Chapter 14 Nuclear Medicine II Flashcards
what is SPECT
single photon emission computed tomography
-tomographic views of distribution of isotopes
SPECT is to gamma camera imaging as chest CT is to chest radiographs
describe how SPECT acquires tomographic images
uses gamma cameras to get projection images, which are then processed to generate tomo images
gamma camera heads rotate around a central axis of rotation
what is distance from camera face to central axis
radius of rotation
what type of collimator is typically used for SPECT imaging?
parrallel hole collimators
what are fan-beam collimators
hybrid of parralel hole (y) and converging (x) holes
-better resolution in x-direction
-limited FOV
-used for brain SPECT
how many planar images does the scintillation camera acquire?
rotates 360 degrees around patient and takes 60 or 120 images obtained at 6 or 3 degree intervals
-scanning time ~ 20 MIN
Where are 180 degree rotations of the scintillation camera common?
cardiac imaging
64^2 matrix size
what did SPECT reconstruction originally use and why was it discontinued?
filtered back projection
-low number of counts yielded noisy and streaky tomo images
what is used for SPECT reconstruction now and why is it better than old method?
iterative reconstruction
more accurate and minimize artifacts
common iterative reconstruction method in SPECT
ordered subset expectation maximization
what type of volume data does SPECT generate?
isotropic
allows transverse, saggittal, and coronal views to be generated
what is frequency filtering in SPECT
-improves quality of SPECT images
-low-pass filter remove high frequencies, yielding smoother but less distinct edges for images
-high-pass filters pass higher frequencies, yielding noisier but sharper images with enhanced edge definition
why does SPECT use 2 or 3 gamma cameras
to reduce time required to acquire projection images
why are elliptical orbits used in SPECT
allows distance to the patient to be minimized
activity used for bone scans
800 MBq 99mTc
120 projections
128^2 acqisition matrix
activity used for cardiac SPECT
800 MBq 99mTc
180 degree rotation
60 projections
64^2 acquisition matrix
activity used for neuroendocrine or neurologic tumour
400 MBq 123I
360 degree rotation
60 projections
64^2 acquisition matrix
activity used for white cell scan
20 MBq 111In
360 degree rotation
60 projections
64^2 acquisition matrix
how often is the photopeak window of the pulse height analysis evaluated?
daily using a source that radiates the whole crystal
how are intrinsic floods performed
without collimator
point source 99mTc
assess performance of NaI and associated light detectors
how are extrinsic floods (uniformity) performedz?
check daily
place large disk of 57Co in front of camera
unacceptable non-uniformities
5%
-typically are 2-3%
how is linearity in SPECT checked?
-quadrant bar phantoms
-weekly
explain high-count uniformity acquisitions
-performed monthly for each camera head and collimator
-200 million for 128^2 matrix
-high count floods are used to obtain uniformity correction factors
how is center of rotation QA done
use pt source or line source
monthly
how is head tilt angle QA done
bubble level
quaterly
how is spatial resolution in air assessed
pt or line source
during acceptance testing
what is Jaszczak phantom used for?
assess spatial resolution, unfiformity, and image contrast
quaterly
is spatial resolution for SPECT worse than planar imaging?
yes, it is poorer because SPECT images are derived from planar images
major benefit of SPECT over planar images
improved contrast from the elimination of overlapping structures
what does SPECT image reconstruction amplify?
-image noise
-non-uniformities
how do non-uniformities appear in SPECT images?
as ring artifacts
how are partial ring artifacts produced
from multi-head SPECT systems when projections are not acquired by all heads over a 360 degree arc
scintillators used for PET
bismuth germanate (BGO)
lutetium oxyorthosilicate (LSO)
gadolinium oxyorthosilicate (GSO)
important PET scintillator properties
photoelectric absorption efficiency
energy resoltuion
light decay time
compare the different PET scintillator properties
BGO has best absorption but worse energy resolution\
GSO and LSO emit more light- better energy resolution
GSO and LSO: shorter decay time- better performed at high count rate
how are PET crystals arranged?
6x6 or 8x8 blocks
36-64 crystals per detector block
each block is coupled to array of PMT, which provide positional info and offer PHA capability
how are detector blocks arranged?
in a ring
several rings are arranged concentrically
how many crystals and blocks in current PET systems
up to 20000 crystals in 400 blocks
what energy range is used to identify annihilation photons in PET
450-550 keV
true coincidence event
two interactions have to occur within a time interval tau
what % of detected photons are accepted by coincidence circuitry?
1%
what is line of response
simultaenous detection of 2 events
what is the line of response data used to create?
sinogram
projection vs angle
what is used to reconstruct images in PET?
iterative reconstruction
time of flight PET
uses difference in arrival times of the 2 annihilation photons
-improves spatial res
what coincidences are there in PET?
true
scatter
random
2D mode vs 3D mode PET
2D mode: septa are present to define planes. Coincidences are not detected between adjacent rings.
3D mode: septa are removed. More sensitive but more scatter and random events. Coincidences are detected between adjacent rings
are septal collimators needed for localization of photons in PET
No
what is FORE in PET
fourier rebinning
used to rebin 3D data into 2D data sets
3D vs 2D PET scanner sensitivity
3D is 6 X higher than 2D
most common PET radionuclide
18F
how is image plane uniformity in PET QAd
-cylinder filled with positron emitter
-every 2 weeks
how is detector calibration done in PET
-positron source in FOV
-frequency is recommended by vendor
how are PET sensitivity counts measured?
counts/MBq
sleeved rod source
annually
how is spatial resolution of PET assessed?
inspect sinogram and image of a point source
check annually
how are count rate performance and scatter fraction of PET assessed?
-line source in polyethylene cylinder
-annually
how is PET system performance evaluated?
-includes uniformity ad hot sphere contrast
-standard ACR phantom
-every 6 months
compare PET images to planar gamma camera images
-PET more sensitive, high counts, lower mottle
-PET uses several million counts, 10X more than planar images
how is ultimate limit of spatial resolution determined?
range of positron
82Rb has worse res than 18F because of longer distance travelled by 82Rb positrons
spatial resolution of PET detectors
5 mm FWHM
PET resolution is worse at edge than center because 511 kEv photons may be detected by adjacent detectors
where do partial volume artifacts occur in PET?
for lesions < 2 X FWHM (i.e. 10 mm lesions)
-activity of small lesions is blurred by imaging system (FWHM)
what do faulty detectors cause in PET?
angled black line in sinogram
faulty block yields thicker angled black strip in sinogram
why do we co-register nuc med images with CT?
improve lesion localization
attenuation correction
how many slices in CT scanner for PET/CT
16 for most
64 for cardiac
how long does it take to do spiral CT scan from eyes to upper thigh?
20 s
CT image acquisition parameters
120 kV
512^2 matrix size
5 mm slice thickness
pitch of 1.5
SPECT and PET acquisition time vs CT acquisition time
20-30 minutes vs 20 s
CTDI for PET/CT CT scan
2 mGy (L) for low dose for attenuation correction and fusion only
15 mGy for disgnostic quality image
why does SPECT show lower activity at patient center vs periphery?
attenuation
CT image is used to correct the attenuation factor and show the true activity distribution
what cm of tissue attenuates half the activity of 99mTc
5 cm attenuates half
10 cm reduces activity to 1/4
most common use of SPECT/CT
myocardial perfusion imaging
where does misregistration artifact come from
SPECT and CT are not acquired simultaneously- any organ motion can yield misregistration artifacts
what body parts are most concerning for creating apparent defects in SPECT due to attenuation
diaphram
breast
in PET and SPECT, how to correspoind attenuation factors with CT image
since CT energy is lower, have to extrapolate attenuation coefficients for PET/SPECT based on known attenuation properties of tissues
-issues with metallic implants
what does attenuation in PET depend on?
TOTAL thickness of tissue
for a large person, what is % loss of counts due to attenuation in PET?
95%
what can cause hot artifacts in CT attenuation corrected PET images?
contrast material
implanted metal objects
increased acitivity near venous structures if intrevaneous contrast is used
hot spots adjacent to strong attenuators should always be confirmed on non-attenuation corrected images
what activity is administered for PET
600 MBq of 18F 90 min prior to iaging
axial coverage in PET for single detector position
20 cm
time for PET scan at each detector position
2-3 min
current PET scans use 5 detector positions to cover body
what organs receive the highest doses
those that take up activity
reduced organ size = higher dose
dose is high for thyroid, spleen, gallbladder, liver
highest organ doses from diagnostic nuc med
10-50 mGy
average effective dose for 99mTc
4 mSv
1.5 mSv for lung scans to 9 mSv for stress cardiac studies
effective doses for PET scan using 800 MBq 18F
8 mSv
if CT portion is diagnostic- get additional 15 mSv
if CT is for anatomy and attenuation correction, get additional 2 mSv
what is nice about beta minus emitters?
beta particle energy is primarily deposited in the organ taking up the radionuclide
what is 131I administered for?
hypothyroidism
thyroid cancer
thyroid dose to a patient receiving 370 MBq od 131I with 50% thyroid uptake is 200 Gy
radionuclides used for therapy
phosphorus-32 -polycythemia vera
strontium-89 - osseous mets
yitrium-90 - liver neoplasms
iodine-131 - thyroid
doses from patients after injection of radipharmaceuticals at 1 m
10 uSv/h for most
100 uSv/h for 18F
50 and 300 uSv/h for 131I hyperthyroidism and cancer
when is NRC notified- unintended radiation exceeds what level?
50 mSv effective dose
500 mGy organ dose
when can 131I patients be released?
activity < 1.2 GBq
breastfeeding cessation?
-not needed for 18F
-24 h for 99mTc
several months for 131I
a week got 67Ga or 111I
where should volatile radionuclides be stored?
fume hoods
dose rate to hands from radiopharmaceutical (lead) syringes
0.2 mGy/h/MBq
syringe shields
reduce extremity doses threefold
how are extremity doses monitored?
ring dosimeter
what do nuc med operators who risk intake of radionuclides undergo for safety?
bioassay (thyroid monitoring for iodine uptake)
what is major source of staff exposure in nuc med?
patients who were administered several hundred MBq of 99mTc
are lead aprons effective in nuc med?
less so than in imaging because of higher gamma ray photon energies
nuc med tech’s effective dose
2-4 mSv/year
what is done to packages with radioactive labels?
monitored for surface contamination
nuc med safety items
wipe tests
daily surveys in areas where written directive procedures are carried out
what to do in event of minor spill
containment
decontamination
notify RSO
what to do in event of major spill
reqire presence of RSO
what activity constitutes a major spill
> 4 GBq of 99mTc
40 MBq of 131 I
In Canada it’s 100 times exemption
how long before radioactive waste can be disposed of with normal waste?
10 half lives
is excreta in sewer system regulated?
no, people can poop as they need to
bone spect most likely uses what kind of collimator and what energy?
low energy and high resolution parralel hole collimators
are collimators used in PET?
No
spatial info is not obtained from parrallel-hole collimators as in planar imaging, bt by detection of annihilation radiation in coincidence
how is spatial info in PET obtained?
use PHA to identify 511 keV photons
in coincidence to generate lines of response
when are personel provided with a dosimeter?
occupational dose limit exceeds 10% of a dose limit
In Canada when you approach 5 mSV per year or 50 mSV per year to hands or feet
number of projection images in SPECT study
60
what radionuclide is used to get extrinsic gamma camera floods?
57Co
what does SPECT image reconstruction do to non-uniformities
amplifies them
is spatial resolution of 82Rb better or worse than 18F?
worse because 82Rb travels further
CTDI for scan for attenuation correction only
2 mGy
less than diagnostic CT scan
average patient effective dose for 99mTc
4 mSv
what would ideal radionuclide emit?
purely beta minus
-electrons would deposit all their energy in the uptake region
spatial resolution of gamma camera
2-3 mm FWHM intrinsic
8-10 mm FWHM with collimator
CT is 100 um in comparison
components of gamma camera
-multihole or pinhole collimator
-NaI scintillator- turns energy from gamma ray into many light photons
-light pipe: directs scintillation photons onto phototube
-PMT: convert energy from scintillator photons into electronic signal
-energy circuit: measures energy of detected photon
arrangement of PMTs in gamma camera
close-packed hexagonal
where does gamma camera position a gamma that underwent compton and PE effect
-camera will think the PE effect occured at a lower energy-some of the energy was lost to compton
gamma camera scintillator efficiency wrt energy
as gamma energy increases, efficiency goes down, because PE is proportional to 1/E^3
gamma camera- what % of scintillation photons reach PMT array?
30%
light photon to photoelectron conversion efficiency in NaI
~ 25%
how does gamma camera relative energy resolution and intrinsic spatial resolution vary with E?
1/root(E)
energy windows for gamma camera
-lower energy counts are from Compton
-min energy is from backscatter out of detector or in patient
-max scatter energy is from backscatter in detector
-multiple scattered photons have very low E
light pipe
-couples PMT to NaI crystal
-thicker light pipe improves linearity but reduces intrinsic spatial resolution
pincushion distortion
increased count density in center of PMT tubes
spatial non-linearity in gamma cameras
-non-random mis-positioning of events
-results from positioning circuits shifting location of an event towards closest PMT
-look-up tables correct for this mis-positioning
-also cause spatial non-uniformities
how to correct for spatial non-uniformities
-acquire image of uniform sheet source and generate correction map
gamma camera collimator material
high Z
high density
collimator geometric efficiency
-for parrallel holes
-fraction of gamma rays passing through the collimator per gamma ray emitted by source
–g is independent of distance from source to collimator (b) exceot for single-hole (1/b^2)
collimator spatial resolution
resolution worsens as source moves away fro collimator face
system spatial resolution
square root of (intrinsic res ^2 + collimator res^2)
-collimator is what limits the system spatial resolution
how to improve intrinsic spatial resolution
-smaller PM tube
-thinner light pipe
-use pre-amplifiers to limit PM tube position summation to only tubes near the interaction event
gamma camera dynamic non-uniformities
-due to different orientations of gamma camera wrt earth’s magnetic field, or temperature changes throughout day
-minimize by magnetically shielding each phototube and through use of on-the-fly tuning
expected uniformity for gamma camera
integral (over whole system) with 3.5% and differential (over ROI) within 2 %
gamma camera count rate performance
front end of gamma camera is paralyzable
back end is not paralyzable
high count rate mode for gamma camera
-adaptive rezoning
-decrease PMT integration time (directly increases error in energy resolution of each event)
-used for 3D cardic imaging, high resolution brain imaging, and PET imaging)
what is adaptive rezoning for gamma camera
-only subset of PMTs closr to location are used to process event
-other zones respond in parrallel fashion
-degrades system energy resolution because less light photons are collected on a per event basis
-degrades spatial resolution since PMTs in one active session are still exposed to light photons from another active session
NEMA
National Electrical Manufacturer’s association
intrinsic energy resolution of gamma camera
FWHM = 9% at 140 keV
gamma camera intrinsic count rate performance
normal = 180 kcps
high mode = 268 kcps
gamma camera system sensitivity
105 counts/s/MBq
high resolution is 73 counts/s/MBq
semiconductor gamma cameras
-energy from interacting gamma ray is converted directly to electronic signal
-more efficient conversion
-better energy resolution
-cadmium zinc telluride (CZT) is semiconductor
blur tomography
activity from defined plane is in focus while planes above or below are out of focus
reconstruction algorithm for rotating gamma camera (SPECT)
filtered backprojection
why does resolution depend on collimator to source distance?
the further away the collimator is, the more voxels the collimator holes will see
SPECT center of rotation error
-misalignment of central image axis with axis of rotation
-causes blurring and donut artifact
SPECT head tilt error
-blurring in coronal and sagittal views of pts far from axis of rotation
SPECT collimator hole misalignment
-collimator holes not parrallel to each other and perpendicular to collimator face- can cause center-of-rotation and head tilt error
SPECT non-uniformity leads to what artifact?
-bull’s eye and ring artifact
-for both static and dynamic non-uniformity
SPECT motion artifacts appear as what?
blurring, streaks
SPECT activity outside of FOV appears as what?
-streaks at edges of reconstructed FOV
SPECT artifacts that cannot be avoided
-system spatial resolution (depends on source to collimator distance b)
-photon attenuation and scatter
compensating for spatial resolution dependence in SPECT
-lesions deep in body not as well resolved as those on surface
-filter with resolution recovery filter
-keep camera head as close to patient as possible
-use converging collimator
-can improve overall spatial resolution but cannot correct for change in spatial resolution with depth
in SPECT- how to compensate for scatter with windows?
-subtract image containing only scattered photons from one containing both scattered and unscattered photons
-use windows to estimate amount of scatter contaminating photopeaks image
holospectral imaging
use several energy windows to create multiple images and use linear alegbra to create scatter-free projection image
-SPECT
can SPECT projection equation with attenuation be inverted using standard filtered backprojection techniques?
no
SPECT- scattered photons- effect on correcting attenuation
-amount of attenuation is less than would be anticipated because scattered photons are detected in place of some of the attenuated, unscattered photons
-2 techniques to account for attenuation- pre-processing and post-processing
-both assume u is constant within a defined boundary and there is no attenuation outside this boundary
problems with SPECT attenuation correction techniques
-assume homogeneous tissue density
-patient contour must be precisely defined
-doesn’t account for collimator response function
-assumes u is independent of depth (u depends on scatter)
-CT SPECT measures u directly for each SPECT slice
type of noise in SPECT planar images
white
SPECT Hahn filter
dampens higher frequency, linear
SPECT Butterworth filter
dampens higher frequency, looks cosine shaped
MTF for perfect response
flat
1
problems with SPECT filtered backprojetion
-accuracy is not good
-slow to acquire
-poor spatial resolution
-better results obtained with alternative inversion technique
-filtered backprojection oversimplies photon detection kernel so that it can invert it- better not to do this
2 processes that photon detection kernel must model in SPECT
-photon propagation from pt of emission to face of collimator, including photon absorption and scatter inside phantom
-camera detection process including photon acceptance probability as function of incident angle and photon energy
MLEM in SPECT
-maximum likelihood expectation maximization algorithm
-iterative technique
-good quality but slow
-error increases for large number of iterations
-based on Poisson noise
-preserves total image counts
-uses a priori info
OSEM in SPECT
ordered subset expectation maximization
-modified from MLEM
-in MLEM, all projection data is used for each iteration
-OS-EM groups projection data into subsets and reconstructed voxel vlues are updated after each subset is evaluated
-faster than MLEM
-includes all advantages of MLEM
-mean square error also keeps decreasing for large number of iterations
crystals in PET compared to SPECT
PET scintillator crystals are thicker (more efficient)
gate window
when a 511 keV photon is detected, a timing gate is opened for a short time interval
-if 2nd photon is detected within this gate window, the 2 events are in coincidence
LOR
line of response
-line between 2 detectors registering coincident events
-assume annihilation event occurred along LOR
probability that 2 random events are detected as LOR in PET depends on what?
increases with count rate
time of flight PET
can figure out timing distance between 1st and 2nd detection and pinpoint where annihilation occurred
width of timing gate window
determined by gantry geometry
-longer than time required for photon to travel across ring of detectors (7-10 ns)
types of events in PET
-true coincident event
-misplaced coincident event
(one or both photons created in a single annihilation event scatter before being detected)
-random event (2 detectors activated by photons from different annihilation events)
formula for true coincident events
true = measured - scattered - random
2D PET mode
only coincident events occuring in same (or possible adjacent) rings are recorded and reconstructed
-absorbing septa are used to prevent photons from other planes from being detected
3D mode PET
absorbing septa are not used and coincident events from any two detectors are recorded and used in the reconstruction
PET data for each patient
1 GB
number of LORs in PET
n(n-1)
n is # of detectors in ring
scintillation crystals for PET
-NaI
-CsF or BaF2- fast but expensive
-BGO- high efficiency for 511 keV photons
-LYSO, LSO, GSO- faster and more efficient than BGO, higher light output = better energy discrimination, faster light decay = higher counting rates
2D PET reconstruction algorithms
-filtered backprojection
-os-em
3D PET reconstruction algorithms
-cone beam and iterative techniques
-filtered backprojection or OSEM with and without attenuation and scatter corrections
factors that affect spatial resolution in PET
-positron range
-non-colinearity
-detector size
explain non-colinearity in PET
Ek of positron must be spent before combinging with electron
-due to Ek of positron and electron, annihilation photons are non-colinear
-variation in emission angle degrades resolution by ~ 2mm at center of whole body
explain detector size effect on spatial resolution
-smaller detectors = better spatial resolution
-compton scatter from one detector to another becomes a problem for small NaI crystals and therefore crystals with higher Z can be used
spatial resolution of current PET scanners
3 mm FWHM
PET artifacts and sources of error
-scatter
-attenuation
-random coincident events
-noise
-deadtime
scatter in PET
-detectors don’t have enough energy resolution to differentiate photons scattered in the patient from unscattered photons
-septa between rings can reduce out-of-plane scattering
-can use techniques to correct for in-plane scatter
-scattered events in PET can appear to come from lines outside the patient’s body (unlike in SPECT)
is attenuation worse for PET or SPECT?
-less of a problem in PET than in SPECT because the 511 keV photons are less attenuated in the tissue than the SPECT photons, and also because 2 photons are detected in coincidence
-since 2phoons are along one LOR in PET, source along the line is equally attenuated independent of depth of annihilation in the patient
how to measure attenuation factor in PET?
-do a transmission scan, prior to injecting the activity into the patient, using a rotating rod source
how to measure random coincidences in PET
-use 2 coincidence windows of the same lenght
-true windown contains both true and randome events
-delayed window only gets random events
-subtract delayed from true
advantages of PET
-better spatial resolution than SPECT (2-3 mm FWHM) but not as good as CT or MRI
-higher sensitivity than SPECT (no collimator) and ring geometry = faster image acquisition
-can correct for attenuation and scatter- images are a quantitative measure of radionuclide
-some radionuclides are biologically important, therefore organic material can be labelled if chemistry is fast enough
advantages of PET
-better spatial resolution than SPECT (2-3 mm FWHM) but not as good as CT or MRI
-higher sensitivity than SPECT (no collimator) and ring geometry = faster image acquisition
-can correct for attenuation and scatter- images are a quantitative measure of radionuclide
-some radionuclides are biologically important, therefore organic material can be labelled if chemistry is fast enough
disadvantages of PET
-requires accelerator to produce radionuclides (they are short lived)
-ring geometry PET scanners are same price as CT scanner
-expensive
