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Multimodality PET/CT Imaging
Paul KinahanImaging Research Laboratory
Director, PET/CT PhysicsDepartment of Radiology
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Types of Images
René Magritte The Treachery of Images 1928
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Types of Images: Projection Imaging
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Types of Images: TomographyImaging
basilar tip aneurysm
tomographic acquisition reconstruction of multiple imagesform image
volume
image processingsimple sophisticated
transaxial or axial view coronal view sagittal view
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Two Types of TomographyʻTomoʼ + ʻgraphyʼ = Greek: ʻsliceʼ + ʻpictureʼ
PET: EmissionCT: Transmission
source
detector
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radiofrequencymicro-wave
TVFMAM
IR UV
Optical(not to scale)
X-raygamma
-raycosmic
-ray
The Electromagnetic Spectrum
Transmission through 10cm of tissue (i.e. water)
0.0
1.0
longer wavelength higher energy
low resolutionregion
(long wavelength)
high resolutionregion
Physics of PET and CT Imaging
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Modality Resolution (mm) TX or EM* ModeX-ray 0.1 – 1.0 TX ProjectionNuclear 10 – 20 EM ProjectionMedicineX-ray CT 0.5 TX TomographicUltrasound 0.3 TX (sound) TomographicMRI 1 EM (RF) TomographicSPECT 10 EM TomographicPET 5 EM Tomographic
*(TX = transmission, EM = emission)
Major Medical Imaging Modalities
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How it works: Positron Emission
Radioactive decay• decays to stable form by
converting a proton to a neutronand ejects a 'positron' to conserveelectric charge
• positron annihilates with anelectron, releasing two anti-colinear high-energy photons
• detection system is ~1-5%efficient, and can be madequantitative
npnp
n
pnp n
pn
pn p
p
pn
p n
pn
p
n
p n npnp
n
pnp n
pn
pn p
n
pn
p n
pn
p
n
p n
~2 mm
18F 18O
~180 deg
E = mc2
= 511 keV
β+
e-
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How it works: Scintillation
high energy511 keV photon
optical photons (~ 1eV)
scintillator(e.g. BGO Denseyet transparent)
currentpulse foreach UVphoton
detected
photomultipliertubes (PMTs)gain of ~ 106
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PET Detector Block
Reflective lightsealing tape
Two dualphotocathode PMTs
• PET scanners are assembled in block modules
• Each block uses a limited number of PMTs to decode anarray of scintillation crystals
gamma raysscintillationlight
signal out toprocessing
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Typical PET Scanner Detector Ring
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Block matrix: BGO crystals6 x 8 crystals (axial by transaxial)Each crystal:
6.3 mm axial4.7 mm transaxial
Scanner constructionAxial:
4 blocks axially = 24 rings15.7 cm axial extent
Transaxial:70 blocks around = 560 crystals88 cm BGO ring diameter70 cm patient port
13,440 individual crystals
Inside GE Discovery STE PET/CT
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How it works: Timing coincidence
Δt < 10 ns?
detector A
detector B
recordpositrondecayevent
scannerFOV
β+ + e-
annihilation
reconstruct imageof tracer uptake
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PET Image Formation Workflow
PrimaryDetection Decoding Detector
corrections
CoincidenceProcessing
DataBinning
DataCorrections
ImageReconstruction
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Quantitative errors in measurement
Lost (attenuated)event
Scattered coincidenceevent
Random coincidenceevent
incorrectly determined LORs
Comptonscatter
no LOR
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Effects of Attenuation: Patient Study
PET: withoutattenuation correction
PET: with attenuationcorrection (accurate)
CT image (accurate)
Enhancedskin uptake
reducedmediastinal
uptake
Non-uniformliver
'hot' lungs
Errors in attenuation correction can dominate image quality
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• CT images are also used for attenuation correction (CTAC) of thePET data
• Note that images are not really fused, but are displayed as fusedor side-by-side with linked cursors
PET/CT Scanner Physiology
X-rayacquisition
Anatomical (CT)Reconstruction
PET EmissionAcquisition
CTImage
Translate CT to PETEnergy (511 keV)
Smooth to PETResolution
Attenuation CorrectPET Emission Data
Functional (PET)Reconstruction
PETImage
Displayof PETand CTDICOMimagestacks
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Basic PET/CT Architecture
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Commercial/Clinical PET/CT Scanner
PET detector blocksthermal barrierrotating CT system
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x-ray tube
patient
couch
detector array
rotating gantrywith tube and
detectors attached
x-ray fan beam
How it works: CT Scan Concept
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CT Scanner in Operation
64-slice CT, weight ~ 1 ton, speed 0.33 sec (180 rpm)
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X-ray CT Tubes• Rotating anode tube
(dissipates heat to allow higher beam currents)
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Modern X-Ray Tube
Large Patient
Large Patient
Electron Collector: reduce off-focal radiation• Lower patient dose
High Peak-Power Target & Bearings• High peak-mA for fast rotation
6862400.35
5712000.35
6002400.4
5002000.4
4002000.5
mAneeded
typicalmAs
Rotationspeed (s)
kVp mA Small Spot mA Large Spot80 10-300 305-675100 10-310 315-770120 10-335 340-800140 10-335 340-715
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What are we looking at?
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Molecular Imaging: Glu Metabolism
FDG-6-PO4 is ‘trapped’and is a marker forglucose metabolic rates*
glucose
glucose 6-phosphate
pyruvate lactate
gylcolysis(anaerobic,inefficient)
TCA(oxidative,efficient)
HOCH2
H 18F
HOH HHO
H
OH
H
radioactivefluorine
O
[18F]fluorodeoxyglucose (FDG)
what wesee
FDG
FDG 6-phosphate
X
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Imaging FDG uptake (PET) & anatomicallocalization (CT)
Function Function+Anatomy and CT-based attenuation correction
Anatomy
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Diagnostic Accuracy of PET/CTexceeds CT or PET only
Weber et al. Nature Reviews Clinical Oncology 2008
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PET with 18F-FDG is used for cancer imaging
• Currently ~ 92% of all PET/CT studies are for diagnosis and staging inoncology imaging
• About 5000 scanners world wide• 2.5 millions scans done annually in US
everythingelse
Cancerdiagnosisand staging
• Neuro• Cardiac• Therapy
planning• Therapy
development
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Castell and Cook, British J Cancer 2008
Pretherapy
1 wkimatinibtherapy
CT PET/CT
PET SUV5 to 1.8
Response to therapy of liver met gastric GISTNo morphological change in the metastasis
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What are the advantages of PET/CT?
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Meikle PMB 2005
Sensitivity
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Quantitation
MR CT
PET US
What do the image values represent?
Linear with position and tracer concentration
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Quantitation
MR CT
PET CT
What do the image values represent?
Linear with positionLinear with position and tracer concentration
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Improvements and/or Artifacts
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• Modified NEMA NU-2 Image Quality Phantom (30 cm x23 cm cross section)
• Sphere diameters:1.0, 1.3, 1.7, 2.2, 2.8, 3.7 cm• 4:1 target:background ratio and typical patient activity• RC = measured / true
Resolution Effects
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2 cm sphere
5 cm sphere
33 cm
profile
Resolution Effects
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Image Reconstruction: Modeling DetectorBlurring
Inter-crystal scattering Parallax error
trueLOR
variabledepth of
interactionassignedline of
response(LOR)
Shape of detector blurring point spread function (PSF)• Radially variant• Asymmetric in transaxial direction• Two-fold symmetric about FOV center
crystalthickness
trueeventcrystal
assigned event crystaldue to scattering
scintillation(Compton scatter)
light collection
annihilationphoton
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Spatially-Variant Image Resolution
standardOSEM
OSEM withdetector blurring
modeled
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Including improved physics modelingin image reconstruction
• In principle can remove detector blurring
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Phantom measurements: ringing artifact
Bai, 2010 IEEE MIC conf record
real?
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Patient shifting• Large change in attenuation going from spine to lung
?
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Breathing Artifacts: Propagation of CT breathingartifacts via CT-based attenuation correction
Attenuation artifacts from CT can dominate true PET tracer uptake values
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Image Smoothing: Noise vs. Resolution
• Always a trade-off in noise vs. resolution• The choice of the best filter to use with the reconstruction
algorithm depends on the clinical task• There are no standards for choice of smoothing
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Effect of changing smoothing
10 mm smoothing4 mm smoothing 7 mm smoothing
RC for 1 cm spheres
0.85
0.92
0.52
0.80
0.40
0.72SNM Chest phantom: True RC is 1.0
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Image Quality
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Image QualityImage quality, for the purposes of medical imaging,can be defined as the ability to extract desiredinformation from an image
- Harrison H. Barrett PNAS, 1993
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Traditional measures: Resolution
• Point-spread function (PSF): Narrower is better• Modulation transfer function (MTF), which is the
absolute value of the frequency-space version of thePSF: Wider is better
• FWHM, FWTM, FW100M, etc - but these can onlyapproximate the PSF
• Bias is related to the PSF, but in a non-trivial way
truth or input scanner output
imaging processPSF
FWHM
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Traditional measures: Noise• Sensitivity: response to very low activity levels• More counts -> lower noise -> Better SNR• Noise Power Spectrum: noise power at each
frequency• Note that apparent noise in a single image, is not
necessarily the same as true noise measured frommultiple images.
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Law of conservation of difficulty• There are always trade-offs: In this case usually
noise vs. resolution or bias• looking at the range of values is important to be fair• looking at the operating point may be the most
important, but can be difficult to determine
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How do you compare images?• define task
– detection– localization– estimation (quantitation)– shape discrimination– combinations of the above, etc.
• measure (quantitate) task performance• these are often time consuming studies and can be difficult to
perform properly• we can in some cases use computer models of human
performance -- so called ‘model’ or ‘computational’ observers --that are based on the human perceptual system
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Noiseless
lesion:background1 : 1.2 : 1.5 : 2
100 kcounts
10 kcounts 2 kcounts
Detectability: Is it there?
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Decreased resolutionNoiseless 100 kcounts
10 kcounts 2 kcounts
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Correlated Noise Introduced byImage Reconstruction
Correlated
Uncorrelated
1M Counts 0.1M Counts
No Noise (reconstructed)
True Object
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Human abdomen simulation with 2cm diam. lesion 2:1 contrast
Resolution Effect of Smoothing vs. Noise
more counts(less noise)
less smoothing(more noise)
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Image QualityImage quality, for the purposes of medical imaging,can be defined as the ability to extract desiredinformation from an image
- Harrison H. Barrett PNAS, 1993
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Diagnostic Accuracy of PET/CTexceeds CT or PET only
Weber et al. Nature Reviews Clinical Oncology 2008
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AcknowledgementsUW Imaging Research Laboratory