Lecture 5 Flashcards
what are we looking for in nuclear medicine imaging
patient is injected with a radiopharmaceutical, and acquire images of the distribution within the patient during the injection look at flow and uptake and after uptake period, there are two types of images: 2D planar images with a gmma camera. adn 3D tomogrphaic images iwth SPECT or PET
what is FDG
it is glucose metabolism, and it is fluorodeoxyglucose. glucose derivative indicator of glucose metabolism. oncology increased glucose metabolism
what is the imaging for membrane lipid synthesis
choline and F choline, which is used for bipsynthesis of phosophotidylcholaine in tumor cell membrane. Malignancy induced increased cellular membrane synthesis. Used in prostate cancer
what is the myocardial blood flow imaging using
N13 ammonia, which is avidly extracted and retained in myocardium. MEasure myocardial blood flow under rest and stress go to coronary flow reseve and dynamic images with absolute quantification of myocardial blood flow
what measures myocardial viability
N13 is ammonia avidly extracted and retained in myocardium and measure myocardial blood flow under rest. FDG also measures myocardial viabiltiy
what is used for alzheimer’s disease
use the pittsburgh compound, which is C11. It binds amyloid
what is one of the shortest half lives of PET and longest
shortest is C11 and longest is gadollinium
what does a modern PET system consist of
multiple rings of crystals, ring diameter, transverse field of view, axial FOC, CT or MRI
why is it good to use PET/CT and PET/MR at the same time
the CT or MR images are used to correct PET images for attenutation. CT or MR images are good for anatomic localization of uptake
what are some of the issues with mutli-modality systems
space within the gantry for PET detectors, PET detecotrs and magnetic field, and using MRI data for attenuation correction of PET images
what is a coincidence detector
it is annhiblation of the positron occured along the line joining the two detectors
what is time of flight
determine where along the line joining the two detectors the annihilation occurred. Faster detection timing for better localization along line and better image quality
what is the difference between collimation in single photon imaging and positron imaging
with single photon imaging, no positional information detected event could have originated from anywhere in object. Absorptive collimation only detect phorons traveling reduced sensitivity and resolution.
positron imaging is electronic collimation provides positional information without reducing sensitivity or resolution
what is electronic collimation
each detector is in electronic coincidence with a group of detectors on the opposite side of the ring
what are the types of coincidence events
true, scatter, random, multiple
what is the photon detector in PET
it absorbs the photon and converts energy into an electrical signal
what is the scintillator crystal
photon despoits energy in scintillator, energy is converted to light photons, and energy convered into light photons.
what is a photon detector
converts light photons into electrical signal
what are the photodetectors in PET
conventional technology with photomultiplier tubes. IT is sensitive to magnetic fileds, and solid state detectors. Compact immune to magnetic field and electrical signal fluctuates with temperature. SIPMs have fast timing characteristics
what are block detectors
these are 2D arrays of scintillation crystals attached to photodetectors. Light guides define the pattern of light distribution in block crystal. Determines light output (sharing) to an array of photodetectors. Can identify whay crystal detected the photon
what is the detector assembly like for PET
crstals to blocks to modules to rings and 54 crystals in a block, 8 blocks in a module and 32 modules in the scanner, and 13824 crystals in the scanner
what is the 2D PET acquisition mode
collimator septa (tungsten) are positioned between rings, reduces scatter and random coincidences, and sensitivity (trues) is reduced.
what is 3D acquisition mode for PET
collimator septa are removed from scanner. Increases sensitivity but increased scatter and random events. MOdern PET have moved to acquisition mode
what is modern PET do
it usually 3d acquisition mode now
what is modern PET do
it usually 3d acquisition mode now
what is the 2D acquisition for collimation
collimator septa between detector rings and limited number of rings in coincidence
what is 3D acquisition for collimation
collimator septa are removed and all rings in coincidence. sensitivitiy gain 500%
does the 3D PET have increased or decreased randoms
increased randoms because larger FOV for 3D acquisition mode means activity outside the scanner may be detected and iamged degradation
does 3D PET need more or less scatter corrections
the 3D PET scanning required more scatter corrections
what are the spatial resolution limits for PET in positron range
positron travels a certain distance befor annihilation. PET system measures the point of annihilation, which is not necessarily the location of decay. For most radionuclides range is not a large factor influencing spatial resolution in clinical PET. Mitigating factosr is that the positron is emitted isotropically does nto always recieve full energy of decay
what are the issues with noconlinearity with PET
annihilation photons are not emitted in exactly opposite directions. Angular deviation is +-0.25. Degradation in resolution 1.6mm for PET system with a 75 cm ring diameter
what are coincidence count rates
trues proportional to acitivity. randoms are proportional to acitvity squared
can you increase dose in order to improce image quality and reduce dose
no
what is the noise equivalent count rate
it is a surrogate estimate of signal-to-noise ratio in an image after taking into account the true, scatter and random coincidences. Used for iamge quality adn dose optimization studies. Caveats not a metric for lesion dtectionl does not consider reconstruction algorithm
what can improve image quality
increasing dose up to the peak NECR and increasing scan duration
what is attenuation do
it reduction in the number of detected photons (comptom scatter), attenuation increases with depth of spource in medium. Loss of quantitative accuracy and somtimes lesion detectability
is attenuation more severe for PET or SPECT
it is more severe for PET
is it easier to correct for attenuation in PET or SPECT
PET because there is an independent of source positrion within the object
how to you correct for attenuation on PET
multiply the PET sinogram by the attenuation correction factor. To correct for attenuation, need to know the distribuiton of attenuation coefficients mu transmission and CT images are maps of the attenutatiokn coefficients of an object
what are coicidence based transmission scans
transmission rod sources rotate around the apteint with low quality CT scans. USe Ge source and positron emitter, and provides mu at 511 keV, and it is accurarate and exact attenutation correction and acquires blank scan without pateint. it is limited by photon flux, high dead time in near detector. Transmission scans almost double total acquisition time. REduce scan time use segmented attenutation correction
what is the formula for ACF
Blank scan/transmission scan
what is CT based attenutation correction
CT images are attenutation maps and a direct measurement of mu. Benefits are fast acquisition and low noise
what is the problem with CT based attenuation correction
overestimation of bone attenuation coefficient and must adjust for different photelectric components in water versus bone
what is MR-based attenuation correction
MR measures properties of protons, not electrons, adn different techniques for body vs. brain. Need to include predefined attenuation maps of coils, non-patient objects
what is MRAC used for
determine tissue, fat, lung, air- attenuation predetermined coefficient no bone component
what is delayed event measurement
use a seocnd, parallel coincidence timing circuit, the signal from one of the detectors is delaed in time, it is not possible to detect true or scatter events in theis delayed coincidence window. Rnadom events only- provides an estimate of randoms in the prompt circuit
what is the second correction of randoms
randoms from singles calculation. Rnadoms rate for an LOR between the detectors. NECR is trues2/trues+scatter+Kxrandoms
what is the scatter fraction calculation
scatter/scatter+trues
what is the best reconstruction algorithm
it is iterative reoncstruction with TOF
what is the time of flight reconstruction
the TOF reconstruction each pixel along the chord is incremented by the probability that the annihilation occured at the pizel. FOr TOF the noise is spread over frewer pixels along the LOR. The result is improved signal to noise ratio
what is a stnadard reconstruction
all the pixels along a chord are incremented by teh same amount
what are the benefits of time of flight
increased SNR, increased contrast, effective gain in sensitivity (decrease acquisition time).
-contrast higher than non-TOF especially for larger phantom. Contrast almost independent of phantom size.
