PET-CT 02/02 Flashcards
What does PET stand for?
Position emission tomography
Why do we use PET?
PET provides information on the metabolic function of organs or tissues by detecting how cells process certain compounds such as glucose.
What is the aim of radiotracers within radionuclide imaging?
To localise their distribution as accurately as possible within the patient by the external detection of radiation emitted from the patient involving gamma rays, annihilation photons or bremsstrahlung radiation.
What is the main advantage of radionuclide imaging?
It is very sensitive to detect very small amounts of tracer and to demonstrate the function of cells, tissues and organs.
What does CT stand for?
xray computer-tomography
When is a radiotracer produced?
A radiotracer is produced when a suitable radionuclide is combined with a pharmaceutical compound or a molecule that targets a particular biological function or process.
How is the behaviour of the radio labelled compound monitored?
By radiation detectors external to the body, allowing the non invasive measurement of in vivo biochemical function, aspects of tissue function and dynamic biological processes.
What is a radionuclide?
An unstable isotope of an element that will spontaneously decay by the emission of particles and/or electromagnetic radiation.
What are the 3 most common emissions from radionuclides?
Alpha and beta particles + gamma rays
How many types of beta particle are there and what are they?
Beta plus (𝛽+) [positron] and beta minus (𝛽-) [beta emission]
What does beta emission (𝛽-) involve?
The decay of some neutron rich isotopes by converting a neutron into a proton, emitting beta minus particles
What does position emission (𝛽+) involve?
The decay of some proton rich isotopes by converting a proton into a neutron, emitting a positron
Which emission is important in diagnosis?
Positron (𝛽+) emission (very important for functional imaging in diagnosis)
What rate does the radioisotope fluorine-18 decay at?
t1⁄2 = 109 min
What happens when 𝛽+ travels in tissue?
It rapidly loses energy and interacts (annihilates) with an electron as its final interaction, producing 2 x 0.511 MeV gamma ray photons
Which emission is the only one with sufficiently penetrating characteristics that enable them to be detected externally to the patient?
Gamma rays
What are the 2 branches of radionuclides?
One uses a relatively limited number of radionuclides that emit gamma rays and is referred to as single-photon imaging.
The other uses radionuclides that decay by emitting positrons - referred to as PET.
Is a positron itself penetrating?
No
How can a positron become penetrating?
By interacting locally with an electron, both annihilating to form 2 photons travelling in opposite directions. These photons are penetrating and can be detected when they leave the body.
When two interactions are simultaneously recorded in a pair of detectors, what is it known as?
Coincidence detection
What is the line joining the two simultaneous interactions is called?
Line of response (LOR)
What happens in the radio labelling process?
A radiopharmaceutical is formed by attaching a radionuclide to a molecule
How are radiotracers introduced into the body?
Intravenous injection, oral ingestion, intradermally, intraarerially etc
What does a gamma ray camera detect?
Annihiliation photons resulting from the decay of positron emitting radionuclides
What do back-projecting LORs produce?
An isotope distribution map (reconstructed images)
What is a commonly used radiopharmaceutical?
Fluorodeoxyglucose (FDG)
What is FDG?
An analogue of glucose, and so follows the same metabolic pathways in the body.
How is FDG useful in detecting cancer?
Most cancer cells metabolize glucose at a much higher rate than normal tissues.
By detecting increased radiolabelled glucose metabolism with a high degree of sensitivity, PET identifies cancerous cells, even at an early stage, when other imaging modalities may miss them
How long is the delay between administration and scanning to allow a good tumour-background ratio of tracer uptake?
A 1-hour delay
What is the structural difference between glucose and FDG?
One OH- group is substituted by 18F
Why are PET and CT combined?
The information from both images together will help in the planning of radiotherapy treatments.
Due to very poor spatial resolution of PET scan it is hard to manually combine (fuse) these two images.
Structural information/ CT units from the CT scan, functional information from the PET scan.
Are PET and CT separate or combined?
Combined in a single apparatus
What can occur as a result of only a single apparatus required?
In a single visit a patient can undergo two high quality diagnostic procedures without the need for repositioning in between.
What is the CT image used for?
Attenuation correction
Providing anatomical detail
How much are scan times reduced from PET along to PET-CT?
45-60mins compared to 25mins
Advantages of PET-CT scanners
- PET/CT offers accurately aligned, clinical quality PET & CT images in a single imaging device.
- Registered imaging has the ability to precisely identify areas of physiologic & non-malignant FDG uptake, increasing diagnostic accuracy.
- Scan times are reduced compared to PET alone.
- Patient has two images performed in a single visit.
- The CT images can be used to correct for physical limitations in PET imaging (attenuation & scatter ).
- Fused images can also be used for radiotherapy treatment planning.
Disadvantages of PET-CT scanners
- PET/CT scanners are expensive.
- Only one of the modalities is being used at any one time, so there is a degree of redundancy in using the scanner.
- Operators will require additional training.
- Extra time must be spent on QC before the scanner can be used in the clinic.
- Artefacts can be created due to patients breathing patterns and movement between the two scans.
For ”difficult” tumours we may use 3 different imaging modalities to contour the GTV, what are these?
CT, MR and PET to create a dose plan
How are PET and CT images best used?
- PET and CT are best used as simultaneous images (from the same scan session).
- It can be possible to fuse separate scan images together although it’s not ideal.
- In radiotherapy, this is not always possible.
- Differences in scanning position between PET and CT scans can be problematic.
Contouring with CT imaging alone
Can be used to ensure more accurate target volume delineation
Also reduces the variation in clinician contouring
Contouring with CT and PET imaging
Often used to determine nodal involvement in almost any site in the body
What do lymph nodes involved with disease look like on a CT scan?
Typically enlarged but is can be subtle as lymph nodes are small structures anyway
Why would we use PET imaging for nodal involvement?
Use PET imaging to determine nodal involvement in almost any site
Aids the staging of disease and treatment volume determination
Small volumes of disease, such as nodes, where FDG uptake is more intense can receive what kind of radiotherapy dose?
A ‘boosted’ radiotherapy dose (sometimes known as ‘dose escalation’)
Often it is involved (with disease) nodes which are boosted
But parts of the primary tumour, can also be boosted for example:
treating whole prostate to a certain px and a smaller part of it to a higher px
boosting part of a brain GTV where imaging shows a higher grade area
A special kind of PET imaging, called TSPO PET, can be used to provide what information?
The location and activity of brain lesions
What radioactive drug is used for TSPO-PET than PET?
A radioactive drug called PK 11195 is used instead of FDG
It binds with the 18 kDa translocator protein (TSPO) which is expressed in high-grade gliomas but comparatively less so in normal brain tissue
TSPO PET can be used to determine cell differentiation and anaplastic transformation in CNS lesions
