2 The basic principle of PET 1. Positron-emitting tracer is injected into the body2. Emitted positrons (+) travel 1 – 3 mm3. Positrons collide with electrons (-) causing an “annihilation”4. Annihilation emits energy in the form of two 511keV energy gamma rays at ~180 degrees5. Gamma rays are detected by opposing detectors6. Energy discrimination (an “energy window”) is used to ensure that each gamma is ~511 keV7. Timing discrimination (a “coincidence time window”) is used to ensure that each gamma ray comes from the same annihilation, hence ensuring accurate localization of the tracer5164237
4 Trues One annihilation Detection within coincidence window E energy windowOne annihilationDetection within coincidence windowEnergy within energy windowtrues = const * activity
5 RandomsTwo annihilationsDetection within coincidence windowEnergy within energy windowRandoms = const * activity * activity
6 Correction of randomsRandoms are related to the single rate of each detectorRandoms are related to the length of the coincidence windowRandoms can be calculated when the singles for each detector are measured, and the coincidence window for each detector pair is knownRandoms can be measured and corrected in real time for each LOR, using a delayed coincidence window with exactly the same length as the “direct” coincidence window
7 Reduction of randoms Relevant parameters: Coincidence window 12 ns random coincidences12 ns6 ns4.5 ns (pico 3D)
8 ScatterOne annihilationDetection within coincidence windowEnergy loss due to scatterBut energy still within energy windowScatter fraction is object dependent!
9 PET event energy spectra PET events are distributed across a range of energy, not only in the 511 keV range. An energy window is employed to reject scatter.ENERGYWINDOW425 – 650ENERGY WINDOW511 keV PHOTONSLSOSCATTERBGOCounts350 – 650100200300400500600700Energy (keV)
10 Correction of scatter Scatter is related to mu map Emission Transmission Scatter CorrectedScatter is related to mu mapScatter is patient dependentScatter needs to be measured for each patientScatter can be estimated by phantoms (but a cylindrical phantom may be a good approximation for the brain; everywhere else it is a very poor estimation)Scatter can be precisely modeled for each patient using the mu map: Watson method
11 Correction of attenuation Patient absorbs some of the 511 keV photonsAttenuation is patient dependentmu map has to be measured for each patientmu map can be measured with external sources137Cs for estimated mu map68Ge for precise definition of mu mapX-ray for high statistics and precise mu map
12 Noise Equivalent Countrate (NEC) Main sources of statistical error in a PET system are randoms and scatterComparison to a system that is resistant to randoms and scatterNEC describes the effective number of counts measured by the PET scanner as a function of the activity in the FOV
13 NEC – clinical performance INJECTED DOSE RANGE185 – 740 MBq 5 – 20 mCi1 hour uptakeBiograph HI-REZ PICO9080Biograph706050Noise Equivalent Count Rate [per sec]2D4030201020.10.20.30.40.5468101214161820Specific Activity kBq/cc [uCi/cc]*Ring difference and energy window unspecified; for Biograph HI-REZ all measurements are clinicalSource: Carney, et Al., “Regionally dependent count rate performance analysisof patient data acquired with a PET/CT scanner,” abstract 364, SNM 2003.
14 Sensitivity Septa employed No septa Low efficiency High efficiency A measure of the number of coincidence events a scanner is able to detect, assuming no dead time. Four to five times improvement with 3D acquisition techniques.2D acquisition mode3D acquisition modeSepta employedLow efficiencyHigher dose requiredLengthy scan timesFewer counts per dose (low count rate)Low scatterNo septaHigh efficiencyLower dose requiredShort scan timesHigher counts per dose (high count rate)High scatter
15 PET•CT ProtocolThe typical protocol begins with a CT topogram to identify the scan range.This is followed by a spiral CT exam of the body part of interest.
16 PET•CT ProtocolThe patient is then automatically positioned for the start of the PET exam.The PET exam is a series of bed positions during which the radioactive emissions are collected.
18 Block detector components 169 crystal elements per detector block4 photomultiplier tubes (PMTs)/detector blockDetector modulePMTChanneledscintillation lightDetector block
19 Attenuation artifacts Conventional CT: 50 cm FOVNote: arms not fully imaged, hardening at edges of field of viewEmission only PETNote: arms fully imagedAttenuation correction PETNote: artifacts in liver and possible lesion distortionReduced image qualityReduced accuracyIncreased artifactsPotential diagnostic impact