-18 results in the emission from the body of the gamma-ray photons that are detected during a PET scan.

[ 2 ]

(c)

Explain how the detection of the gamma-ray photons is used to produce an image of the tissue being examined.

[ 2 ]

[Maximum number: 4]

(a)

Positronium is highly unstable, and after a very short period of time it becomes gamma radiation.

[ 4 ]

(i)

Describe how gamma radiation is formed from the two particles in positronium.

[ 3 ]

(ii)

State one medical application of the process described in (c)(i).

[ 1 ]

[Maximum number: 4]

An electron, at rest, has mass $m_{\mathrm{e}}$ and charge -q.
A positron is a particle that, at rest, has mass $m_{\mathrm{e}}$ and charge +q.
A positron interacts with an electron. The electron and the positron may be considered to be at rest.
The outcome of this interaction is that the electron and the positron become two gamma-ray ( $\gamma$ -ray) photons, each having the same energy.

(a)

Calculate, for one of the $\gamma$ -ray photons:

[ 2 ]

(i)

the photon energy, in J
energy =

[ 2 ]

$E=m c^{2}$

C1

$\begin{aligned} =9.11 \times 10^{-31} \times\left(3.0 \times 10^{8}\right)^{2} =8.2 \times 10^{-14} \mathrm{~J} \end{aligned}$

A1

(b)

State and explain the direction, relative to each other, in which the $\gamma$ -ray photons are emitted.

[ 2 ]

All question bank results loaded

Covered syllabus points

24.3 PET scanning question 1

Question (a)

Question (b)

Question (c)

24.3 PET scanning question 2

Question (a)

Question (i)

Question (ii)

24.3 PET scanning question 3

Question (a)

Question (i)

Question (b)