Radionuclide imaging and therapy

• What is a radioctive tracer?

  A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body.
• Why are gamma emitters used as radioactive tracers?

  The gamma can easily penetrate the body tissue meaning it can leave the body and is weakly ionising in comparison with alpha or beta radiation.
• What is the role of the molecules that are bonded to the radioactive isotope (radiopharmaceutical product)?

  To be a good tracer, the molecule must not impact the body's regular function, but gather in tissues.

  This enables different molecules to be used to make the tracer gather in specific organs.
• What are the 3 common radioactive tracers used?

  Technetium - 99m (metastable)

  Iodine - 131

  Indium - 111
• How do we know where radioactive tracers are in the body?

  Once they have been injected/injested they are absorbed by the organ or tissue and appear on the screen of a gamma camera as a bright area.
• Give examples of how different lables impact where bright areas would form on a gamma camera.

  Labelling glucose highlight areas of higher respiration - allows us to identify where tumours are.

  Labelling white blood cells shows the location of an infection the body.

  Labelling red blood cells highlights areas of decreased blood supply.
• Compare the type of radiation emitted by Technitium-99m, Iodine-131, Indium- 111

  Technitium 99m emits gamma only (weakly ionising)

  Iodine-131 emits beta minus and gamma (more harmful)

  Indium-111 emits gamma only (weakly ioning)
• Compare the half-lives of Technitium 99m, Iodine-131 and Inium-111

  Technetium-99m - 6 hours

  Iodine-131 - 8 days

  Indium-111 - 68 hours
• Compare the energy of gamma photons in Technitium 99m, Iodine-131 and Inium-111.

  Technitium-99m - 140 keV

  Iodine-131 - 360 keV

  Indium-111 - 170 keV and 250 keV
• Compare the uses of Technitium-99m, Iodine-131 and Indium-111 tracers.

  Technitium-99m - can be used to investigate organs, blood and tumours.

  Iodine-131 - to treat overactive thyroids and thyroid cancer

  Inium-111- to label cells and diagnose blood disorders and rare cancers.
• What is a molybdenum-technetium generator?

  a device that produces technetium from molybdenum.
• Why is technetium-99m produced from molybdenum?

  As technetium-99m has a half life of 6 hours to allow low irradiation to the patient, this means it has to be produced in the hospital when needed. Therefore, a hospital can use Mo-99 which has a half life of 66 hours and a molybdenum-technetium generator to produce Tc-99m when needed.
• How is Tc-99m extracted from the generator?

  A saline solution passes over the radioactive materials, the Tc-99m enters the solution which can be extracted and given to the patient before their scan.
• What does PET stand for?

  Positron Emission Tomography
• What is PET?

  Imaging tissues and organs by measuring the metabolic activity of the cells and tissues.
• What is given to the patient before the PET scan?

  The patient is injected with a beta-plus emitting isotope such as fluorine-18.
• How does the PET scan work?

  • The body is surrounded by a ring of gamma cameras.
  • The positrons annihilate with electrons in the patient producing two identical gamma photons travelling in opposite directions.
  • The delay time between the two photons is used to determine the location of the annihilation due to the tracer (F-18).
• What is the energy of each gamma photon produced by the annihilation in PET scanner?

  E=hf=mc^2
• How is diagnosis carried out using PET scanning?

  • The gamma photons travel in opposite directions to conserve momentum.
  • They hit the detectors in a line - the line of response.
  • The more photons from a particular point, the more tracer that is present in the tissue. This appears as a bright point on the image.
  • An image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons.
• What is the biological half-life of a substance?

  The time taken for the concentration of a substance in the body to decrease by half.
• What is the Equation for effective half life?

  $1/T_E =$$1/T_p +$$1/T_B$
• Why is the effective half-life always shorter than the shortest half-life out of physical half life and biological half-life?

  When the physical half-life is short it is because the body is also excreting the chemical to which the radionuclide is attached.

  When the biological half-life is short it is because the radionuclide is also decaying while the body is excreting the radiopharmaceutical at this fast rate.
• What component of the gamma camera absorbs gamma rays produced by annihilation?

  thin lead tubes called collimators.
• How do collimators ensure the sharpest and highest resolution images are produced?

  Only photons moving parallel to the collimator are absorbed, this improves sharpness as any scattered photons are excluded.

  Collimators must be narrow and long to ensure more gamma rays are absorbed and so more electrons will be produced. This improves image quality as more electrons contributing to the electric pulse output will increase the resolution of the image.
• How does the scintillator work?

  When gamma photons are incident on the scintillator, an electron in the crystal is excited to a higher energy state.

  As the excited electron travels through the scintillator, it excites more electrons.

  When these electrons deexcite, the lost energy is transmitted as visible light photons.
• What is the function of the photomultiplier tubes in a gamma camera?

  The photomultipler detects the faint flashes of light from the scintillator, converts them into voltage pulses and amplifies the electrical signal.
• How are images formed on a computer from the gamma camera?

  • An image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons
• What can low-energy X-Rays treat? How are low-energy X-rays used to externally treat a patient?


  Skin cancer

  The X-rays are directed at surface tumours, but are low-energy so do not penetrate deep into the body. This minimises the risk of damage to deeper tissues.
• What are high-energy X-rays used to treat? What is the process for external treatment?

  Tumours deep inside the body.

  External beam radiotherapy (EBRT) - the X-rays from the external source destroy cancer cells during division.
• How are metal filters used to limit exposure to healthy cells during high-energy X-ray radiotherapy?

  During EBRT, the patient is held in an aluminium mold to make sure only the target tissue receieves the radiation dose. Also by filtering through aluminium, the low energy X-rays are removed which could potentially damage tissue on the surface of the patients body rather than a tumour deep inside the body.
• How does using X-rays from different directions limit exposure time?

  The beam of X-rays matches the dimensions of the tumour - this is known as conformal radiotherapy.

  The LINAC (which produces the x-rays) can be rotated around the patient, ensuring the targeted area receieves a much larger dose of the radiation than the surrounding healthy tissue.
• What is internal radiotherapy?

  When a radioactive implant is placed next to, or into the tumour.

  The implant irradiates the tumour.
• What are the advantages and disadvantages of radioactive implants?

  Advantages: the source of the radiation can be placed as close to the cancer as possible.

  Disadvantages: A small amount of healthy tissue is likely to be exposed to ioning radiation.
• What type of radiation should be used in an implant?

  The radioisotope must not penetrate far from the implant site to avoid irradiating healthy tissue further from the tumour. This means beta radiation (or LOW ENERGY gamma) as it can penetrate the seed's metal casing, but cannot penetrate much further than the beyond the implant site.
• Why is EBRT more appropriate for treating a tumour in the brain than using a radioactive implant?

  Radioactive implants have to be mechanically placed in the body. This would be challenging and healthy tissue surrounding the tumour would become irradiated imparing brain function.

  However, EBRT means that the largest dose of radiation is localised to the tumour and the X-ray beam is shaped to fit the exact dimensions of the tumour limiting damage to healthy tissue. Also, there is no surgery required to irradiate the tumour.
• What imaging methods are there?

  CT scan

  External X-rays

  MRI

  PET

  Ultrasound

  Ingested gamma source
• What are the advantages and disadvantages of the CT Scan?

  advantages: show different tissue types, high resolution (0.5mm)

  disadvantages: expensive, large radiation dose.
• What are the advantages and disadvantages of external X-rays?

  Advantages: cheap, fast and easy operation, high resolution (0.5mm).

  Disadvantages: cannot distinguish different tissues, cannot image some organs.
• What are the advantages and disadvantages of MRI?

  Advantages: Detailed images- can distinguish types of tissue, non-invasive, no radiation exposure. Disadvantages: Expensive, long scan times, some people may feel claustrophobic.
• What are the advantages and disadvantages of PET scans?

  Advantages: Detailed images, shows the function of organs, good for imaging the brain. Disadvantages: Expensive, exposure to radiation, does not work for every organ.