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27 Cards in this Set
- Front
- Back
- 3rd side (hint)
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How does PET differ from standard NM procedures?
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Shows metabolism and blood flow on a cellular level. It is the only imaging modality to do so
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What is fMRI?
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functional MRI, state of the art imaging modality that shows function based on regional blood flow
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What is FDG?
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fluorodeoxyglucose
How does it work? |
It gets trapped in the cells during glucose cycle
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How does finger exposure of F18 compare to Tc99m?
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5 times more
Tc-99m - 4mrem/sec I-131 - 3mrem/sec F-18 - 19mrem/sec |
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What are the 2 methods where proton rich isotopes become stable?
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1. positron emission - changes proton into neutron
2. electron capture - changing proton into neutron |
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What are PET detectors designed to pick up?
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2 511 keV photons 180 deg apart
How are they created? |
positron interaction with electron causes annihilation resulting in 2 511 keV photons traveling 180 deg apart
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Is collimation necessary on a PET scanner?
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detectors are looking for 2 511 keV positron annihilation photons detected in coincidence making collimation unnecessary but septal rings used on 2D imaging to reduce cross-talk scatter
What is this called? |
ACD - annihilation coincidence detection
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What is the arrangement of crystals and PMT's on a PET scanner?
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groups of crystals are arranged in a ring around the patient, each group has one PMT. There are septal rings between detectors.
Why are these septal rings sometimes removed? and what problem does this cause? |
When doing 3D, scatter is increased
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What are the 3 coincidence types for PET?
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scattered
random true |
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Why is a high count rate required to get a good PET scan?
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for every coincidence event at least 100 are discounted
What is recorded when a coincidence event takes place? |
1. position of the detectors
2. energy information |
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Why do random coincidences present the largest problem?
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Trues increase in direct proportion to an increase in activity while randoms increase with the square of the activity
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In 2D mode _________ between rings is used, which ______ counting and lowers __________, while in 3D mode no _________ is used and any LOR is used, while in 2D the same of adjacent ring is only used. 3D has __ times the sensitivity but ______________________ also increase. With the greater count rate in 3D _______________________ are required.
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In 2D mode shielding between rings is used, which limits counting and lowers sensitivity, while in 3D mode no shielding is used and any LOR is used, while in 2D the same of adjacent ring is only used. 3D has 5 times the sensitivity but random and scatter coincidences also increase. With the greater count rate in 3D newer/faster algorithms are required.
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What is the most widely used detector crystal for PET, that has the greatest sensitivity?
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BGO - bismuth germanate
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Why is the sensitivity greater with PET and by what factor?
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10-20 times because little or no collimation is used
How does this effect the dosage? |
relatively lower doses are required
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How is compton scattering limited in PET scanning?
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Attributed to the composition of detectors, BGO, which has high density and atomic number, which reduces scatter events.
What type of event predominates? |
photoelectric events
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True or False: BGO has a better light yield than NAI(Tl) crystals
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False, NaI(Tl) has the best light yield despite their lower stopping power and other problems
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What is the time of decay in reference to light produced by crystals?
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amount of time for light output to return to 0 after scintillation is initiated
Time of decay is related to what variable in the detector? |
Dead time
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List from best to worst the following crystals in the characteristic of dead time and give the associated value:
NaI(Tl), BGO, LSO and GSO |
LSO - 40
GSO - 60 NaI(Tl) - 230 BGO - 300 |
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Why is the spatial resolution somewhat limited in PET but not by the camera?
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Positrons are traveling prior to annihilation so 180 deg path is not exactly true
What angular spread does this result in? and what is the final resolution capability of a PET scanner? |
plus or minus 0.3 deg
8 mm |
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Name the 9 areas of FDG distribution in a normal scan?
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1. Brain
2. Breast 3. Bowel 4. Heart 5. Liver 6. Salivary 7. Stomach 8. Thyroid 9. Urinary |
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Isotopes that have positron decay have excess ________ and lie _____ the neutron/proton line of stability and are all produced in an ____________
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Isotopes that have positron decay have excess protons and lie below the neutron/proton line of stability and are all produced in an accelerator
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Proton rich isotopes decay by what 2 methods?
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1. electron capture
2. annihilation reaction |
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The average energy of the positron is how much of the maximum?
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1/3
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What does a sinogram map?
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The angle and displacement of each LOR event in the slice
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What is the drawback to using delayed time window method for estimating and correcting the number of randoms in PET acquistion?
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makes for a noise correction
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3D PET has _______ the sensitivity of 2D but ________ more scatter
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3D PET has 4-5x the sensitivity of 2D but 15-40% more scatter
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What makes time of flight PET possible?
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shorter decay time of crystals and faster electronics
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