PET Scanning (Cambridge (CIE) A Level Physics)

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

(i) State what is meant by a tracer.

[2]

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

[1]

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

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.

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

PET scanning uses a tracer and involves annihilation. 

Explain what is meant by 

(i) a tracer,

[1]

(ii) annihilation.

[1]

For the annihilation process 

(i) State the names of the particles involved.

[1]

(ii) Calculate the total energy released in one annihilation event.

[2]

(iii) Calculate the wavelength of each gamma photon released.

[2]

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

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

Positron emission tomography (PET) is useful in medical diagnosis. PET can be used to locate tumours or areas of increased activity within the brain.

(i) Describe the principles of PET.

[4]

(ii) Explain how PET scans are able to locate tumours or areas of increased activity within the brain.

[2]

The decay of the nuclei in the tracer material results in the emission of a positron. Two of the detectors, directly opposite to each other, are triggered as they each receive a gamma photon from the annihilation of the positron.

Suggest why the annihilation 

(i) occurs close to the point where the positron is emitted,

[2]

(ii) results in two gamma-ray photons of equal energy moving in opposite directions.

[2]

Three tracer isotopes, rubidium-82, fluorine-18 and iodine-131, are being considered for use in a PET scanner. The incomplete nuclear decay equations are shown below.

(i) Complete the nuclear decay equations for rubidium-82, iodine-131 and fluorine-18.

[3]

(ii) State and explain which tracer would not be suitable for use in a PET scanner.

[2]

It is suggested that a scanner could be designed to detect photons produced from the annihilation of a proton and an antiproton.

Calculate: 

(i) the energy released in the proton–antiproton annihilation.

[2]

(ii) the wavelength of the γ-ray photons produced in the annihilation.

[2]

Positron Emission Tomography (PET) and ultrasound scans are both used in medical diagnosis.

Compare the effectiveness of these scans in terms of

(i) the images and the information that can be obtained,

[6]

(ii) patient safety.

[4]

Table 1.1 gives the properties of two radionuclides.

Table 1.1

By considering the information in Table 1.1, suggest which of these nuclides is more suitable for use as a tracer in a PET scan.

Fluorodeoxyglucose (FDG) is a radiopharmaceutical used for PET scans. It contains radioactive fluorine-18 (F-18), which decays by positron emission with a half-life of 110 minutes.

In preparation for a scan scheduled for 12:00 p.m., a radiologist prepares a sample of F-18 FDG at 5 a.m. earlier in the day. For this scan, the radiologist determines the optimum activity of fluorine-18 would be 10 mCi when the scan starts at 12:00 p.m.

Determine the activity, in GBq, of the dose of fluorine-18 that the radiologist should prepare at 5 a.m. 

1 mCi = 3.7 × 10⁷ Bq

The patient is scheduled to come in an hour earlier, at 11:00 a.m. to receive their injection of F-18 FDG.

At this facility, the maximum activity limit of F-18 FDG that can be injected into patients is set at 14 mCi.

The radiologist forgets to factor this into their calculations when preparing the dose of F-18 FDG and discovers that it has an activity of 15.3 mCi at 11:00 a.m. when the patient arrives.

Determine the earliest time, to the nearest minute, at which the sample of F-18 FDG may be injected into the patient.

About 9.9% of the mass of F-18 FDG is fluorine-18. The molar mass of fluorine-18 is 0.018 kg mol^–1.

(i) Calculate the initial mass, in kg, of F-18 FDG given to the patient at the time of injection.

[3]

(ii) Determine the fraction of F-18 FDG remaining in the patient's body 6 hours after the initial injection. 

[2]

(iii) Suggest why the actual fraction would likely be lower than the value you calculated in (c)(i).

[1]

Both computed tomography (CT) scans and positron emission tomography (PET) scans can produce detailed pictures of the organs and tissues inside the body. Both techniques share many similarities and differences.

In recent years, it has become much more common for doctors to use a PET-CT scan which is a machine able to perform both scans simultaneously. This way, doctors can combine the images from both scans to create an enhanced picture of the tissue or organ being studied.

(i) Describe the major differences between a CT scan and a PET scan.

[4]

(ii) Discuss the benefits and risks of using a PET-CT scanner instead of using either scan separately in medical diagnosis.

[2]

Table 1.1 shows some information about two radionuclides commonly used in PET scanning.

Table 1.1

Under certain conditions, a gamma photon may be converted into an electron and a positron.

Deduce, with suitable calculations, if the decay of one or both radionuclides in Table 1.1 can result in both the annihilation and the creation of electron-positron pairs.

In practice, it is observed that the majority of gamma photons emitted have an energy of 511 keV and travel at exactly 180° to each other, regardless of the initial energy of the positron.

When fluorine-18 is used as a tracer, a very small proportion of the gamma photons have energies greater than 511 keV and do not travel at exactly 180° to each other. 

When oxygen-15 is used as a tracer, a slightly larger proportion of the gamma photons have energies greater than 511 keV and do not travel at exactly 180° to each other. 

(i) Explain these observations.

[6]

(ii) Suggest a potential issue in the detection of the gamma rays if the majority of photons possessed energies greater than 511 keV.

[2]

A tracer contains radioactive fluorine-18 and has an initial activity of 450 MBq. A typical PET scan using fluorine-18 takes 30 minutes. 

Determine the number of photons produced during a PET scan using fluorine-18. Assume that all photons produced in annihilation events are detected by the detector.

A typical PET scan using oxygen-15 takes 6 minutes.  

Determine the initial activity of oxygen-15 required to produce the same number of photons as the scan using fluorine-18.