Introduction To Positron Emission Tomography Collimator

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October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenPlanar and SPECT Cameras SummaryIntroduction to PositronEmission TomographyStephen Bowen, Ph.D.Nuclear Medicine Basic Science LecturesOctober 18, 2011srbowen@uw.edu System components:– Collimator Detector Electronics Collimator– Types: Parallel, Converging, Diverging, Pinhole, Multi-pinhole– Performance: Penetration, Resolution, Efficiency Detector:– Components: Scintillator crystal, Optical spacer, PMTs– Performance: Efficiency, Intrinsic (spatial) resolution, Energy resolution Acquisition modes:– Frame vs List mode– Static (time-averaged), Dynamic (TAC), Gated (cardiac / respiration) Camera QA corrections:– Uniformity, Linearity, Photo-peak window, Multi-energy registration SPECT QA/QC:– Center-of-rotation, Head tilt, uniformityPET Definition Positron– Uses positron (! ) emitting radio-isotopes to label physiologictracers (e.g. glucose metabolism, cell proliferation, hypoxia)– Positrons are unstable in that they annihilate with electrons,resulting in two anti-parallel photons each with energy 511 keV– PET scanners measure coincident annihilation photons andcollimate the source of the decay via coincidence detection Emission– The source of the signal is emission of annihilation photons fromwithin the patient, as opposed to photons transmitted through thepatient in x-ray imagingPositron AnnihilationParent nucleus:unstable due to excessive P/N ratio(18F, 11C, 13N, 15O, 124I)(18O, 11B, 13C, 15N, 124Te) ! !protondecays toneutronP NPN N PP NP NN P!positronemissione 1 mmee !neutrino also emitted(inconsequential to PET) Tomography– Three-dimensional volume image reconstruction through collectionof projection data from all angles around the patientpositron may scatter!positron annihilates withan electron:mass energy is converted toelectromagnetic energy resulting intwo anti-parallelannihilationphotons1

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenTomographic Data AcquisitionEmission Coincidence Detectiondetector itimeAll coincidence events acquired over time allowsdynamic imagingdetector jiRandom ratedeterminedfrom i, j singlesrates!Group coincidence data into parallel projections(LOR) for tomographic reconstructiondetector i-jcoincidencejcoincidence eventsSort LOR into sinogramsand/or savelist-mode dataLORCoincidence Events: Signal and NoisePET detectors seek simultaneous gamma ray absorptions(“simultaneous” within 5-10 ns)PET signal componentsP T S atterRandomT ! "t # rij ! activityR ! "t # ri # rj ! activity dence:one or both photons changedirection from a scatter beforedetectionanti-parallel photons traveldirectly to and are absorbedby detectorsphotons from different nucleardecays are detectedsimultaneouslyNOTE: scattered and random coincidence lines-of-response need not pass through object!ProjectionAngle!rij photonpairdetectionrateindetectorpixelsi,jri singlephotondetectionrateinpixeli S and R has to be estimated and removed Estimation challenges– R estimation accurate and efficient (singles method)– S estimation can have significant errors (e.g. lung)2

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenPET Acquisition: 2D vs. 3D ModeForm of collimation (septa) that separate axial slices in 2D PET- reduces scattered and random events (also reduces trues!)PET Contrast and Quantitation Noise from Scattered Coincidence– Predominantly Compton scatter. Gamma rays scatter off of electrons,change direction and lose energy. results in misplaced events due to change in photon direction (loss ofcontrast) energy discrimination can eliminate scatter (but not all) correction based on scatter equations, scatter object (CT), measureddata Noise from Random Coincidence– Random events proportional to singles rates squared– Mean random events estimated in two ways: measured with delayed coincidence window (direct measure, highnoise due to random rate) calculated based on system singles rates (low noise singles-basedcalculation) Attenuation of Signal– Gamma rays are absorbed in the patient Variability due to heterogeneity of attenuating tissue– Correctable with properly aligned attenuation mapdetector crystalssepta & end shieldingscatter &randomsblocked2D PET uses axial septa3D PET uses no septaSignal and Noise EstimatesPET Contrast: 2D vs. 3D modeScatterFraction(SF)SF SSF T SST S2D 20%3D 34%NEC DSTE Count Rates: NEMA Cylinder Phantom400T2T S !RDSTE Measured S1000NoiseEquivalentCounts(NEC)T2NEC T S !R2DT2DR510Phantom activity (mCi)15603DNEC40202DS0NEC rate (kcps)300Count rate (kcps)TTSNR !! (P)T S !R2DNEC80SignaltoNoiseRatio(SNR)2000510Phantom activity (mCi)1520" ! t "tAscan Ainj ! e ( scan inj )FDGoncologypatientactivity:60 minAscan (10 "15mCi) !1 110 min# 7 "10mCi23

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenAnnihilation Photon Attenuation PET/CT Attenuation Correction (AC)Anti-parallel gamma ray coincidence detection means that attenuation isindependent of position along any line of �Attn.ofphoton1:P1 e µ (x)dx0aAttn.ofphoton2:P2 e aTotalattn.ofcoincidencepair:PC P1P2 e µ (x)dx µ (x)dxx′sameCT,re- ‐sampledtoPETresolution0PET/CT Scanners" CT scan used for PET AC" CT image is downsampled to PET resolution"– Advantages" Fast acquisition" Low image noise"– Disadvantages" Higher dose" Attn. coeff. measured with poly-energeticphotons 140 keV"Consequences of CTAC! More accurate xʼPET/CT ScannersClinicalPET/CTPET Detector BlockMicroPET/CT PETscannersareassembledinblockmodules scintillationcrystalssignalouttoprocessingTwo dual photocathodePMTsReflective lightsealing tapegammaraysscintillationlight4

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenInside GE Discovery STE PET/CTPET Spatial Resolution Positron Physics– Positron Range– Photon Non-colinearityResolution components add inquadrature DetectorsBlock matrix: BGO crystals""6 x 8 crystals (axial by transaxial)""Each crystal:"" "6.3 mm axial"" "4.7 mm transaxial""Scanner construction""Axial:"" "4 blocks axially 24 rings"" "15.7 cm axial extent"– Response function2222Rsystem Rpos.phys. Rdet Rsampl Rrecon Ring Geometry– Non-uniform LOR sampling– Depth-of-interaction"Transaxial:"" "70 blocks around 560 xtals"" "88 cm BGO ring diameter"" "70 cm patient port" Reconstruction Filters13,440 individual crystals"Detector Signal DecodingLightSharingRelative PMTs signal heightsdepend on crystal 00.511.522.5Maximum positron kinetic energy (MeV)data from Derenzo, et al. IEEETNS 33:565-569, 1986 Photon non-colinearity– Non-colinearity: Rnon-colin 0.0022 x Ring Diameter– Clinical scanner: Diam. 80 - 90 cm; Rnon-col. 2 mm– Small animal scanner 15 - 20 cm; Rnon-col. 0.4 mm33.5Radial Positron range maximum energy of isotope scatter mediumPositron rms range (mm)2.5ACSignalDecodingEnergy, E A B C DAxial position, Z (C D) / ETransverse position, X (B D) / ERadial position: not determined (no DOI)TransversePositron Physics ResolutionAxialACBDPMTs5

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenPET Ring Geometry Effect onResolutionDetector ResolutionData Sampling Error: Coincidence lines-of-response are not uniformly spaced across a ring detector Interpolate to uniform spacing, or account for non-uniformity in reconstructionw!centerDepth-of-Interaction error:entrance position andtrue line-of-responsew/2!edgephoton penetration Peaks for different crystals at different positions"detection position andassigned line-of-response Window center and width adjusted for each crystal"Resolution Effect of Smoothing vs. Noisewith FBPHuman abdomen simulation with 2cm diam. lesion 2:1 se)PET Sensitivity1. Absorption efficiency of detectors– scintillation crystal attenuation coefficient– scintillation crystal thickness– detector response uniformity2. Solid angle coverage of object by detectors– PET ring diameter– smaller diameter pro: increases solid angle and sensitivity, reduces systemcost con: leads to DOI resolution degradation con: limits patient size– PET ring axial lengthlarger axial extent pro: increases solid angle and sensitivity con: increases system cost6

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenDetector Sensitivity vs. ometric Efficiency vs. anner axisPET scanner sensitivity scaleswith the number of detectablecoincidence events, which inturn scales as !max.axialend planeaxial center planeThis results in lower sensitivity atthe end of any PET te,dead- ‐timeFull qphoto- ‐sensormatching,manufacturingcostQA for PET Scanners: Evaluation ofPerformance icalManufacturersAssociation(NEMA)Standard Sensitivity - both system and per transaxial slice(measured with a line ness,t:typicallyBGOscannersuset 3cm,LSOscannersuset urce!mLimitedq!!!!!!!!!edge 0oPET scanner axisGraph from “Emission Tomography”,Eds. Wernick, Aarsvold, pg.186PET Image Formation sCoincidenceProcessingDataBinningDataCorrections Spatial resolution - measured with a point source and ananalytical image reconstruction algorithm at severalpositions in the scanner FOV (x,y,z resolution) Uniformity - measured with a uniform cylinder of activity Count rate - measured with a decaying line source in asolid, cold cylinder Dead time correction accuracy - measured from the countrate data Scatter fraction - measured from the count rate data Attenuation correction accuracy, contrast performance from a non-cylindrical phantom with hot and cold spheres.ImageReconstruction7

October18,2011Email:srbowen@uw.eduNuclear Medicine Basic Science LecturesStephenBowenAnalytic ReconstructionBackprojectionIterative ReconstructionFiltered Backprojectionf (0 )f ( k)initialimageestimatemeasureddatap(k) Hf (k) ncompute estimatedprojection dataFrom WikiBooks Basic Physics of Digital Radiography FBP assumes linear projections and does not account for manysources of variability in LOR Backprojection leads to streak artifacts in PET imagesReconstructed PET/CT images ppp ( k)comparemeasured andestimatedprojection dataf (k) f (k 1)update imageestimate basedon ratio ordifferenceThere are many ways to:– model the system (and the noise)– compare measured and estimated projection data– update the image estimate based on the differences between measuredand estimated projection data– decide when to stop iteratingModern Times: Time-of-Flight Time-of-flight capability is now offered in many new PET scanners"– Measure time difference of detection of coincidence gammas"No ACkVCTAC-CT– Defines a line segment in space, shorter than distance between detectors"– Improves image signal to noise that is a function of the object gmentlength!x cDt/2!xc speedoflightDt timingresolutionAC: Attenuation CorrectionOS-EM: Ordered Subsets Expectation MaximizationFBP: Filtered Back-Projection!x 7.5cmfortheDt 0.5nstypicalofTOFPETscanners8

Nuclear Medicine Basic Science .eduPET Introduction Summary PET concept– Physics of positron emission, photon annihilation, coincidence detection PET components– 2D collimated vs. 3D acquisition mode, detector block PET resolution– Positron range, detector response, line-of-response sampling, depth-of-interaction– Take home 1: clinical PET resolution 5 mm, small animal PET 1 mm PET quantitation– CT attenuation correction– Take home 2: separable attenuation correction makes PET more quantitativethan SPECT or MRI PET sensitivity– Absorption efficiency, geometric efficiency– Take home 3: PET sensitivity 103 greater than SPECT, 106 greater than MRI PET image formation– Acquisition– Reconstruction9

Emission - The source of the signal is emission of annihilation photons from within the patient, as opposed to photons transmitted through the patient in x-ray imaging Tomography - Three-dimensional volume image reconstruction through collection of projection data from all angles around the patient Positron Annihilation 1 mm

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