PET Imaging Flashcards
What is PET?
Positron emission tomography
== a type of nuclear medicine procedure that measures metabolic activity of the cells of body tissues
What is NMR imaging?
Nuclear magnetic resonance imaging
What is nuclear medicine?
The use of radioactive tracers (radiopharmaceuticals) to obtain diagnostic information or perform targeted radiotherapy
What is a radiopharmaceutical?
A compound or drug which targets the organ or structure of interest
Pharmaceutical (traces physiology/localises in organs of interest)
+ (by biochemical bonding)
Radioactive nuclide (emits radiation)
What is a radionuclide?
An unstable form of a chemical element that releases radiation as it breaks down and becomes more stable
The bit emitting the radiation that allows us to see where the drug has gone (for diagnosis) or emits the radiation which kills the cells we’re trying to treat
What is the common radionuclide used in PET?
Fluorodeoxyglucose (FDG)
Radioactive atom combined with glucose
Glucose analogue - follows the same metabolic pathway as glucose until inside the cell at which point metabolism stops and FDG is trapped
Activity in the cell builds up in proportion to rate of glucose metabolism
What is the structure of FDG?
A radioisotope which is an analog to glucose with the positron-emitting radionuclide fluorine-18 substituted for the normal hydroxyl group
How is FDG used to detect tumours?
FDG is a modified form of glucose which can be absorbed by tissues
Cancerous tissues have high rates of metabolism and therefore will uptake lots of glucose
Once the glucose has entered the cancerous tissue, positrons will be emitted. They then collide with free electrons in annihilation near the decay event and give off gamma rays which are detected by the scanner indicating the location of the tumour
What determines the distribution of the radiopharmaceutical?
Now whether this radiopharmaceutical goes to the bones, a specific tumour type, the lungs or brain obviously depends on what the drug or compound exactly is
What is a positron?
Positively-charged electron
What is the theory of PET?
Works by using a scanning/detection device to detect photons emitted by a radionuclide in the organ or tissue being imaged
Radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the organ during metabolic processes
Positrons are emitted by the breakdown of the radionuclide as they are unstable
Gamma rays called annihilation photons are created when positrons collide with electrons near the decay event
The scanner then detects the annihilation photons, which arrive at the detectors in coincidence at 180 degrees apart from one another
What is the line of coincidence?
PET is based on the detection, in coincidence, of 2 photons created upon annihilation of a positron
The line of coincidence is the 180 degrees plane where the 2 photons hit either side of the detector
What is a true event?
A true event is recorded when the 2 annihilation photons are detected within a time coincidence window
i.e., the 2 photons hit either side of the detector at the same time
What is a random event?
Within this scheme it is unavoidable that two uncorrelated photons might be detected sufficiently close in time to be mistakenly identified as a coincidence
This constitutes an accidental coincidence, also called random coincidence (or just random)
What is the basis of PET imaging?
The basis behind PET imaging is being able to detect these simultaneous gamma rays
What is the reconstruction of PET images based on?
Based on knowing that the annihilation happened somewhere along a line joining these 2 detection points
This is the sort of high-quality, high-resolution whole body image we can get with PET
What is the structure of a PET scanner?
Based on a ring of independent scintillation detectors, all connected to each other via coincidence circuitry so that we can simultaneously detect the 2 back-to-back gammas produced when the positrons annihilate
What are the type of images that can be produced with PET?
Static or dynamic
Dynamic = series of images
Static = dynamic images combined and smoothed
What are recent/future advances of PET scanning?
Bigger FOV (field of view) (multi-ring) = means more of the body can be imaged at once which massively reduces imaging time
Design of detector block = a lot of work being done in terms of the design of the detector block
i.e., configuration of the individual scintillation crystal/PM tube units - design of these is critical in terms of the spatial resolution you can acheive
What is FOV?
Field of view
What is a problem with PET imaging?
Attenuation - loss of signal due to absorption or scatter of photons in tissue
Photons don’t leave the body without any sort of interaction
How do we correct for attenuation?
X-rays from a CT scan are used to construct an attenuation map of density differences throughout the body that can then be used to correct for the absorption of the photons emitted from FDG decay
Compensates for the effect of signal loss due to scatter and absorption
Essential for quantitation
Vast majority of PET scanners are PET-CT –> attenuation correction is straightforward
PET-MR needs to use convoluted methods - is more challenging
What are the PET acquisition parameters?
- Required axial field of view (e.g., half-body, brain etc)
- Scan duration per bed position
- Bed overlap in subsequent bed positions
What is a cyclotron?
A machine used to make short-lived radioactive isotopes that can be used for medical imaging or research
What are the common isotopes used in PET and what are their half life?
15O - 2 min
13N - 10 min
11C - 20 min
18F - 110 min
Why is fluorine (18F) the most used radionuclide in PET?
It has a half life of 110 min which is the longest out of the common isotopes in PET and therefore the signal isn’t reduced as quickly as other isotopes
Only one of the common isotopes that can feasibly be produced at a site remote from the PET scanner
What has caused the recent explosion in PET imaging?
The introduction of FDG, an excellent tracer based on F-18 radionuclide caused the recent explosion
How does a cyclotron produce PET isotopes?
The positron emitting nuclides used in PET are all cyclotron produced
A beam of positively charged particles are blasted into a stable target material to produce these proton-weighted nuclides which will tend to decay via positron emission so as to even up the balance of protons and neutrons in the nucleus
What is a problem with cyclotron production?
They are very very expensive and very few cyclotrons
How are tracers sourced without a cyclotron?
Commercially available tracers found at Alliance Medical and PETNET (commercial suppliers)
FDG is the most commonly used PET tracer supplied by commercial suppliers
What are clinical uses of FDG?
Clinically can help with differential diagnosis of AD, epilepsy localisation, tumour characterisation
Can be used to map patterns of glucose metabolism for normal/diseased brain functions
What other PET tracer can be useful for the diagnosis of AD?
Amyloid e.g., F18-florbetaben
Binds to beta amyloid plaques in the brain
Clinically can help with differential diagnosis of AD
In research can be used e.g., to assess the impact of anti-beta amyloid drugs
