What discovery was the foundation of PET imaging?
Of the positron and the principles of positron annihilation by Anderson 1932
What invention enabled the production of radioactive tracers?
Cyclotron 1930
Who invented the positron imaging device?
Sweet and Brownell 1953
What is the most commonly used raioactive tracer?
FDG- fluorodeoxyglucose which acts as a naturally occuring glucose in the body
What produces the gamma rays for the PET images?
FDG binds to the naturally ocurring glucose in the body and reacts with the positron which targets the electron, when hits an electron at the right speed, they combine and destroy each other, producing the gamma rays
Outline how emission and annihilation produce gamma rays
- FDG positron is emitted
- The carbon atom emits the positron
- It propogates a particular region of the brain
- Target the electron causing the positron annihilation which is decayed into two gamma rays
- Emitted in two opposite directions with equal intensities
- Has 2 gamma photons of 511keV (kilo electron energy)
What is the average travelling distance of the positron determinant of?
The maximum spatial resolution of PET (around 1mm)
What are the components of the PET scanner?
- Patient table
- Detector blocks in a detection ring
- Coincidence processing unit
- Sinogram/listmode data
- Image reconstruction
What is the detector block comprised of?
- Scintillator crystals (absorb the gamma rays)
- Photo multiplier (amplify the gamma signal then send it the processing unit)
What detects the gamma rays at two ends of the table?
- Detection rings
- Radioactive tracer targets the brain areas and emits the gamma rays/photons in two directions which are detected by the detection rings and sent to the processing unit
What is the PET resolution dependent on?
Scanner Diameter:
Smaller detectors provide higher spatial resolution because they can pinpoint the origin of gamma photons with greater accuracy.
Larger diameters result in poorer resolution because the photons must travel farther, increasing the chance of error in localisation.
Detector Rings:
The closer the detector rings are to the object, the better the resolution because they reduce the distance over which the gamma rays travel.
Proximity minimises inaccuracies due to photon scattering or misregistration.
What indicates increased neural activity in PET?
Concentration of gamma ray emission indicates increased neural activity in the brain
How are the unstable radioisotopes created?
- By nuclear reaction in a cyclotron
- Negative hydrogen atoms are accelerated at high speed to 11 MeV by D-shaped electromagnets
- The beam is extracted from the cyclotron by passing through a thin carbon foil, which strips off the electrons, leaving protons
- The protons are swept out of the beam and hit the target (usually lead) to form the isotopes
- The nature of the target (material) determines the radionuclide produced
How is the isotope/tracer created?
Radionuclides that emit positrons are chemically added to biological molecules which can then be introduced into the body
What are the most commonly used isotopes for PET?
- Carbon 11
- Nitrogen 13
- Oxygen 15
- Flourine 18
- The radiotracers do not lose thier biological activity i.e. H215O behaves like H216O
What 3 components are considered in a radioisotopes nature?
When being selected for PET
- Their half-life in mins (usually)
- Gamma photon max energy (MeV)
- Thier range in water
What isotope source is usually used to calibrate the PET scanner?
68Ge
Why is half-life important in PET?
Too long half-lives are bad for the subject as it would still receive a radioactive dose after the scan is over
The short half lives of the isotopes reduce the radiation exposure of the patient
What problems does the isotopes having a short half-life pose?
- Problems with storing and maintenance
- Most PET facilities create their own isotopes on site which is resource intensive and expensive
What are 3 advantages and 1 Limitation of PET?
+:
- Only quantitative techniques able to put numbers in vivo on metabolic or pharmacological processes dynamically
- Almost infinite numbers of parameters can be imaged
- Non-invasive technique other than the injection of the radiotracer into the body
-:
- The right target (physiological pr pathological biomarker) and the radiotracer must be available or developed
What are 3 disadvantages of PET?
- Radioactivity: each subject is limited to a maximum of 12 doses of 15O-labelled tracers or two doses of 18F-labelled tracers (in one session).
- The short half life of PET tracers require a cyclotron (expensive and staff consuming)
- Bad temporal (around 30sec) and spatial (around 2-8mm) resolution
What is a practical application if PET in clinical diagnosis of AD?
- Amyloid beta and tau are the molecules that accumulate in the brain and reduce the function of neurons
- Amyloid-PET can be used to show the extent of amyloid protein in the clinic for diagnosis
- Tau-PET has been developed more recently and is used in research settings
- Chapleau et al 2022
How has PET been applied to investigate the auditory cortex in the brain?
- Injection of (18F)FDG in patients with hearing impairment
- An auditory task is performed to stimulate activity in the superior temporal gyrus (auditory cortex)
- Moteki et al 2011
