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A Compact 16-Module Camera Using 64-Pixel CsI(Tl)/Si PIN Photodiode Imaging Module

A Compact 16-Module Camera Using 64-Pixel CsI(Tl)/Si PIN Photodiode Imaging Module. W.-S. Choong 1 , G.J. Gruber 1 , W.W. Moses 1 , S.E. Derenzo 1 , S.E. Holland 1 , M. Pedrali-Noy 1 , B. Krieger 1 , E. Mandelli 1 , G. Meddeler 1 , N.W. Wang 1 , C.S. Tindall 1 , and E.K. Witt 2.

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A Compact 16-Module Camera Using 64-Pixel CsI(Tl)/Si PIN Photodiode Imaging Module

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  1. A Compact 16-Module Camera Using 64-Pixel CsI(Tl)/Si PIN Photodiode Imaging Module W.-S. Choong1, G.J. Gruber1, W.W. Moses1, S.E. Derenzo1, S.E. Holland1, M. Pedrali-Noy1, B. Krieger1, E. Mandelli1, G. Meddeler1, N.W. Wang1, C.S. Tindall1, and E.K. Witt2 1Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2Capintec, Inc, Ramsey, NJ 07446

  2. Bulky Long imaging distance Intrinsic resolution ~ 3.5 mm Significant dead area around the periphery DE/E ~ 9% FWHM Compact Short imaging distance Intrinsic resolution ~ 2 - 3 mm Small dead area around the periphery DE/E < 15% FWHM Motivation Anger Camera Compact Camera

  3. Approaches to Compact Cameras • Discrete scintillator/photodiode camera: gamma rays interact in an array of optically-isolated scintillation crystals that are coupled 1-to-1 to a photodiode array • Solid-state camera: gamma rays interact directly with a pixellated solid-state detector (e.g. CdZnTe) • Position-sensitive photomultiplier tube camera: gamma rays interact in one or more scintillation crystals which are then read out by PSPMT.

  4. LBNL Low-Noise Photodiode Arrays • Low leakage current (<50 pA/pixel @ 50 V) • 3 pF/pixel capacitance • 80% quantum efficiency for 540 nm light • 98.5% good pixel yield during fabrication 64 3x3 mm2 pixels (edge and corner pixels are 30% and 50% smaller area respectively)

  5. Photodiode Fabrication • Made on 300 m thick, high resistivity (>10 k-cm), n-type silicon • Low leakage current is achieved by depositing a thick layer of phosphorus-doped polysilicon to getter impurity atoms • p+ junctions are fabricated by ion-implanting boron • n+ back-side contact is made by re-depositing a thin layer of polysilicon after etching off the thick polysilicon gettering layer • A layer of indium tin oxide (ITO) is deposited on the back-side to serve as an anti-reflective (AR) coating optimized for 540 nm light (the peak output of CsI(Tl) scintillator crystal) Reference: S.E. Holland, N.W. Wang, and W.W. Moses, “Development of low noise, back-side illuminated silicon photodiode arrays,” IEEE Transactions on Nuclear Science, vol. 41, pp. 443-447, 1997

  6. PETRIC Readout IC • Input: 64 photodiode signals plus control signals that modify ASIC behavior • Front end: 64 channels of low-noise charge-sensitive preamplifiers and adjustable shaper amplifiers • “Winner Take All” (WTA): reduces the 64 amplified, shaped signals to a single “winner” channel (the one with the greatest amplitude) • Output: the 1 analog “winner” plus 6 digital bits identifying the winning pixel Reference: M. Pedrali-Noy, G.J. Gruber, B. Krieger, E. Mandelli, G. Meddeler, W.W. Moses, et al., “PETRIC-A Positron Emission Tomograph Readout Integrated Circuit,” IEEE Transactions on Nuclear Science, vol. 48, pp. 479-484, 2001

  7. Components of Module Surface mount connectors Low-noise Si PIN photodiode array EMF shield for PETRIC PETRIC CsI(Tl) array Ceramic PCB Surface mount capacitors Getek PCB

  8. Complete 64-Pixel Module • PETRIC readout: 64 front end channels followed by “Winner Take All” circuitry that selects the channel with the greatest amplitude • Custom printed circuit boards: provide electrical connections to the photodiodes and the PETRIC • Custom Si PIN photodiodes: very low leakage current (<50 pA/pixel) => low electronic noise • CsI(Tl) crystals: commercial 3 x 3 x 5 mm3 crystal array Capacitors EMF shield Connectors 14 mm 24 mm 24 mm

  9. 16-Module Camera Design • A scintimammography imaging camera constructed from tiling 16 individual 64-pixel modules • Separate WTA multiplexes signals from up to 16 modules WTA, peak detect, interface I/O

  10. Motherboard: Block Diagram

  11. PETRIC I/O Signals

  12. Motherboard: Timing Diagram

  13. Photograph of the 16-Module Camera Comparator Readout timing circuitry Peak detector WTA-3 Four 64-pixel module Cable to NI-DAQ card Photodiode bias voltage

  14. Pulse Height Spectrum (Tc99m) The numbers in parenthesis are the corresponding fwhm energy resolution.

  15. Energy Resolution Inner Pixels Edge Pixels

  16. Energy Resolution(From a High SNR Module) Inner Pixels Edge Pixels

  17. Energy Resolution(From a Low SNR Module) Inner Pixels Edge Pixels

  18. System Spatial Resolution(with Line Source) In Air 10 cm Acrylic Block All-Purpose Collimator 8000 cps/mCi, 23.5 mm thick High-Sensitivity Collimator 16,200 cps/mCi, 23.5 mm thick Collimator

  19. System Spatial Resolution

  20. Noise Sources (7% - 10% fwhm) (2% - 3% fwhm) (8% - 10% fwhm)

  21. Electronic Noise (w/o PD) Module #1 Module #2

  22. Electronic Noise (w/ PD) Module #1 Module #2

  23. Light Collection Inhomogeneity(Measured w/ PMT, Excited by 511 keV) Center = 147 Center = 137 Center = 123 Center = 157 Note:All crystals have similar performance if re-wrapped with Teflon tape.

  24. Future Directions • Working with Capintec, Inc under a CRADA (Cooperative Research and Development Agreement) project to build a 25-module and 100-module gamma camera using smaller pixel (2.4 mm x 2.4 mm) photodiode arrays. • Design and fabricate a new WTA I.C. with 32 input channels and a peak detect to be used in the gamma camera.

  25. Canberra Photodiode Arrays LBNL Photodiode Canberra Photodiode 64 2.4x2.4 mm2 pixels (edge and corner pixels are 32% and 54% smaller respectively) 19.6 mm 19.6 mm

  26. Canberra Photodiode Arrays: Performance

  27. 32-Channel WTA IC with Peak Detect

  28. Conclusions • Have implemented a compact 16-module gamma camera • Have demonstrated the working of the 16-module gamma camera (at least with 4 modules) with good SNR (12.4% and 14.4% on the inner and edge pixel respectively for 140 keV emissions of Tc99m) • Have measured system spatial resolution with an all-purpose and high-sensitivity collimator • This technology is promising and may soon be available for breast cancer and axillary lymph nodes imaging • Working with Capintec, Inc to build a 25-module and 100-module gamma camera using 2.4 mm x 2.4 mm pixel photodiode array

  29. Funding • U.S. Department of Army • U.S. Department of Energy • National Institute of Health • Capintec, Inc

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