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Here is the categorized question bank based on the syllabus subtopic 24.3 PET scanning, formatted for Obsidian.

1. Understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied

Oct/Nov 2025 P44 Q8.a (2/3)

Oxygen-15 (${}_8^{15}\text{O}$) is radioactive and has a half-life of 2.04 minutes. The decay of oxygen-15 produces positrons. For this reason, oxygen-15 is sometimes used as a tracer in positron emission tomography (PET scanning).

State what is meant by a tracer.

Mark Scheme

• (radioactive) substance introduced into the body (B1)
• substance absorbed by the tissues being studied (B1)

Examiner Comments

• Candidates found it difficult to give a complete and correct meaning of a tracer. The radioactive substance was often referred to, but was stated without mention of the need to introduce it into the body.
• Some candidates implied the substance or the positrons were already inside the body.
• Absorption into the tissue ‘being studied’ or equivalent wording was rarely stated.

Oct/Nov 2023 P42 Q9.a.i (1/2)

Fluorine-18 (${}_9^{18}\text{F}$) 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).

State what is meant by a tracer.

Mark Scheme

• material introduced into the body and (position in body) can be detected or absorbed by the tissue (being studied) (B1)

Examiner Comments

• Many candidates gave part of the answer but not a full answer as to what is meant by a tracer.
• A significant minority of weaker candidates attempted to describe how a tracer is used rather than answering the question that was asked.

Feb/Mar 2022 P42 Q11.a (1/2)

Positron emission tomography (PET scanning) obtains diagnostic information from a person. The information is used to form an image.

PET scanning uses a tracer. Explain what is meant by a tracer.

Mark Scheme

• substance containing radioactive nuclei that is introduced into the body or substance containing radioactive nuclei that is absorbed by the tissue being studied (B1)

2. Recall that a tracer that decays by ${\beta }^{+}$ decay is used in positron emission tomography (PET scanning)

Oct/Nov 2025 P44 Q8.b (3/3)

The equation for the decay of oxygen-15 is: ${}_8^{15}\text{O}{\to }_Q^P\text{X}{+}_S^R{\beta }^{+}+\text{Z}$ where X is the nucleus formed during the decay and Z is another particle.

(i) State the values of the integers $P$, $Q$, $R$ and $S$. (ii) State the name of particle Z.

Mark Scheme

• (i) $P=15$ and $R=0$ (A1)
• (i) $Q=7$ and $S=(+)1$ (A1)
• (ii) (electron) neutrino (B1)

Examiner Comments

• (i) The majority of the candidates gained full credit.
• (ii) The most common incorrect answer was ‘neutron’.

Oct/Nov 2023 P42 Q9.a.ii (2/2)

Fluorine-18 decays to a nuclide of oxygen (O) according to: ${}_9^{18}\text{F}{\to }_Q^P\text{X}{+}_8^R\text{O}$

State the symbol of the particle that is represented by X and the values of $P$, $Q$ and $R$.

Mark Scheme

• X = ${\beta }^{+}$ or ${\text{e}}^{+}$ and $P=0$ (B1)
• $Q=1$ (or $+1$) and $R=18$ (B1)

Examiner Comments

• This question was a straightforward exercise in balancing a nuclear equation.
• Some candidates did not know the symbol for a positron, and many others did not know the effect on proton number and nucleon number of beta-plus decay.

Oct/Nov 2022 P42 Q10.a.i (2/3)

Positron emission tomography (PET scanning) involves the detection of gamma-radiation in order to identify the position of origin of positrons in the body.

Positrons are not naturally present in the body. Explain how positrons come to be present in the body during PET scanning.

Mark Scheme

• introduction of tracer (into the body) (M1)
• containing a ${\beta }^{+}$ emitter (A1)

Examiner Comments

• The use of a ${\beta }^{+}$-emitting tracer was generally well known by candidates, and most candidates were able to articulate it well enough to be awarded at least partial credit.

3 & 4. Understand that annihilation occurs / Explain that positrons annihilate with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions

Oct/Nov 2025 P44 Q8.d (3/5)

The particles that are emitted from the body and detected outside it during PET scanning are not positrons but another type of particle.

(i) State the name of the particles that are detected. (ii) Explain how these particles are formed inside the body.

Mark Scheme

• (i) gamma photons (B1)
• (ii) positron collides with electron (in body) (B1)
• (ii) annihilation results in their masses becoming photon energy (B1)

Examiner Comments

• (i) Many candidates gave incomplete answers such as ‘photons’, ‘gamma rays’, ‘gamma particles’, ‘gamma radiation’ etc. Others gave incorrect answers, most commonly ‘electrons’.
• (ii) The description of gamma photon production often missed out that it is the total mass of the positron and electron that is converted. Candidates often either did not mention mass at all, or wrote that ‘mass’ was converted without making it clear that it is the mass of the particles. It was not always clear that the mass is converted into the energy of photons (and not just energy in general).

Oct/Nov 2023 P42 Q9.b.i (2/4)

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.

Mark Scheme

• positrons (emitted in the decay) and electrons annihilate (B1)
• mass of particles becomes energy of gamma photons (B1)

Examiner Comments

• Most candidates were able to state that positrons encounter electrons and undergo annihilation.
• The idea that this involves all of the mass of the particles becoming the energy of the gamma photons was less well articulated by many candidates.

Oct/Nov 2022 P42 Q10.a.ii (3/4)

Explain how positrons cause the emission of gamma-radiation from the body during PET scanning.

Mark Scheme

• positron interacts with electron (B1)
• (pair) annihilation occurs (B1)
• mass of particles converted into gamma photons (B1)

Examiner Comments

• This was another well-answered question, with the pair annihilation of the positron with an electron being familiar to most candidates.
• Candidates did not always make clear that pair annihilation involves the conversion of the whole mass of the positron and electron into the energy of the gamma photons.

Feb/Mar 2022 P42 Q11.b (2/3)

PET scanning involves annihilation. (i) Explain what is meant by annihilation. (ii) State the names of the particles involved in the annihilation process.

Mark Scheme

• (i) a particle interacting with its antiparticle so that mass is converted into energy (B1)
• (ii) electron(s) and positron(s) (B1)

5. Calculate the energy of the gamma-ray photons emitted during the annihilation of an electron-positron pair

Oct/Nov 2022 P42 Q10.b (4/8)

Show that the wavelength of the gamma-radiation that is detected during PET scanning is approximately $2.4\text{ pm}$. Explain your reasoning.

Mark Scheme

• (annihilation of electron and positron) produces two photons (B1)
• $E=(\Delta )m{c}^2$ (B1)
• $E=hf$ and $f=c/\lambda$ or $E=hc/\lambda$ (B1)
• $\lambda =\frac{2\times 6.63\times 10^{-34}\times 3.00\times 10^8}{2\times 9.11\times 10^{-31}\times (3.00\times 10^8{)}^2}=2.4(3)\times 10^{-12}\text{ m}$ or $2.4(3)\text{ pm}$ (full substitution and answer with unit needed) (B1)

Examiner Comments

• The more able candidates were usually able to correctly use $E=m{c}^2$ and $E=hc/\lambda$ to demonstrate the calculation of the $2.4\text{ pm}$ wavelength value.
• Often missing from responses was an explanation that the masses of the two particles involved become the energy of two gamma photons.
• Another common error was to use the de Broglie wavelength equation as a starting point, and to calculate momentums of electrons apparently travelling at the speed of light.

Feb/Mar 2022 P42 Q11.c (3/2)

(i) Calculate the total energy released in one annihilation event in (b). (ii) Calculate the wavelength of each gamma photon released.

Mark Scheme

• (i) $E=2m{c}^2=2\times 9.11\times 10^{-31}\times (3.00\times 10^8{)}^2$
• (i) $E=1.64\times 10^{-13}\text{ J}$ (A1)
• (ii) $\lambda =\frac{2hc}{E}=\frac{2\times 6.63\times 10^{-34}\times 3.00\times 10^8}{1.64\times 10^{-13}}$ (C1)
• (ii) $\lambda =2.43\times 10^{-12}\text{ m}$ (A1)

6. Understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration can be created by processing the arrival times

Oct/Nov 2023 P42 Q9.b.ii (2/4)

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

Mark Scheme

• arrival times of photons are processed (B1)
• image built up of tracer concentration in the tissue (B1)

Examiner Comments

• There was much confusion with other types of medical imaging in responses seen to this question, with image formation in ultrasound scanning and CT scanning being commonly discussed.

Feb/Mar 2022 P42 Q11.d (3/4)

Explain how the gamma photons are used to produce an image.

Mark Scheme

• Any 3 from: (B3)
• the two gamma photons travel in opposite directions
• gamma photons detected (outside body / by detectors)
• gamma photons arrive (at detector) at different times
• determine location of production (of gamma)
• image of tracer concentration in tissue produced