How are positons produced?
proton+ ► positron+ + neutron + neutrino
Produce them in cyclotrons generally
What is FDG and why do we use it?
Fleuro Deoxy Glucose, (a Fl instread of Oxygen version of glucose) is used as a tracer beacuse all cells need glucose, in particular the brain. Cancer cells use lots of glucose.
How do you image positrons?
interaction
detection
what material wouold you use for detection?
- positron meets electron and annihilates, matter is converted into two 511kev gamma rays travelling 180deg to each other. They are detected with a ring of detecters around the patient.
- Crystal scintillation, PMT amplification, discriminator, counter
- If events are co-incident the LOR is recorded (i.e. the line adjoining the two events.
- For detection, NaI crystal you can use few large crystals. LSO is high sensitivity and fast recovery.
Describe the process of filtering in PET detection
- Coincedent (5-20ns delay)
- Correct energy (511keV gammas)
- Can have true, random and scatter.
What corrections do you need for PET detection?
- Correction map for non-identical detectors
- Missing photons casued by pulse-pile up and dead time
- Attenuation correction uses CT or transmission scan to correct for each LOR.
2 Fundamental Limits on PET spatial resolution?
What is a typical value for a PET scanner?
Fundamental reason is maximum path length of positron which depends on it’s energy (18F =2.4mm). can cause 0.2-0.6mm loss in spat res.
If the gammas are not produced at exactly 180deg (residual momentum) can casue 2mm loss in spat res.
LIMIT is approx 3mm, more practically 4.
3 PET reconstruction methods
- Fbp - consistent & rigorous but images don’e often look as good
- Iterative methods - better images but requires more comuting power and success depends on number of iterations. Can you things like ordered subsets to speed up.
- mutual information from anatomical CT or MR to use additional information in the reconstruction. useful for PET-MR.
Define standardised uptake value
what’s a problem with it?
What would an SUV value be affected by in an image?
What else do you correct for?
SUV = Activity concentration (Bq/ml) / (injected dose (Bq) / Body weight (g))
unit of g/ml
problem is that body weight is not always the best predictor, can have other things like BSA or lean body mass
SUVmax,mean,min can be affected by the signal:noise ratio, the coundary of the ROI in which you measure, and the heterogeneity in the ROI.
Also aply biochemical correction for blood glucose level by multiplying by Glucoseplasma(mmol/L)/5(mmol/L).
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3 main medical PET applications
- Cancers - diagnosis, RT planning
- Brains - epilepsy, dementia, tumours
- Hearts - blood flow, plaques, metabolism
Advances in PET detection
- Time of flight - measure time lag between detection of two gammas, helps to determine the position along the LOR. Reduction of distortion.
- Point spread function - correct for angle at which gammas approach crystal detectors. Gamma travels in crystal, if it isn’t coming in parallell, ir may go further and penetrate another crystal, misplacing the LOR.
- Improved scintillation crystals
- PET-MR
- Advantages - simultaneous imaging information, high spatial res from MR, high sensitivy from PET, low dose, motion correction, reduced throughput for patients.
- Disadvantages - compromise PET and MR performance, attenuation correction, limited choice of coils, safety, cost
How do you do attenuation correction with :
- Standalone PET
- CT
- MRI
Standalone PET - do a transmission scan, gamma source rotated around patient to build up correction map. Crude image with segmentation.
X-ray CT - low dose c-rays used to make image suitable for a correction map
MRI - use sequences to simulate a CT image, needs segmentation as image intensity dose not directly relate to attenuation.
