introduction to pet imaging
DESCRIPTION
Introduction to PET Imaging. Jack L. Lancaster, Ph.D. Statistical Parametric Mapping Course Fall 2008. RIC RESOURCES. Image Analysis Division Function (Statistical Parametric Maps) Anatomy (spatial measures) Image Normalization Databases (BrainMap/TD) Electrophysiology Division EMG, ERP - PowerPoint PPT PresentationTRANSCRIPT
Introduction to PET Imaging
Jack L. Lancaster, Ph.D.Jack L. Lancaster, Ph.D.
Statistical Parametric Mapping Course Fall 2008Statistical Parametric Mapping Course Fall 2008
RIC RESOURCESRIC RESOURCES
• PET DivisionPET Division– GE 4096 PET (~10 mm)GE 4096 PET (~10 mm)– CTI/Siemens (~ 7 mm)CTI/Siemens (~ 7 mm)– Rodent PET ( ~ 4 mm)Rodent PET ( ~ 4 mm)– Primate PET (~ 3 mm)Primate PET (~ 3 mm)– 2x20 MeV Cyclotrons & 2x20 MeV Cyclotrons &
Radiochemistry LabsRadiochemistry Labs• MRI DivisionMRI Division
– Siemens Trio 3T (<1 mm)Siemens Trio 3T (<1 mm)– Siemens TIM Trio 3TSiemens TIM Trio 3T– 2 Bruker 7T2 Bruker 7T- Bruker 11.7TBruker 11.7T
• Image Analysis DivisionImage Analysis Division
– Function (Statistical Parametric Function (Statistical Parametric Maps)Maps)
– Anatomy (spatial measures) Anatomy (spatial measures)
– Image NormalizationImage Normalization
– Databases (BrainMap/TD)Databases (BrainMap/TD)
• Electrophysiology DivisionElectrophysiology Division
– EMG, ERPEMG, ERP
– irTMS, TMS/PETirTMS, TMS/PET
– ModelingModeling
Cyclotron Production of O-15 Cyclotron Production of O-15 Water for CBF StudiesWater for CBF Studies
14N(d,n)15O
Target Nucleus
Accelerated Particle(deuteron - 1p & 1 n)
Emitted Particle
Product Nucleus
Atomic Numbers: N = 7O = 8.
O-15 Water for CBF StudiesO-15 Water for CBF Studies• O-15 water is an inert tracer. It is freely diffusible
(approximately 95% extraction fraction in primates under normal blood flow conditions) across the blood-brain barrier; thus, the delivered tracer can diffuse quickly into the extravascular space.
• Due to the small size of the water molecule, the distribution of O-15 water in the brain reflects the tissue perfusion at the capillary level.
• Administered as bolus IV injection in subjects arm. • In order for images to better reflect the local cerebral blood flow
distribution, a short imaging interval (~90 seconds after bolus arrives) is used to help lessen the amount of clearance of the tracer.
• Multiple injections with different tasks for CBF studies.
PET Brain ImagingPET Brain Imaging
RadionuclideRadionuclide Half-LifeHalf-Life ++ fraction fraction Max Max ++ EnergyEnergy
O-15O-15 123 sec123 sec 1.001.00 1720 keV1720 keV
F-18F-18 110 min110 min 0.970.97 635 keV635 keV
C-11C-11 20.4 min20.4 min 0.990.99 960 keV960 keV
N-13N-13 9.96 min9.96 min 1.001.00 1190 keV1190 keV
All radionuclides are cyclotron produced
detector
detector
PETPETPPositron Emission Tomographyositron Emission Tomography
• What do we want to detect in PET?What do we want to detect in PET?– 2 photons of 511 keV in coincidence, coming in a 2 photons of 511 keV in coincidence, coming in a
straight line from the same annihilationstraight line from the same annihilation
e-
e+
unstable nucleus emits positron positron annihilates with electron
two 511 photons are emitted
simultaneously in opposite directions
TRUE coincidence
Types of CoincidenceTypes of Coincidence• True coincidenceTrue coincidence is the simultaneous detection of the two emissions resulting from a single is the simultaneous detection of the two emissions resulting from a single
decay event. decay event. Increases with system sensitivity.Increases with system sensitivity.
• Scatter coincidenceScatter coincidence is when one or both photons from a single event are scattered and both is when one or both photons from a single event are scattered and both are detected. are detected. Increases with body size, energy of scattered radiation is less than 511 keV. Increases with body size, energy of scattered radiation is less than 511 keV.
• Random coincidenceRandom coincidence is the simultaneous detection of emission from more than one decay is the simultaneous detection of emission from more than one decay event. event. Increases with radioactivity level and sensitivity of detectors.Increases with radioactivity level and sensitivity of detectors.
True Scatter Random
Want to form images using only true coincidence.
• PET scannerPET scanner– TTyypicalpical configuration: configuration:
• whole-body (patient port 60 cm; axial FOV~15 cm)whole-body (patient port 60 cm; axial FOV~15 cm)
• Detectors - BGO scintillation crystals with PMTsDetectors - BGO scintillation crystals with PMTs
• cylindrical geometrycylindrical geometry
• 32 rings of detectors32 rings of detectors
• >500 detectors/ring (>18,000 total )>500 detectors/ring (>18,000 total )
• several millions of Lines Of Response (LORs) several millions of Lines Of Response (LORs)
True True
O-15 water – cerebral blood flow (CBF)
T-1/2 ~ 2 minutes
F-18 FDG - glucose metabolism
T-1/2 ~ 110 minutes
PET CT
PET data acquisitionPET data acquisition• Organization of dataOrganization of data
– True counts in LORs are accumulatedTrue counts in LORs are accumulated
– In some cases, groups of nearby LORs are grouped In some cases, groups of nearby LORs are grouped into one average LOR (“mashing”)into one average LOR (“mashing”)
– LORs are organized into projectionsLORs are organized into projections
etc…
Scatteredcoincidences component
Attenuation
Random coincidencescomponent
Detector efficiency
effectsTrue
coincidencescomponent
PET image reconstructionPET image reconstruction• Data corrections are necessaryData corrections are necessary
– the measured projections are not the same as the the measured projections are not the same as the projections assumed during image reconstruction projections assumed during image reconstruction
Object(uniformcylinder)
projectionmeasured
projectionassumed
integral of the activity along the line of
response
PET data acquisitionPET data acquisition• 2D and 3D acquisition modes2D and 3D acquisition modes
septa
2D mode (= with septa)
3D mode(= no septa)
In the 3D mode there are no septa: photons from a larger number of incident angles are accepted, increasing the sensitivity.
Note that despite the name, the 2D mode provides three-dimensional reconstructed images (a collection of transaxial, sagittal and transaxial slices), just like the 3D mode!
PET image reconstructionPET image reconstructionSinogramObject
r
SinogramObject
r
PET image reconstructionPET image reconstruction
SinogramObject
r
PET image reconstructionPET image reconstruction
SinogramObject
r
PET image reconstructionPET image reconstruction
r
SinogramObject
PET image reconstructionPET image reconstruction
SinogramSinogram
SPECT: 360º (1 photon)
PET: 180º (2 opposite photons)
PET Image ReconstructionPET Image Reconstruction• 2D Reconstruction2D Reconstruction
– Each parallel slice is reconstructed independently (a 2D sinogram originates a 2D slice)Each parallel slice is reconstructed independently (a 2D sinogram originates a 2D slice)
– Slices are stacked to form a 3D volumeSlices are stacked to form a 3D volume f(x,y,z)f(x,y,z)
2D reconstruction
Plane 5
Slice 5etc
etc
2D reconstruction
Plane 4
Slice 4
2D reconstruction
Plane 3
Slice 3
2D reconstruction
Plane 2
Slice 2
2D reconstruction
Plane 1
Slice 1
Sinograms Tomographic Images
Key Factors in Spatial Key Factors in Spatial ResolutionResolution
RRdetdet - Detector resolution which relates to size, spacing of - Detector resolution which relates to size, spacing of
detectors in the ring. Independent of radionuclide.detectors in the ring. Independent of radionuclide.
RRrangerange - Resolution component due to range of positron - Resolution component due to range of positron
within tissue of interest.within tissue of interest.
RR180180 - Resolution component due to non-colinearity of - Resolution component due to non-colinearity of
511 KeV photons (~0.5˚). 511 KeV photons (~0.5˚).
€
Rsys ≈ Rdet2 + Rrange
2 + R18002
RR180180 ~ 0.0022xD where D is distance in mm between ~ 0.0022xD where D is distance in mm between
detectors that are in coincidencedetectors that are in coincidence
Example:Example: whole-body PET systemwhole-body PET system
D = 80 cm D = 80 cm RR180180 ~ 2 mm. ~ 2 mm.
RRrangerange depends on positron energy (O-15 max energy = 1720 depends on positron energy (O-15 max energy = 1720
keV, F-18 max energy = 635 keV) and tissue, but for soft keV, F-18 max energy = 635 keV) and tissue, but for soft tissuetissue
FWHM: FWHM: O-15O-15 0.501 mm0.501 mm
F-18F-18 0.102 mm0.102 mm
€
Rsys ≈ Rdet2 + Rrange
2 + R18002
RRdetdet improves with smaller detectors and better imager designs.improves with smaller detectors and better imager designs.
RRrangerange is a function of max energy of positron and tissue type. Does is a function of max energy of positron and tissue type. Does
not change with different systems (whole-body vs. small animal).not change with different systems (whole-body vs. small animal).
RR180180 ~ 0.0022xD decreases with diameter of rings.~ 0.0022xD decreases with diameter of rings.
€
Rsys ≈ Rdet2 + Rrange
2 + R18002
Spatial Resolution Summary
PET Imaging EvolutionPET Imaging EvolutionHuman brain
Animal PET~1998
Monkey brain
Image credits: CTI PET Systems (Now Siemens)
Image credits: Crump Institute, UCLA ~4 mm FWHM~4 mm FWHM
~7 mm full width half maximum of ~7 mm full width half maximum of point spread function (FWHM)point spread function (FWHM)
Noise In PNoise In PETET Images Images• Noise in PET images is dominated by the counting statistics of the Noise in PET images is dominated by the counting statistics of the
coincidence events detected.coincidence events detected.• Noise Noise can becan be reduced at the cost of image resolution by using an apodizing reduced at the cost of image resolution by using an apodizing
window on ramp filter in image reconstructionwindow on ramp filter in image reconstruction (FBP algorithm) (FBP algorithm)..
105 106 107 counts
Unapodized ramp filter
Hanning window, 4mm
Hanning window, 8mm
Speech Production
Verb Generation
Petersen, Fox et al., Nature, 1988
PET Cerebral Blood Flow (CBF) PET Cerebral Blood Flow (CBF) O-15 Water as TracerO-15 Water as Tracer
Difference in task Difference in task images images CBF CBF
Left hand movementLeft hand movement Right hand movementRight hand movement
LR
Left primary motor cortex
Left supplementary motor area
Right primary motor cortex
Right supplementary motor area
Activation pattern during paragraph reading in patients with Idiopathic PD hypophonia. A: Pre treatment and B: Post treatment.
L-SMA
Bilateral SMA
A
B
Z=56 X=52
Right primary mouth motor cortex
Right primary mouth motor cortex
Right auditory cortex
Plastic Scintillating Fiber Rodent Plastic Scintillating Fiber Rodent Arterial Monitoring SystemArterial Monitoring System
PET/CTPET/CT
CT PET CT+PET
General Electric Medical SystemsGeneral Electric Medical Systems
Spatial NormalizationSpatial Normalizationof Brainsof Brains
Global (GSN) - 4x4 affine transform matrixGlobal (GSN) - 4x4 affine transform matrix
Regional (RSN) – Discrete Deformation Regional (RSN) – Discrete Deformation Vector FieldVector Field
Spatial transforms to correct for differences Spatial transforms to correct for differences in brains before group analysesin brains before group analyses
Fitted AC-PC LineFitted AC-PC Line
CCCC TNTN SCSC CBCB
Global Spatial Normalization Global Spatial Normalization of Anatomical MRIof Anatomical MRI
GSN using Convex Hull TemplateGSN using Convex Hull Template15O-water 18F-FDG MRI
B4
After
Talairach Convex Hull TemplateTalairach Convex Hull Template 3 different brains
Axial section views following GSN of MRI and O-15 PET images of single Axial section views following GSN of MRI and O-15 PET images of single brain image and averages from brain image and averages from 12 subjects12 subjects. .
MRIMRI
PETPET
AverageAverageSingle StudySingle Study
Z = +1 mm
Talairach Reference SystemTalairach Reference System
• AC-PC line (y-axis)AC-PC line (y-axis)
• AC as originAC as origin
• Bounding BoxBounding Box
– 136 x 172 x 118 mm136 x 172 x 118 mm
• Right-handed Right-handed coordinate systemcoordinate system
Origin(AC)
BrainMap BrainMap DatabaseDatabase
Web Based SearchesWeb Based Searches
http://http://www.brainmap.org:9000/www.brainmap.org:9000/
bmapWeb/Experiments.jspbmapWeb/Experiments.jsp