1Positron Emission TomographyPhysics, Instrumentation, Data AnalysisCarl K. Hoh, MDDepartment of RadiologyUCSD Medical CenterPET Radiation Detectors• Theoretical ideal scintillation material– High density (Z)– Efficient scintillator (high & quick light output)• Newer scintillators (more efficient)– Leutetium orthosilicate (LSO)– Gadolinium orthosilicate (GSO)– Use in 3D mode2Crystals used in PETScintillators Effective Z Decay Index of Relative Peak wavelength (µs) refraction light yield (nm)Sodium iodide 50 0.23 1.85 100 410Bismuth germanate 74 0.30 2.15 13 480Lutetium 66 0.04 1.82 65 420oxyorthosilicateGadolinium 59 0.06 1.85 25 430oxyorthosilicate Barium fluoride 52 0.62 1.49 13 3100.0006 3 220PET Scanner Design• Smaller individual crystal size = betterspatial resolution• Physical limit to size of photomultiplier tube• Crystal blocks– Cut block detector type– Reflector block detector type• Buckets or modules3Block DetectorBGO Crystal cut in 4 x 8 elements4 Photomultiplier tubesBlock Detector DesignsBGO cut-block detectorBGO reflection design block4Distribution of light from a single crystal element to 4 PMTsPulse strengthPhotonLight PipeBlock DetectorImage of the Xpos and Yposdetermined from the fourphotomultiplier tubes outputs.Block detector with 8x8elements and 4photomultiplier tubes5Output Map from each crystal element in a blockExample: 3 Ring of Detector Blocks6Three Rings of Detector Blocks1238 elements per block in the axial direction3 rings in an Siemens HR scanner= 24 elements in axial FOV.AxialFOV =15 cm2D and 3D Detector Configurations• 2D mode: use of septa– Limit axial FOV of events to 1-2 elements– Reduce the number of scatter• 3D mode: retract or no septa– Increase sensitivity x 4-10– But also increase scatter and random events– Decreased sensitivity at end of axial FOV– High count rate: dead time problems– Huge acquired data sets7One block8 direct planes7 indirect planesAxial directionAxial directionseptasepta2D Planar Acquisition (septa)One block2D Planar Acquisition1 238 elements per block in the axial direction3 rings in an Siemens HR scanner= 24 elements in axial FOV.3 x 8 = 24 direct planes3 x 7 = 21 + 2 indirect planesTOTAL = 47 image planesAxialFOV =15 cm83D Acquisition (septa retracted)One blockOne blockOne blockOne blockOne blockOne blockUCSD2D vs 3D Image Acquisition2D (20 min)F DOPA (6 mCi)3D (20 min)UCSD9BGO systems• Relatively poor energy resolution• Need wide energy window: 300-650 keV• Relatively high scatter detection• Need for scatter correctionCoincidence Detection: Trues,Random, & Scatter Events• True event: pair of annihilation gamma rays– 6-12 nanosecond event timing window• Random event: detection of gamma rays from twodifferent annihilation events– Delayed event timing window• Scatter event: due to Compton effect in tissues– 15% of data in 2D mode10Bucket+CMAddress latchAddress latchFourBlocksperBucketMRealTimeSorterMultiplexerMultiplexerSummerComparitorPET vs SPECT Images• Spacial Resolution: 4 mm vs > 12 mm• Count Rate: 50-100 vs 1 (< 1% of pt activity)• Acquisition: simultaneous (ring) vs rotating lineprofile11Reconstruction Algorithms• Filtered Back Projection– Simple– Quick– Streak artifacts• Iterative Reconstruction– Need fast computerFiltered Back Projection Iterative ReconstructionPET Image Reconstruction12Filtered Back ProjectionImage ReconstructionIterative ReconstructionFiltered Back ProjectionImage ReconstructionIterative Reconstruction13Attenuation Correction byTransmission Imaging• Tissue attenuation more visible on PET– Annhilation photon pairs essentially “see” fullthickness of the body• Measured Attenuation– By transmission imaging• Calculated Attenuation– in some head/brain scansPET: Types of Coincident EventsTrue eventScatter eventRandom eventAdds low spatial frequency image noiseUCSD14Fan Pattern of Possible Lines Of ResponseParallel LORs form Projection Image15Projection Data in Sinogram FormatSinogramrθpositionangleUCSDProjection Data Stored as a Projection Data Stored as a SinogramSinogramyxProjectionrθ16I = Io e-µ2rI = Io e-µrrrrAttenuation EffectsSPECT vs PETSPECTPETAttenuation of a pair of annihilation photons17Transmission image from a Rotating TransmissionRod SourceRotatingTransmissionSourceGe-68Blank ScanTransmissionScanFDG injectionUptake period (40min)Position patientEmissionScanPatient offEvents in a Static Brain Acquisitionwith Measured Attenuation1 2 3 4 5 6 7 840 min5 minUCSD18Attenuation CorrectionIrIrRod SourceIrBlank SinogramTransmission Sinogram Attenuation SinogramAttenuation CorrectionIrIrEmission SinogramAttenuation Sinogramx=IrAttn. Corr. Sinogram19FDG Brain Scan (10 mCi)Artifacts with Measured AttenuationCorrection due to Patient MovementMeasured atten. correctionNo correction appliedAttenuation artifact from motionAfter NormalizationAfter NormalizationNormalizationNormalizationBefore NormalizationBefore NormalizationNormalization corrects for variations in Normalization corrects for variations in crystal geometric and detection efficienciescrystal geometric and detection efficiencies(rotating(rotatingplane or rodplane or rodsource)source)UCSD20PET Quality ControlScan Type When to Perform1. Daily Check Scan (blank) Every day2. Bucket Setup When system is drifting3. Normalization Weekly to monthly,or after bucket setup4. Phantom calibration On a new 68Ge phantomUCSDPET Quality ControlDaily Check ScanReferenceBlank SinogramDailyBlank Sinogram2 hr or 200M counts?=If > + 2.5 S.D. : recommend normalizationIf > + 5.0 S.D. : recommend serviceFrom prior normalizationrod sourcesUCSD21PET Quality ControlBucket Setup (2hrs)Adjust constant fraction discriminatorsAdjust x-y position of the position profileAdjust the time alignment (electronics)Adjusts for different type of PMTsReports bad detector blocksneedle sourceUCSDPET Quality Control3D and 2D Normalization (~6.5 hrs)Acquire calibration scan (30 mins)Crystal Efficiency scan (4 hrs or 50M cnts)Calculates Normalization fileSet to the default normalization file.Reconstructs calibration scan with attn corr2D standard blank scan (2hrs or 200M cnts)Computes ECF (correction factor)Phantom SourcePhantom Source: 0.5 mCi for 3D3.0 mCi for 2D2268Ge Cylinder Phantomt1/2 271daysTransaxial Imageof PhantomROIDetermination of Calibration Factorcylinder filled with water and 68Ge activityT1/2 = 271 daysCylinder well counts well-cntscc secCylinder well
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