Diagnostic Methods in Medicine (OCR A Level Physics)

Fluorine-18 is a common radioactive isotope used in positron emission tomography (PET). Fluorine-18 emits positrons. A patient is injected with a radiopharmaceutical containing fluorine-18.

Describe how a PET scanner is used to locate an area of increased activity within the patient.

[4]

The half-life of fluorine-18 is 110 minutes.

Calculate the time t in minutes for the activity of the radiopharmaceutical to decrease to 30% of its initial activity.

t = ................................... minutes [3]

PET scanners are not available in all hospitals. This is because fluorine-18 requires expensive on-site particle accelerators and fluorine-18 has a very small ‘shelf-life’. Suggest the impact this may have on the treatment and diagnosis of patients in the country.

[1]

The medical tracer fluorine-18 is used in positron emission tomography (PET). Fluorine-18 is a beta-plus emitter with a short half-life.

Describe how the fluorine-18 nuclei are located in a patient using a PET scanner.

[4]

The nuclear reaction below shows how the isotope of fluorine-18 () is made from the isotope of oxygen-18 ().

The oxygen-18 nucleus is stationary and the proton has kinetic energy of 0.25 × 10⁻¹¹ J. The binding energy of the nucleus is 2.24 × 10⁻¹¹ J and the binding energy of the nucleus is 2.20 × 10⁻¹¹ J. The proton and the neutron have zero binding energy.

i) Explain why a high-speed proton is necessary to trigger the nuclear reaction shown above.

 [2]

ii) Estimate the minimum wavelength of the gamma ray photon ().

= ..................................................... m [3]

iii) Fluorine-18 is a positron emitter. Name a medical imaging technique that uses fluorine-18 and state one benefit of the technique.

 [2]

Describe how the components of a computerised axial tomography (CAT) scanner can produce high-quality images of the internal structures of a patient.

[4]