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X-ray Imaging Using Single Photon Processing with Semiconductor Pixel Detectors

This article explores the origins and advancements in x-ray imaging using single photon processing with semiconductor pixel detectors. It discusses the benefits of quantum imaging, the requirements for medical imaging detectors, and the applications of Medipix1.

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X-ray Imaging Using Single Photon Processing with Semiconductor Pixel Detectors

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  1. X-ray ImagingUsingSingle Photon ProcessingwithSemiconductor Pixel Detectors

  2. The Origins... • High energy physics: • unambiguous reconstruction of particle patterns with micrometer precision • low input noise due to tiny pixel capacitance WA 97, RD 19 (CERN): 208Pb ions on Pb target 7 planes of silicon pixel ladders; 1.1 M pixels Bettina Mikulec Vertex 2002, 8 Nov. 2002

  3. Electronics CMOS technology advances steadily; Moore’s law Sensors new materials to increase stopping power and CCE; main problem: inhomogeneities! Hybrid Pixel Detectors Bettina Mikulec Vertex 2002, 8 Nov. 2002

  4. if  Q > process signal Single Photon Processing • Quantum imaging • Example: photon counting  Q has to correspond to a single particle! Bettina Mikulec Vertex 2002, 8 Nov. 2002

  5. Quantum Imaging - Advantages • Noise suppression • high signal-to-noise ratio; dose reduction • low rate imaging applications • Linear and theoretically unlimited dynamic range • Potential for discrimination of strongly Compton scattered photons (for mono-energetic sources) or e.g. fluorescence X-rays • Energy weighting of photons with spectral sources possible • higher dose efficiency; dose reduction Bettina Mikulec Vertex 2002, 8 Nov. 2002

  6. Medical Imaging Detector requirements (sensor and electronics) depend on diagnostic X-ray imaging application. Example: mammography • spatial resolution 5-20 lp/mm • high contrast resolution (<3%) • uniform response • patient dose <3 mGy • imaging area: 18 x 24 (24 x 30) cm2 • compact and easy to handle • stable operation • no cooling • digital • cheap Moore and direct detection quantum processing sensors to be improved high DQE (sensor + q.p.) to be solved ??? Bettina Mikulec Vertex 2002, 8 Nov. 2002

  7. Medipix1 square pixel size of 170 µm 64 x 64 pixels sensitive to positive input charge detector leakage current compensation columnwise one discriminator 15-bit counter per pixel count rate: ~1 MHz/pixel (35 MHz/mm2) parallel I/O 1 m SACMOS technology (1.6M transistors/chip) Medipix2 square pixel size of 55 µm 256 x 256 pixels sensitive to positive or negative input charge (free choice of different detector materials) pixel-by-pixel detector leakage current compensation window in energy discriminators designed to be linear over a large range 13-bit counter per pixel count rate: ~1 MHz/pixel (0.33 GHz/mm2) 3-side buttable serial or parallel I/O 0.25 m technology (33M transistors/chip) Medipix1 / Medipix2 Bettina Mikulec Vertex 2002, 8 Nov. 2002

  8. 12249 m 13907 m Medipix1 / Medipix2 the prototype… the new generation! Bettina Mikulec Vertex 2002, 8 Nov. 2002

  9. Medipix1 Applications Examples: • Dental radiography • Mammography • Angiography • Dynamic autoradiography • Tomosynthesis • Synchrotron applications • Electron-microscopy • Gamma camera • X-ray diffraction • Neutron detection • Dynamic defectoscopy • General research on photon counting! Bettina Mikulec Vertex 2002, 8 Nov. 2002

  10. Applications Dynamic Autoradiography: (INFN Napoli): Mammography (INFN Pisa, IFAE Barcelona): Mo tube 30 kV; Medipix1; part of a mammographic accreditation phantom Medipix1; 14C L-Leucine uptake from the solution into Octopus vulgaris eggs (last slice in time: 80 min) Bettina Mikulec Vertex 2002, 8 Nov. 2002

  11. Applications Dental Radiography (Univ. Glasgow, Univ. Freiburg, Mid-Sweden Univ.): Sens-A-Ray commercial dental CCD system (Regam Medical) Medipix1 160 Gy 80 Gy 40 Gy Bettina Mikulec Vertex 2002, 8 Nov. 2002

  12. Medipix1 - SNR Pixel-to-pixel non-uniformities: optimum for counting systems: Poisson limit  N optimum SNR = N /  N determined SNR for Medipix1 taking flood fields (Mo tube) covering the entire dynamic range of the chip:  SNRuncorr(max.) ~30 using a flatfield correction  Medipix1 follows perfectly the Poisson limit! Red curve = Poisson limit SNRuncorr Bettina Mikulec Vertex 2002, 8 Nov. 2002

  13. SNRuncorr Medipix1 - SNR SNRuncorr • differences in the raw SNR, but with flat field correction the Poisson limit is ALWAYS reached • BUT: flat field correction dependent on energy spectrum! • working in over-depletion reduces charge sharing effects  flat field corrects mainly sensor non-uniformities! Bettina Mikulec Vertex 2002, 8 Nov. 2002

  14. Medipix1 Flat Field Studies 2 kinds of non-uniformities: ‘waves’ and fixed pattern noise 17 V detector bias (under-depleted) 35 V detector bias (fully depleted) ‘waves’ due to bulk doping non-uniformities raw image wrong flat field; inverse ‘waves’, BUT: single pixel inhomogeneities smeared out  fixed pattern noise! flat field corrected Bettina Mikulec Vertex 2002, 8 Nov. 2002

  15. 8 V 24 V 32 V 48 V 64 V 80 V 16 V 4 V 12 V Si Wave Patterns • vary detector bias voltage from under- to over-depletion • divide flat field map @Vbias with map @100 V Bettina Mikulec Vertex 2002, 8 Nov. 2002

  16. Si Wave Patterns Section of the correction map for different detector bias: • ‘waves’ move in under-depletion; stable in over-depletion • amplitude decreases with bias, but waves don’t disappear completely Remark: images can be corrected for these non-uniformities Bettina Mikulec Vertex 2002, 8 Nov. 2002

  17. Dose Optimization • Dose optimization for specific imaging tasks: example: accumulation of single X-ray signals during X-ray of an anchovy Bettina Mikulec Vertex 2002, 8 Nov. 2002

  18. Summary Medipix1 • The Medipix1 prototype chip allows to study the photon counting approach • Comparison to charge integrating systems turned out to be sometimes difficult due to the larger pixel size of Medipix1 • Most of the problems encountered were due to sensor non-uniformities (e.g. locally varying leakage currents) and bump-bonding quality • Medipix1 turned out to be a tool to study the attached sensor; even silicon sensors show non-uniformities • The flat field correction was intensively studied and allows to minimize the pixel-to-pixel variations down to the Poisson limit over the full dynamic range of the chip. The energy dependence of the flat field correction has to be further investigated. • The experience with Medipix1 lead to many improvements implemented in the Medipix2 ASIC. Bettina Mikulec Vertex 2002, 8 Nov. 2002

  19. Medipix2 Characterization • all the reported measurements were done using the electronic calibration (injection capacitor + external voltage pulse). • The 8 fF injection capacitor nominal value has a tolerance of 10%. • The dedicated Muros2 readout system had been used Bettina Mikulec Vertex 2002, 8 Nov. 2002

  20. Medipix2 Characterization adjusted thresholds ~110 e- rms unadjusted thresholds ~500 e- rms Bettina Mikulec Vertex 2002, 8 Nov. 2002

  21. Medipix2 Characterization • Threshold linearity in the low threshold range: Bettina Mikulec Vertex 2002, 8 Nov. 2002

  22. Medipix2 Characterization • threshold at 2 ke- • injection of 1000 pulses of 3 ke- • matrix unmasked Bettina Mikulec Vertex 2002, 8 Nov. 2002

  23. Summary of the Electrical Measurements Bettina Mikulec Vertex 2002, 8 Nov. 2002

  24. Conclusions • Miniaturization of CMOS technology allows for small pixel sizes and increased functionality. • A new single photon processing chip Medipix2 consisting of a 256 x 256 matrix of 55 m square pixels has been produced and successfully characterized. • The potential of quantum imaging for various applications is still far from being fully explored. • Quantum imaging in the medical domain: • rather complete systems are required to convince end users • MTF and DQE curves as well as comparative phantom images are necessary for approval (see e.g. FDA) • A lot of progress has been made to achieve large areas; as yet no satisfactory solution for most medical applications • There is a trend in some applications towards object characterization in addition to simple transmission images need energy information  colour X-ray imaging Bettina Mikulec Vertex 2002, 8 Nov. 2002

  25. ‘Wishlist’ sensors: high absorption efficiency and improved homogeneity reliable ASIC-to-sensor connections tiling: large areas without dead space ASIC: • small pixel size with charge sharing solutions (modern CMOS technologies!) • low-noise front-end with appropriate sensor leakage current compensation; sensitive to electron and hole signals • very fast front-end for time-resolved studies • a precise threshold above noise • a multi-bit ADC/pixel for energy information (optimum weighting!) • large dynamic range • …??? cost! Bettina Mikulec Vertex 2002, 8 Nov. 2002

  26. Medipix1 Flat Field Studies a phantastic tool to studysensor inhomogeneities… • vary detector bias voltage from under- to over-depletion • calculate corresponding flat field from flood images (1st row) • divide with correction map from 100 V detector bias data (2nd row) Bettina Mikulec Vertex 2002, 8 Nov. 2002

  27. Medipix1 Flat Field Studies Bettina Mikulec Vertex 2002, 8 Nov. 2002

  28. Medipix2 Characterization adjusted thresholds ~110 e- rms mean ~1100 e- spread ~160 e- rms unadjusted thresholds ~400 e- rms Bettina Mikulec Vertex 2002, 8 Nov. 2002

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