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CAIE A-Level Physics A2 24.3 Pet Scanning Questions

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

Syllabus
2028–2030
Course
Physics 9702
Level
A2

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 introduced24.3.2—That a tracer that decays by β+ decay is used in positron emission24.3.3—Annihilation occurs when a particle interacts with its antiparticle and that24.3.4—That, in PET scanning, positrons emitted by the decay of the tracer24.3.5—The energy of the gamma-ray photons emitted during the annihilation of an24.3.6—The gamma-ray photons from an annihilation event travel outside the body and

Question 1

[Maximum number: 5]

Fluorine-18 (918 F)\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

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

Question (a)

(a)

State what is meant by a tracer.

[ 1 ]

Question (b)

(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 ]

Question (c)

(c)

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

[ 2 ]

Question 2

[Maximum number: 2]

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.

Question 3

[Maximum number: 8]

Question (a)

(a)

Positron emission tomography (PET scanning) makes use of a tracer containing a radioactive material that decays by positron emission.

[ 3 ]

Question (i)

(i)

State what is meant by a tracer.

[ 2 ]

Question (ii)

(ii)

State the name of the particles that are emitted from the body and detected by the detectors during PET scanning.

[ 1 ]

Question (b)

(b)

Explain how the particles in (a)(ii) are created from positrons.

[ 3 ]

Question (c)

(c)

Positrons can be artificially created by a process in the laboratory that is the reverse of the process in (b). This process creates both a positron and an electron moving at the same speed in opposite directions.

Suggest why two of the particles in (a)(ii) are needed to create one positron.

[ 2 ]
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