Xray Imaging Using Single Photon Processing with Semiconductor

X-ray Imaging Using Single Photon Processing with Semiconductor Pixel Detectors B. Mikulec* M. Campbell, E. Heijne, X. Llopart, L. Tlustos CERN, Medipix Collaboration * now with the University of Geneva, Switzerland

The Origins. . . u High energy physics: n n unambiguous reconstruction of particle patterns with micrometer precision low input noise due to tiny pixel capacitance WA 97, RD 19 (CERN): 208 Pb ions on Pb target 7 planes of silicon pixel ladders; 1. 1 M pixels Bettina Mikulec Vertex 2002, 8 Nov. 2002

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

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

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

Medical Imaging Detector requirements (sensor and electronics) depend on diagnostic X-ray imaging application. Example: mammography u spatial resolution 5 -20 lp/mm u high contrast resolution (<3%) u uniform response u patient dose <3 m. Gy u imaging area: 18 x 24 (24 x 30) cm 2 u compact and easy to handle u stable operation u no cooling u digital u cheap Bettina Mikulec Moore and direct detection quantum processing sensors to be improved high DQE (sensor + q. p. ) to be solved ? ? ? Vertex 2002, 8 Nov. 2002

Medipix 1 / Medipix 2 Medipix 1 u u u square pixel size of 170 µm 64 x 64 pixels sensitive to positive input charge detector leakage current compensation columnwise one discriminator Medipix 2 u u u u 15 -bit counter pixel u count rate: ~1 MHz/pixel (35 MHz/mm 2) u parallel I/O 1 m SACMOS technology (1. 6 M transistors/chip) u Bettina Mikulec u u u 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 pixel count rate: ~1 MHz/pixel (0. 33 GHz/mm 2) 3 -side buttable serial or parallel I/O 0. 25 m technology (33 M transistors/chip) Vertex 2002, 8 Nov. 2002

Medipix 1 / Medipix 2 the prototype… 12249 m 13907 m the new generation! Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 1 Applications Examples: u Dental radiography u Mammography u Angiography u Dynamic autoradiography u Tomosynthesis u Synchrotron applications u Electron-microscopy u Gamma camera u X-ray diffraction u Neutron detection u Dynamic defectoscopy u General research on photon counting! Bettina Mikulec Vertex 2002, 8 Nov. 2002

Applications Mammography (INFN Pisa, IFAE Barcelona): Dynamic Autoradiography: (INFN Napoli): Mo tube 30 k. V; Medipix 1; part of a mammographic accreditation phantom Medipix 1; 14 C L-Leucine uptake from the solution into Octopus vulgaris eggs (last slice in time: 80 min) Bettina Mikulec Vertex 2002, 8 Nov. 2002

Applications Dental Radiography (Univ. Glasgow, Univ. Freiburg, Mid-Sweden Univ. ): Sens-A-Ray commercial dental CCD system (Regam Medical) Medipix 1 160 Gy Bettina Mikulec 80 Gy 40 Gy Vertex 2002, 8 Nov. 2002

Medipix 1 - SNR Pixel-to-pixel non-uniformities: optimum for counting systems: Poisson limit N optimum SNR = N / N determined SNR for Medipix 1 taking flood fields (Mo tube) covering the entire dynamic range of the chip: Red curve = Poisson limit SNRuncorr(max. ) ~30 using a flatfield correction SNRuncorr Medipix 1 follows perfectly the Poisson limit! Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 1 - SNRuncorr 8. 5 ke. V with adj. (35 V det. bias) 29. 8 without adj. (35 V det. bias) SNRuncorr flat field corrects mainly sensor non-uniformities! Bettina Mikulec 11. 7 ke. V 12. 4 ke. V 18. 8 7 with adj. (17 V det. bias) 19. 2 with adj. (80 V det. bias) 30. 7 § 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 Vertex 2002, 8 Nov. 2002

Medipix 1 Flat Field Studies 2 kinds of non-uniformities: ‘waves’ and fixed pattern noise 17 V detector bias (under-depleted) ‘waves’ due to bulk doping non-uniformities 35 V detector bias (fully depleted) raw image flat field corrected Bettina Mikulec wrong flat field; inverse ‘waves’, BUT: single pixel inhomogeneities smeared out fixed pattern noise! Vertex 2002, 8 Nov. 2002

Si Wave Patterns 16 80 24 32 48 64 12 8 4 V u u 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

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

Dose Optimization u 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

Summary Medipix 1 u u u The Medipix 1 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 Medipix 1 Most of the problems encountered were due to sensor nonuniformities (e. g. locally varying leakage currents) and bump -bonding quality Medipix 1 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 Medipix 1 lead to many improvements implemented in the Medipix 2 ASIC. Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 2 Characterization u u u all the reported measurements were done using the electronic calibration (injection capacitor + external voltage pulse). The 8 f. F injection capacitor nominal value has a tolerance of 10%. The dedicated Muros 2 readout system had been used Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 2 Characterization adjusted thresholds ~110 e- rms unadjusted thresholds ~500 e- rms Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 2 Characterization u Threshold linearity in the low threshold range: Bettina Mikulec Vertex 2002, 8 Nov. 2002

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

Summary of the Electrical Measurements Electron/Hole Collection Gain ~12 m. V/ke- Non-linearity <3% to 80 ke- Peaking time <200 ns Return to baseline Electronic noise <1 s for Qin <50 ke- n. THL ~100 e- n. THH ~100 e- Threshold dispersion THL ~500 e- THH ~500 e- Adjusted threshold dispersion THL ~110 e- THH ~110 e- Minimum threshold ~1000 e- Analog power dissipation Bettina Mikulec ~8 W/channel at 2. 2 V supply Vertex 2002, 8 Nov. 2002

Conclusions u u Miniaturization of CMOS technology allows for small pixel sizes and increased functionality. A new single photon processing chip Medipix 2 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: n n 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) u A lot of progress has been made to achieve large areas; as yet no satisfactory solution for most medical applications u 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

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

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

Medipix 1 Flat Field Studies Bettina Mikulec Vertex 2002, 8 Nov. 2002

Medipix 2 Characterization adjusted thresholds ~110 e- rms mean ~1100 espread ~160 e- rms unadjusted thresholds ~400 e- rms Bettina Mikulec Vertex 2002, 8 Nov. 2002