PET CT Flashcards
What does PET stand for?
Positron Emission Tomography
What is PET CT?
Tomographic imaging technique also based on injecting a radiotracer which emits positrons, which have a short half-life.
Why is CT combined with PET?
to improve spatial resolution and fuse morphological and functional information
What is a disadvantage of the short half life of the radiotracer used in PET?
Cyclotron needs to be on site, this is expensive to run
What is a positron?
an antielectron
What does the distance traveled by the positron depend on?
The distance the positron travels in tissue depends on its initial energy and the number of
interactions with the atoms in the tissue
What are the principles of PET?
Emitted positrons annihilate with electrons in the body resulting in the formation of two gamma rays, with an energy of 511 keV each travelling in opposite directions.
The two gamma rays emitted from the patient have trajectories that are 180 degrees apart & strike solid-state detectors which are positioned in a series of complete rings around the patient.
What do the detectors commonly consist of?
1000’s of small crystals of bismuth germanate (BGO) and are coupled to PMT’s
Other scintillation crystals used for PET more recently include..
lutetium oxyorthosilicate (LSO) or gadolinium orthosilicate (GSO)
Why isn’t NaI used as the scintillator like in SPECT?
NaI scintillator (SPECT) is not used as its detection efficiency for the high energy gamma rays is too low
What happens when the gamma rays hit the detector?
When gamma rays hit the crystals, photons are produced, which are then converted into an amplified electrical signal, by PMTs
What are the most commonly used radionuclides in PET?
Fluorine-18 (F18)
It is combined with deoxyglucose as fluorodeoxyglucose (FDG). This is a tracer for glucose
metabolism.
Why is FDG used?
FDG is taken up by metabolically active cells.
It is not able to take part in glycolytic pathway because it cannot be phosphorylated
Accumulates in the metabolically active cells. The more active a cell is, themore will accumulate.
Does PET use a collimator?
PET does not require a collimator and, therefore, eliminates the weakest link in the SPECT image
formation process.
What is coincidence detection used for?
Coincidence detection is used to distinguish photons arising from positron annihilation, based on temporal discrimination.
How is PET more advantageous than SPECT?
in terms of spatial resolution, statistical quality and quantitative accuracy
What are the ideal qualities of the scintillator?
High linear attenuation coefficient for the 511 keV photons
High ratio of Photoelectric to Compton interactions
Large number of light photons produced per gamma-ray interaction
Short scintillation light decay time
Why are photomultiplier tubes (PMT) used?
PMTs have been the preferred photodetector because their high gain
results in an electrical output with a good signal to noise ratio.
What is coincidence counting, & why is it important?
If two opposite detector elements detect a gamma ray photon simultaneously it is considered a coincidence count.
As well as this, A line of response (LOR) joining the two detectors is assumed to intersect the unknown location of the
annihilation event
What is Time of Flight?
It has long been appreciated that the difference in the detection times of the two annihilation photons provides a mechanism for precisely localizing the site of individual positron-electron annihilations.
Recent addition which has improved image quality & spatial resolution
the benefit of time of flight in PET imaging:
in large patients where high attenuation and scatter mean that image quality is usually the poorest.
Why is scatter correction needed in PET?
because of the limited energy resolution of PET systems
What can be used to correct scatter in PET?
Pulse height analyser, as only photons within the required energy window are counted as true events.
Generally, the scattered photons will have lower energy than the photons traveling directly to the detector
What is the primary role of PET CT?
Primary tumour assessment and staging, Radiotherapy planning, treatment response assessment
