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The 3D Axial PET Concept

The 3D Axial PET Concept. Positron Emission Tomography - Principle and intrinsic limitations - State of the art Hybrid Photon Detectors: Principle, performance and fabrication The 3D axial PET camera - concept - strong and critical points - discussion of the key components

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The 3D Axial PET Concept

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  1. The 3D Axial PET Concept Positron Emission Tomography - Principle and intrinsic limitations - State of the art Hybrid Photon Detectors: Principle, performance and fabrication The 3D axial PET camera - concept - strong and critical points - discussion of the key components (scintillator, photo detector, electronics, integration) - Brain PET study: geometry and first performance estimates http://www.cern.ch/ssd/HPD Christian Joram, CERN, PH-Department

  2. Positron Emission Tomography • Principle and fundamental limits • Patient injected with positron (b+) emitting radiopharmaceutical , e.g. FDG, marked with 18F. • b+ annihilates with e- from tissue, forming an annihilation photon pair (back-to-back, 511 keV) • 511 keV pairs detected in scintillator crystals via time coincidence • b+ emission point lies on line defined by detector pair (line of record LOR, chord) • Reconstruct 2D image using Computed Tomography • Use stacked detector rings to obtain a 3D volumetric image. • + many mathematical tricks (filtering, density corrections, …) Standard PET geometry FDG structure

  3. Factor Shape Resolution (FWHM) d d/2 Detector Crystal Width 0 (individual coupling) 2.2 mm (Anger logic, empirically) PMT1 PMT2 Anger Logic 180º ± 0.25º 1.8 mm (depends on det. ring radius) Photon non-collinearity 0.5 mm, 18F 4.5 mm, 82Rb Positron range Courtesy of B. Moses, Mattinata 2002 1.25 (in-plane) 1.0 (axial) multiplicative factor Reconstruction Algorithm

  4. L dp a Standard PET geometry The parallax dilemma • Crystals need to have a minimum thickness L • Efficiency for pair detection • la = photon attenuation length of crystal • L = la e2 ~ 40%, L = 2 la e2 ~ 75%, … • la = 1-2 cm (depending on material, see below) • A standard PET does not measure the depth of interaction (DOI) in the crystal. • This introduces a parallax error • The resolution in the off-center region degrades significantly • Solution: reduce L (bad e2) or measure DOI or invent a different geometry

  5. Scatters Randoms Noise Equivalent Count Rate Resolution is important, but also sensitivity and image quality matter ! Sensitivity = True Event Rate / µCi / cm3(measures detector efficiency) Determined by geometry, choice of scintillator, data acquisition… Trues But the signal is subject to background from Compton scattering and random coincidences.

  6. T  T2/R ~ const. R 2 100 80 No Dead Time (DT) 60 With DT NEC (a.u.) Tdead  e–(t) Rdead  2e–(t) 40 20 With DT & Coincidence Processor Limit Total throughput const. NEC ~ 1/ρ2 0 0.0 2.0 4.0 6.0 8.0 10.0 Activity  (a.u.) The NEC rate depends on many factors. More activity and efficiency does not always help ! Courtesy of B. Moses, Mattinata 2002

  7. AFOV 25 cm Pulse Shape Discrimination = Phoswich approach PMT1 PMT2 LSO Dt = 7 ns 7.5 GSO or LSO 31 cm 7.5 8  8 matrix • 8 panels with 9  13 blocks • 2  64 crystals per block • Crystal dimensions: 2.1  2.1 x 7.5 mm DOI is known with a precision of 5 mm (FWHM), 10 mm without phoswich. However 15 mm detector length correspond only to about 1 la. Efficiency e2 ~ 40% Concrete example: The High Resolution Research Tomograph (HRRT) (CTI, MPI, Karolinska …) Currently the “reference” in brain PET K. Wienhard et al., IEEE Trans. Nucl. Sci. 49 (2002) 104–10

  8. HRRT performance • spatial resolution (FWHM) • transaxial: 2.4 - 2.9 mm • axial: 3-4 mm • Energy resolution: 17% • Sensitivity: 4.5 cps/kBq • NEC: 140 kcps measured by moving a 18F point source (1 mm Ø) over a cylinder of 20 cm diameter and 20 cm length cylinder of 20 cm diameter and 20 cm length filled with 18F dissolved in water to an activity of 0.35 mCi/ml.

  9. Time-Of-Flight PET – an old idea recently revived (Si-PM ! ) • Idea • Use very fast scintillators and photodetectors (<3 ns FWHM) • Use TOF information of photons to constrain source distribution • Benefits • Smaller coincidence time  lower background  higher peak NECRR • Reduced noise (less ‘false’ intersections) • Simultaneous recording of emission / transmission (ext. source) from W.W. Moses IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 50, NO. 5, OCTOBER 2003, p. 1325

  10. light quantum photocathode e - focusing D V electrodes silicon sensor + FE electronics segmented silicon sensor Hybrid Photon Detectors ( HPD ) • Background: • Applications in LHC experiments require photon detectors featuring • High sensitivity in visible and UV range • Ability to see single photons • high speed (remember 40 MHz bunch crossing) • High filling factor (>70%) • Large area coverage (~m2)  Development of large area Hybrid Photon Detectors Combination of sensitivity of PMT with excellent spatial and energy resolution of silicon sensor

  11. Bialkali photocathode h e- Si Sensor 2048 pads (1 x 1 mm2) 16 front-end chips Ceramic PCB The 5-inch Pad HPD

  12. Same spectrum measured with PMT (schematically) Single photon imaging with 2048 Channels. 1 p.e. Electronics noise well separated from signal 2 p.e. counts 3 p.e. signal amplitude (a.u.) Signal definition and energy resolution. S/N ~ 10.

  13. Turbo Pump HPD fabrication Facilities and infrastructure for the fabrication of large HPDs (up to 10”Ø) have been developed at CERN. All ingredients for photodetector production are available: • Design/simulation • Photocathode processing • (bialkali, Rb2Te, CsI) • Glass / ceramic tube manufacturing • Indium sealing technique

  14. New 3D axial concept Axial arrangement of camera modules based on matrices of long crystals read out on both sides by HPDs The 3D axial PET camera Conventional concept Illustration: F. Schönahl, HUG Rings of block detectors

  15. Main advantages of the concept • Full 3D reconstruction of g quanta without parallax error • x,y from silicon pixel address • z from amplitude signal ratio of the 2 HPD’s •  Precise Depth of Interaction DOI measurement •  No limitation in detector thickness  improved sensitivity. • Measurement of light yield on both sides of crystals • Negligible statistical fluctuations in HPD •  Very good γ energy resolution • 3D reconstruction provides possibility to recuperate part of g’s which underwent Compton scattering in the scintillator crystals •  Compton enhanced sensitivity HPD1 z y x HPD2

  16. PEM (PE mammography), in compressed mode, is another interesting option. Critical aspects of the concept • z-resolution, if obtained by light ratio, scales with crystal length • σz ~ L • Axial field of view limited to about 10 - 15 cm • Little experience in fabrication of long crystals (cost) The concept seems to be very promising for brain PET, where both high spatial resolution and high sensitivity are required.

  17. YAP:Ce LSO:Ce LuAP:Ce LaBr3:Ce Density ρ (g/cm3) 5.55 7.4 8.34 7.13 5.3 Effective atomic charge Z 32 66 65 46.9 75 Scintillation light output (photons / MeV) 18000 23000 ~10000 ~61000 ~9000 Wavelength of max. emission (nm) 370 420 370 356 480 Refractive index n at wavelength of maximum emission 1.94 1.82 1.95 ~1.88 ~2.15 Bulk light absorption length La (cm) at 370 nm ~14 ~20 Principal decay time (ns) 27 40 38 30±5 300 Mean γ attenuation length at 511 keV (mm) 22.4 11.5 10.5 ~20 ~11.6 Photo fraction at 511 keV (%) 4,5 32.5 30.5 41.5 15 Energy resolution at 663 keV 4.5 8 2.9 Scintillation crystals for the 3D axial concept • Criteria to be taken into account:light yield, absorption length, photo fraction, self absorption, decay time, availability, machinability, price. BGO • YAP is OK for proof of principle, however suffers from low Z (high absorption length, low photo fraction) All preliminary performance estimates are based on YAP (availability!) • LaBr3, L(Y)SO and LuAP are the really interesting candidates.

  18. Planned geometry for proof of principle 4 camera modules R=160 mm EuroMedIm workplan ~2005

  19. A camera module based on HPD PCR5 52 mm Baseplate (from Pad-HPD) 2 Silicon sensors (208 pads 4×4 mm2 ) 32 mm 208 crystals (YAP) 3.2 × 3.2 × 100 mm3 HPD PCR5 Ceramic envelope with sapphire entrance window

  20. First fully operational PET HPD E. Chesi et al., NIM A 564,2006, 352-363

  21. Experimental set-up for PET-HPD tests VME DAQ Readout card Si sensor (300 mm) 208 pads (4×4 mm2) -UPC = 0 -20 kV vacuum pump (turbo) P < 10-5 mbar Pulsed LED (blue) sapphire collimator MgF2 mirror

  22. Mapping from photo cathode to silicon sensor has very good linearity. Mapping was measured in a previous prototype with 1 mm pixel size. Deviations from linearity only ~ 60 mm RMS!

  23. Hit Distributions from Light Spot This lego plot shows that a threshold of ~ 20 fC (~ 50 / 20 p.e. at 10 / 20 kV) eliminates easily any dark current hits  Background free images

  24. Total Charge Summed from All Pads above Threshold Histograms of the total charge for two different HPD acceleration voltages. Left: UC = -10 kV ; Right: UC = -20 kV. The mean of charge distributions left and right is not a factor 2 apart since there is more energy loss in dead layer of pad sensor at lower electron energy and also more pads with partial energy above threshold at higher electron energy

  25. Mean Charge m and s/m of charge distributions as function of Cathode Voltage • Note: • mean charge is linear but intercepts at ~6 keV due to energy loss in dead layer. This can be improved in next sensor production run. Expect intercept at ~0.5 keV, which will considerably improve the charge gain in the HPD • s/m reaches almost a plateau around 17 keV since energy straggling becomes small. • One can estimate an energy loss of ~ 1.6 keV at 20 kV with nearly negligible straggling. • Gain at 20 kV is 5090 • From s/m = (ENF/N)1/2 • N = 507 photo electrons ( ~ expected Np.e. per HPD from a 511 keV g in LYSO) The absolute gain of the chain can now be calculated: 0.94 fC/(ADC count) for chip1 Mean charge m (left axis) and ratio of Gaussian width to mean charge σ/μ (right axis) versus cathode voltage UC (kV).

  26. Full ring scanner Possible configuration for a Brain PET R = 170 mm • 34 cm inner diameter • 10 cm axial length • 2496 crystals • 24 HPDs • total detection volume • 2556 cm3 • F coverage 66% • W coverage 18% z y x

  27. Resolution x,y s ~2.2 mm reconstructed reconstructed Resolution z s ~4.5 mm Resolution x,y s ~1.5 mm source source 3D axial PET geometry Crystal dimensions: 3.2 x 3.2 x 100 mm Material: YAP:Ce 33 point sources, no background 90 mm 5 mm 100 mm (0,0,0) (0,0,0) transaxial (x,y) plane axial (x,z) plane

  28. Resolution x,y s~6 mm FWHM reconstructed reconstructed Resolution x,y s~1.5 mm FWHM source source Standard PET geometry Crystal dimensions: 3.2 x 3.2 x 30 mm 33 point sources, no background 90 mm 100 mm (0,0,0) (0,0,0) transaxial (x-y) plane axial (x-z) plane

  29. Performance estimates 3D Axial Brain PET (with LSO/LYSO) • (HRRT) • Detected photoelectrons for a γ of 511 keV: ~500 - 600 per HPD (f(lb) (?) • Energy resolution: ~10 % (FWHM) (f(lb) (17) • Spatial resolution in transaxial plane: 1.5 - 2.2 mm (FWHM) (2.4-2.9) • Spatial resolution in axial direction: ~4-5 mm (FWHM) (f(lb) (3-4) • Coincidence interval: ~ 5-10 ns (2 ns?) • Compton gain: ~ 2 (1) • Sensitivity ~ 5.5 cps/kBq (4.5) • NEC: 130 kcps (?) (140) • HPD based Brain PET should beat HRRT in spatial and energy resolution and should at least be comparable in other disciplines.

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