27.5 Pet Scanner Flashcards
What radioactive isotope is used in PET SCANNERS
Fluoruine 18, which decays by beta plus to produce a positron, oxygen 18, an electron neutrino, and also a gamma photon
What does the fluorine 18 produce that we end up detecting?
It emits positrons, and about 1mm away from point of emissions this will ANNIHILATE with an electron, which releases TWO GAMMA PHOTONS in oppsite directions
NOT THE GAMMA PHOTONS PRODUCED DURING BETA + DECAY OF FLUORINE 18
Why are the gamma photons produced when positron annihilates with electron OPPSOSITE EACH OTHER
this is due to conservation of momentum
These two gamma are oppsite Esch other, and so detectors can work out where annihilation happened based on the difference in arrival times of the gamma photons , and thus produce an image
What medical tracer is used ik pet scanners typically and why?
Called FDG, basically glucose but one atom removed snd fluorine 18 is added in
Now body will still think it’s glucose, so this is absorbed by respiring tissues, and thus we can see the function of these tissues specially
Why is using fdg and thus pet good for modelling tumours?
Tumours have HIGH RATE OF RESPIRATION
Thus will think it’s absorbing hella glucose, when actually fdg,
Thus we can model the function of the tumour very well using PET
Again how does a pet scan work exactly
1) patient ingests the medical tracer
2) the tracer will emit positrons and close to emission will anhilare to produce two gamma photons travelling oppsite each other due to conservation of momentum
3) as the patient is surrounded by a RING OF gamma cameras which all have own scintillator and photomultiplier tibes
4) based on the delay time between both phtons, the position of annihilation can be accurately determined by a computer which received the signals
5) this is processed to form an image of both function and location of the organ
What’s in the gamma camera ring?
No collimator this time as they will be coming at an angle, but this Problem is fixed because the angles are consistent for oppsite photons
What are the advantages of a pet scan?
1) 3d image of functioning organs can be produced and cross section even tho resolution is bad
2) Same As gamma camera Technique advantaged too
3) good for non invasive technique to identify tumours etc due FDG thing
Disadvantages of pet scan?
1) the radioactive flurione must be produced ONSITE and is expensive
2) these scans tend to exist in specialist hospitals only , so inconvenient
3) inages produced are of low resolution
4) always some sort of ionisation involved and that’s effort
Why does the radioactive tracer need to be produced on-site again
Why is this a disadvantage
Because half life so low
2) as specialise particle on-site accelerator needed it’s hella expensive
Pet vs gamma
Pet vs cat
Pet vs general
1) 3d , but wayyy more expensive
2) live real time images, shows function , x ray shows anatomy tho , both 3d but gamma isn’t
3) pet is non invasive way fo identifying tumours etc, but hella expensive and inconvenient and has ionising radiation
