PET/CT basic principles Flashcards
What does a PET scanner consist of?
Ring of scintillation detectors arranged so pairs of detectors on opposite sides of the ring operate in coincidence with one another
What are the 4 steps of the scintillation process?
Electrons in valence band can absorb energy by interaction of photoelectron or Compton scatter electron with an atom & get excited into conduction band
electron de-excites by releasing scintillation photons & returns to ground state
Impurities or activators are added to modify the band structure
scintilliation is in visible range
what converts the incident light into a large electrical signal?
Photomultiplier tube
What technology converts light directly into a digital signal?
Digital photon counting technology
Why is the digital photon counting technology better than traditional analog photomultipliers?
System improves signal-to-noise ratio which enables improved sensitivity & image clarity
What does 1:1 coupling of crystals to digital detectors double?
sensitivity gain
volumetric resolution
quantitative accuracy
What are the 4 main properties of scintillator which are crucial for PET application?
High stopping power to stop 511 keV gamma rays
decay time of excited states must be short
high light output
good intrinsic energy resolution
What is the basis of coincidence detection & coincidence imaging?
Simultaneous emission of two photons in opposite directions
What do simultaneous pulses from detectors indicate?
Annihilation occurred somewhere along the path between 2 detectors, referred to as line of response
What does the no. of coincidence events occurring between detectors indicate?
How much activity there was in the LOR
When are radial artifacts visible (star effect)?
When count rates are low or a region of interest with low activity is near an organ with high activity
How are artifacts caused by low count data avoided?
by using alternative reconstruction method that calculates the activity image iteratively in an algebraic fashion
What is the size of the detector related directly to?
Spatial resolution (smaller detector = better spatial resolution)
What causes blurring in the image?
because the annihilation site and not the site of radioactive atoms are imaged by PET
What is regarded as a true coincidence event
if 2 gamma rays from a single annihilation event enter the PET detector without undergoing any significant interactions within the imaging field of view
What are 2 artificial coincidences?
random
scatter
What is the recorded count rate a combination of?
Trues
randoms
scatter
What do random and scatters degrade?
Image both qualitatively and quantitatively
What is attenuation?
Loss of true events due to scatter and absorption
What are the characteristics of resulting images when attenuation occurs?
artificially-depleted radioactivity deep in the body while outer contour of the body shows an artificially high amount of radioactivity
what is the CT scan used for in PET scans?
used to correct PET data for loss of counts due to attenuation
What do the attenuation values depend on?
Energy
What is a major advantage of the PET/CT scanner?
Provides an anatomical image which can be superimposed on the functional PET image
What are truncation artifacts?
If a part of the patient’s anatomy extends beyond the boundaries of transverse CT FOV the part of the anatomy is not reconstructed
What are respiration artifacts?
Mismatches of breathing patterns are particularly severe with breath-hold techniques
What are 3 other potential artifacts?
CT contrast agents
metal implants
beam hardening
What does time of flight make use of?
The small difference in time between the detection of the 2 photons emitted from the same positron annihilation event
What is the time difference due to?
Different distances travelled
Is the location of annihilation site known?
No it must be recovered by image reconstruction
How do you calculate SUV?
(activity in ROI (MBq)/vol(ml)) / injected activity (MBq)/patient weight (g)
What areas will have SUV’s > 1?
areas with higher average uptake
higher the SUV greater the risk of disease
What 5 things can influence SUVs?
Reconstruction algorithm
Blood glucose level
ROI used
Uptake period
Patient motion/breathing
What is the partial effect due to and what does it affect?
Due to low spatial resolution
affects quantitative accuracy of PET/CT images
What is incorporated into the reconstruction to address partial volume effect?
Point spread functions (PSFs) in the field of view
What does PSF-based reconstruction reconstruction cause?
edge artifacts & a high degree of overestimation of radioactivity in small regions
When is PET in radiotherapy most widely used?
For NSCL - identifies collapse region/consolidation
What happens to GTV in many cases?
It is reduced which opens up the potential to dose escalate
How is PET/CT used in dementia?
As an indicator to differentiate Alzheimer’s disease from other types of dementia in its early stages
How is PET/CT used in epilepsy?
Localization of epileptopgenic foci
How is AI used in PET?
Assist in classifying tumours/areas in brain with less/more uptake than expected
How is PET/CT used in cardiology?
For coronary artery assessment - evaluates significance of individual stenoses and provide valuable prognostic information
What does the combined PET multi-detector CT system allow for in cardiology?
Direct comparison of a PET study with CT coronary angiography performed in the same sitting
In cardiac sarcoid imaging why is it crucial for patients to follow an appropriate diet?
To suppress myocardial glucose utilization
What are 2 new applications of PET/CT for inflammation?
Patients with psoriasis
Patients with HIV
What is the purpose of the iMAR algorithm?
Increases the confidence of the interpretation of the PET/CT scan & influences SUV
What is the purpose of respiratory motion correction?
AI does automatic amplitude-based gating which virtually freezes respiratory motion
What are 2 challenges of dual modality PET/MRI scans?
Radiofrequency interference between 2 systems
PET detectors working in high magnetic fields
What are 2 advantages for dual modality PET/MRI scanners?
reduces overall radiation dose
reduce positron range effect
What data is used to perform AC of PET/MRI?
MR data
What are the 5 different image classes of MR/
air
lungs
fat
fat-nonfat tissue mixture
non fat
What are 4 advantages of PET?
Differentiate malignant from benign
determine extent and stage of cancer
detect unsuspected metastatic disease
differentiate between residual scar tissue, radiation necrosis and tumour recurrence
What are 4 limitations of PET?
Range of cancers do not accumulate
FDG also accumulates in inflammatory lesions
lesion < 4mm not seen
Scan takes 10-15 mins
not a screening test
