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Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?

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Presentation on theme: "Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?"— Presentation transcript:

1 Innovation is in our genes. 1 Siemens Medical Solutions Molecular Imaging What are PET basics?

2 Innovation is in our genes. 2 Siemens Medical Solutions Molecular Imaging The basic principle of PET 1. Positron-emitting tracer is injected into the body 2. Emitted positrons ( +) travel 1 – 3 mm 3. Positrons collide with electrons ( -) causing an annihilation 4. Annihilation emits energy in the form of two 511keV energy gamma rays at ~180 degrees 5. Gamma rays are detected by opposing detectors 6. Energy discrimination (an energy window) is used to ensure that each gamma is ~511 keV 7. 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 tracer

3 Innovation is in our genes. 3 Siemens Medical Solutions Molecular Imaging Coincidence

4 Innovation is in our genes. 4 Siemens Medical Solutions Molecular Imaging Trues E energy window One annihilation Detection within coincidence window Energy within energy window trues = const * activity

5 Innovation is in our genes. 5 Siemens Medical Solutions Molecular Imaging Randoms Two annihilations Detection within coincidence window Energy within energy window Randoms = const * activity * activity

6 Innovation is in our genes. 6 Siemens Medical Solutions Molecular Imaging Correction of randoms Randoms are related to the single rate of each detector Randoms are related to the length of the coincidence window Randoms can be calculated when the singles for each detector are measured, and the coincidence window for each detector pair is known Randoms 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 Innovation is in our genes. 7 Siemens Medical Solutions Molecular Imaging Reduction of randoms Relevant parameters: Coincidence window coincidence window random coincidences 12 ns 6 ns 4.5 ns (pico 3D)

8 Innovation is in our genes. 8 Siemens Medical Solutions Molecular Imaging Scatter One annihilation Detection within coincidence window Energy loss due to scatter But energy still within energy window Scatter fraction is object dependent!

9 Innovation is in our genes. 9 Siemens Medical Solutions Molecular Imaging 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. ENERGY WINDOW 350 – 650 ENERGY WINDOW 425 – 650 Counts Energy (keV) SCATTER 511 keV PHOTONS BGO LSO

10 Innovation is in our genes. 10 Siemens Medical Solutions Molecular Imaging Correction of scatter Scatter is related to mu map Scatter is patient dependent Scatter needs to be measured for each patient Scatter 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 Emission Transmission Scatter Corrected

11 Innovation is in our genes. 11 Siemens Medical Solutions Molecular Imaging Correction of attenuation Patient absorbs some of the 511 keV photons Attenuation is patient dependent mu map has to be measured for each patient mu map can be measured with external sources 137Cs for estimated mu map 68Ge for precise definition of mu map X-ray for high statistics and precise mu map

12 Innovation is in our genes. 12 Siemens Medical Solutions Molecular Imaging Noise Equivalent Countrate (NEC) Main sources of statistical error in a PET system are randoms and scatter Comparison to a system that is resistant to randoms and scatter NEC describes the effective number of counts measured by the PET scanner as a function of the activity in the FOV

13 Innovation is in our genes. 13 Siemens Medical Solutions Molecular Imaging NEC – clinical performance *Ring difference and energy window unspecified; for Biograph HI-REZ all measurements are clinical Source: Carney, et Al., Regionally dependent count rate performance analysis of patient data acquired with a PET/CT scanner, abstract 364, SNM INJECTED DOSE RANGE 185 – 740 MBq 5 – 20 mCi 1 hour uptake Specific Activity kBq/cc [uCi/cc] Noise Equivalent Count Rate [per sec] 2D Biograph HI-REZ PICO Biograph

14 Innovation is in our genes. 14 Siemens Medical Solutions Molecular Imaging Sensitivity 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. Septa employed Low efficiency Higher dose required Lengthy scan times Fewer counts per dose (low count rate) Low scatter No septa High efficiency Lower dose required Short scan times Higher counts per dose (high count rate) High scatter 2D acquisition mode 3D acquisition mode

15 Innovation is in our genes. 15 Siemens Medical Solutions Molecular Imaging PETCT Protocol The 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 Innovation is in our genes. 16 Siemens Medical Solutions Molecular Imaging PETCT Protocol The 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.

17 Innovation is in our genes. 17 Siemens Medical Solutions Molecular Imaging scatter correction attenuation correction PET Recon Spiral CT: seconds CT PET Survey WB PET: min PET CTPET CT Recon Fused PETCT FUSION PETCT scan protocol

18 Innovation is in our genes. 18 Siemens Medical Solutions Molecular Imaging Block detector components 169 crystal elements per detector block 4 photomultiplier tubes (PMTs)/detector block Detector block PMT Detector module Channeled scintillation light

19 Innovation is in our genes. 19 Siemens Medical Solutions Molecular Imaging Attenuation artifacts Conventional CT: 50 cm FOV Note: arms not fully imaged, hardening at edges of field of view Emission only PET Note: arms fully imaged Attenuation correction PET Note: artifacts in liver and possible lesion distortion Reduced image quality Reduced accuracy Increased artifacts Potential diagnostic impact

20 Innovation is in our genes. 20 Siemens Medical Solutions Molecular Imaging ACPlus Attenuation Correction Extended 70 cm transverse FOV Super fast attenuation scanning Exceptionally high statistics Unmatched attenuation image quality Highest accuracy attenuation correction Conventional attenuation scan ~120 sec scan time 10 6 counts FULL FOV Conventional CT attenuation scan ~10 sec scan time counts TRUNCATED FOV Siemens ACPlus ~10 sec scan time counts FULL FOV (NOT TRUNCATED)

21 Innovation is in our genes. 21 Siemens Medical Solutions Molecular Imaging Standard PET: filtered backprojection COINCIDENCE TIMING WINDOW (4.5 nsec) DETECTOR ELECTRONICS GANTRY CROSS SECTION

22 Innovation is in our genes. 22 Siemens Medical Solutions Molecular Imaging Standard PET: filtered backprojection COINCIDENCE TIMING WINDOW (4.5 nsec) GANTRY CROSS SECTION DETECTOR ELECTRONICS

23 Innovation is in our genes. 23 Siemens Medical Solutions Molecular Imaging Time of flight DETECTOR ELECTRONICS COINCIDENCE TIMING WINDOW (4.5 nsec) T, TIME DIFFERENCE OF DETECTION CONVENTIONALTOF Source: Conti, et al., IEEE 2004

24 Innovation is in our genes. 24 Siemens Medical Solutions Molecular Imaging Complex schematic of a PETCT


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