PET physics for medical students (summary) Flashcards
Just a quick note: as I haven’t had physics in high school this whole lecture is relatively new to me. So I will go into a little bit more detail based on the whole lecture and some youtube clips. I’ll put a * on the flashcards that contain questions that are not necessarily based on the summary.
:)
- What is a positron?
- What is a neutron?
- What is a photon?
- An electron that is positively charged (e+)
- A subatomic particle that has a neutral charge (and a mass slightly greater than that of a proton)
- An elementary particle that is a quantum of the electromagnetic field (such as gamma rays)
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What is the definition of antiparticles?
A subatomic particle having the same mass as one of the particles of ordinary matter but opposite electric charge and magnetic moment. In regard to PET physics, β+ (positron) and e- (electron) are antiparticles of each other.
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What is the definition of annihilation?
A reaction in which a particle and its antiparticle collide and disappear, releasing energy.
Note: the most common annihilation on Earth occurs between an electron and its antiparticle, a positron.
What is β-decay (also called positron emission)?
A type of radioactive decay in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron (β+) and an electron neutrino.
Explain what happens from the moment when a radioactive isotope is intravenously injected to the moment gamma rays are produced.
- A radioactive isotope (e.g. F-18) is intravenously injected into the body and due to certain biochemical properties (e.g. form of sugar), the radioactive isotope will travel to where the cancer is located.
- Arround the cancer, β-decay of the radioactive isotope occurs causing the release β+ particle.
- The body (and whole universe) is filled with electrons and therefore, the β+ particle will easily collide with a nearby electron in the body. Since β+ and electrons are antiparticles of each other, the two will annihilate each other.
- Due to the annihilation, 100% of the mass of the β+ particle and electron will be turned into electromagnetic energy in the form of gamma rays. Specifically, the annihilation will cause the formation of 2 gamma rays that will travel in opposite directions (180 degrees) to each other.
PET is based on the principle of the conservation of momentum.
What is meant with the conservation of momentum?
By knowing that the total momentum of the positron-electron system is zero just before they annhilate, we can determine the momentum of the resultant gamma rays.
Name the primary reason why PET has better spatial resolution than other nuclear techniques.
Due to coincidence detection
What is meant with coincidence detection?
An event is recorded in PET only if the two annihilation photons are both detected by the camera within a short time interval (i.e. coincidence timing window).
Why is spatial resolution of PET limited to about 5 mm?
Due to the random positron path before annihilation and gama emission occurs. A positron travels around 1 mm (number based on youtube clip) before losing nearly all his energy, hence the tracer is likely to have been around 1 mm of the point of annihilation.
What is PET attenuation?
In PET imaging, an annihilation of an emitted positron with an electron produces two 511 KeV photons that move in opposite directions. These photons travel through the tissue before reaching PET detectors. The absorption and scatter caused by the photon-tissue interaction leads to photon attenuation
What is a crucial requirement for PET imaging so that an image can be quantified/created?
Correction for attenuation within the body.
By what device is correction for PET attenuation calculated?
CT scanner
Why is a PET scanner always combined with a CT? Give two reasons.
- For anatomical localisation of the PET image.
- For correction of attenuation of the radiation within the patient’s body.
Why is 89-Zirconium (half-life of 3.3 days) used for monocloncal antibodies?
Because these antibodies need around 2 days to be taken up by their target.
