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Investigation of the LabPET TM Detector and Electronics for Photon-Counting CT Imaging

1st European Conference on Molecular Imaging Technology Marseille 9-12 May 2006. Investigation of the LabPET TM Detector and Electronics for Photon-Counting CT Imaging. Philippe Bérard a , J. Riendeau b , C. M. Pepin a , D. Rouleau a , J. Cadorette a , R. Fontaine b , R. Lecomte a.

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Investigation of the LabPET TM Detector and Electronics for Photon-Counting CT Imaging

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  1. 1st European Conference on Molecular Imaging Technology Marseille 9-12 May 2006 Investigation of the LabPETTM Detector and Electronics for Photon-Counting CT Imaging Philippe Bérarda, J. Riendeaub, C. M. Pepina, D. Rouleaua, J. Cadorettea, R. Fontaineb, R. Lecomtea a Department of Nuclear Medicine and Radiobiology b Department of Electrical and Computer Engineering Université de Sherbrooke, Sherbrooke, Québec, Canada

  2. 2003 - 2004 1 x 1 mm2 single L(Y)SO pixel PET detector Standard NIM electronics 20 sec/proj 5 sec/proj 10 sec/proj Am241g 59.5 keV 1mm3 source 74 MBq 1 week ! 50 sec/proj 40 sec/proj 30 sec/proj IEEE NSS/MIC Portland, October 2003 (2 x 2 x 10 mm3). Academy of Molecular Imaging, March 2004, Orlando FL (1 x 1 x 10 mm3).

  3. Main Purpose Molecular Imaging Involves highly specific probes 1.3 1.7 1.0 2.0 2.4 0.75  Develop a combined multi-modality scanner capable of obtaining anatomical & functionaldata using a single apparatus relying on the same detection system.  Conduct successful follow-up investigations of gene expression, disease progression and therapeutic outcome in small animal models. Investigation of photon-counting mode CT performed using LabPET™ detectors and digital electronics. See poster # 191 Lecomte et al. Preliminary report on the LabPET™, a high-performance APD-based digital PET scanner for small animal imaging.

  4. Dose issue in follow-up studies Effects of radiation dose on small animals • LD50/30 for mouse: ~ 5 – 7.5 Gy • Doses in the range 5 - 200 cGy : induce cell resistance to subsequent therapeutic doses of radiation (“adaptive response”*) • Doses in the range 1 - 20 cGy : induce therapeutic effects on certain tumor cells PET/CT dose objective: lower than 1 cGy Micro CT typical dose ranges 10 - 40 cGy (100 µm resolution, σ~ 25 HU)** Lower spatial resolution: Dose ~ Δx-4, PET scintillator ~ 2 x 2 mm2 pixels *G.P. Raaphorst and S. Boyden, “Adaptive response and its variation in human normal and tumor cells”, Int. J. Radiat. Biol. 75, pp. 865-873, 1999. **Lee et al. A flat-panel detector based micro-CT system: performance evaluation for small-animal imaging, Physics in Medicine and Biology, no.48, 2003. Ford, N. L., Thornton, M. M., Holdswoth, D. W. Fundamental image quality limits for microcomputed tomography in small animals, Med. Phys., vol. 30, no. 11, pp. 2869-2877, 2003.

  5. Photon counting in CT • Advantages of photon counting CT: • Each event has equal weight independent of energy • Elimination of weight factor proportional to energy of integration imaging • Closer to optimal weighting of E-3 * • Threshold detection allows discrimination of noise and scatter Requirements : • Detector: high light output scintillators to detect low-energy X-rays, • fast fluorescence to avoid pulse-pile-up • count-rate > 106 - 107 counts/s, the higher the better * R.N. Cahn et al., “Detective quantum efficiency dependence on X-ray energy weighting in mammography”, Med. Phys. 26 (12), pp. 2680-2683, December 1999.

  6. Potential PET/CT Scintillators Short decay time  to avoid pulse pile-up and dead time in the front-end processing electronics High light output  low X-ray energy threshold Photon-counting CT scintillator requirements: Suitable for PET

  7. PET/CT Detector • LYSO best combination of : • high attenuation coefficient • high light output • fast decay time  PET and CT imaging in photon- counting mode reflector APD scintillator The high photoelectric absorption  confines spatial resolution to the crystal of interaction

  8. PET/CT Configuration 2. Carbon nanotube cold cathode 1. Rod anode microfocus Oxford instruments 3. Normal microfocus x-ray tube with off focal spot geometrical configuration and reconstruction. See TomXGam and PIXSCAN

  9. Materials and Methods Stepping motors move X-ray source and phantom Step and shoot acquisition Motor controller 115200 steps over 360o Data counts transferred by serial link Acquisition with a limited number of detectors

  10. LabPETTM Electronics Free-running ADCs APD Detector Module 16-ch CSP ASIC (0.18 µm) APD Bias Regulators FPGA DSP LabTEPTM 1 ‘fast track’ module See poster # 180 Fontaine et al. Digital signal processing applied to crystal identification in positron emission tomography dedicated to small animals.

  11. Digitized CSP signal from X-ray source 65 kV, 20 mA ~ 55 keV event 0.6 mm Cu Sample value dxd = 23 cm Sample number • Signal rise time ~90 ns (at 50% ADC maximum dynamic range) • Fast baseline restorer to enable fast count rate • A veto with a duration of 8 samples was programmed in the FPGA after threshold crossing to detect maximum Resulting non-paralyzable dead-time of 156 ns limits the count rate to 5.6 x 106 events/sec/channel.

  12. Electronics count-rate X-ray source and LabPET™detector 10 % dead time at 1.6 x 106 counts/s 25 % dead time at 2.0 x 106 counts/s Paralyzable component from the detector ! CSP saturation Charge integration time Scintillator decay time m = recorded count rate n = true interaction rate  = system dead time: 156 ns Non-paralyzable model

  13. CT Image Acquisition Operation characteristics Voltage : 65 kV Current : 20 μA Filter : 0.6 mm Cu Distance x-ray source – phantom : 11.5 cm Distance phantom - detectors : 11.5 cm nb detectors : 16 Linear sampling : 2 mm (detector width) 180 projections , 0.1 second/projection  25 minutes scan time 1 second/projection  45 minutes scan time Parallel FBP reconstruction algorithm : Nyquist cutoff frequency

  14. Energy resolution of detector Higher light output desirable Better SNR Better energy resolution  Lower counting threshold Energy spectrum used for image acquisition. Landmark for small animals : 20-30 keV mean energy.

  15. Spatial Resolution PSF PSF FWHM FWTM 250 μm rod PSF : FWHM 1.03 mm FWTM 2.50 mm

  16. Spatial Resolution MTF MTF10%sys exp = 0.71 lp/mm G. T. Barnes, M. V. Yester, M. A. King, Optimizing computed tomography (CT) scanner geometry, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

  17. Spatial Resolution MTF a = 50 mm MTFsys = MTFfoc MTFdet MTFsam MTFalg MTF10%sys exp = 0.71 lp/mm G. T. Barnes, M. V. Yester, M. A. King, Optimizing computed tomography (CT) scanner geometry, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

  18. Spatial Resolution MTF MTFsys = MTFfocMTFalg MTFdet MTFsam MTF10%sys exp = 0.71 lp/mm G. T. Barnes, M. V. Yester, M. A. King, Optimizing computed tomography (CT) scanner geometry, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

  19. Spatial Resolution MTF MTFsys = MTFfocMTFdetMTFsam MTFalg MTF10%sys exp = 0.71 lp/mm G. T. Barnes, M. V. Yester, M. A. King, Optimizing computed tomography (CT) scanner geometry, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

  20. Spatial Resolution MTF MTFsys = MTFfocMTFdetMTFsamMTFalg MTF10%sys exp = 0.71 lp/mm MTF10%sys theo = 0.73 lp/mm G. T. Barnes, M. V. Yester, M. A. King, Optimizing computed tomography (CT) scanner geometry, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

  21. Noise 3.5 cm diameter cylinder filled with water 1 sec/projections Ring artifacts !!!

  22. Noise σ~ D-0.5 GORE, J. C., TOFTS, P. S. Statistical limitations in computed tomography, Phys. Med. Biol., vol. 23, no. 6, pp. 1176-1182, 1978.

  23. Contrast phantom 2 1 6 3 5 4 Measured dose (TLD): 0.7 mGy 3.5 cm diameter Plexiglas phantom filled with 5 mm diameter rods Several tissue-like materials can be discriminated with sufficient accuracy.

  24. Conclusion APD-based inorganic scintillator detectors and fast digital multi-channel pulse processing electronics, as developed for the LabPETTM, are suitable to detectand count individual X-rays in the diagnostic energy range. • Resolution : 1.03 mm • Noise : 6.8 HU • Dose : 0.7 mGy

  25. Future Work • Add more detector, more analog and digital electronics • Image artefact : CT adapted image reconstruction algorithm • Higher resolution : magnification, higher sampling, smaller detector pixel size Objectives : ~ 1 μl PET resolution, < 500 μm CT resolution

  26. Acknowledgments • Thanks to: • Murray Davies and Henri Dautet of PerkinElmer Optoelectronics, Vaudreuil, QC for providing the detectors used in this study

  27. Thank You for your attention !

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