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Investigation of Photon Counting Pixel Detectors for X-Ray Spectroscopy and Imaging

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Investigation of Photon Counting Pixel Detectors for X-Ray Spectroscopy and Imaging Investigation of photon counting pixel detectors for X-ray spectroscopy and imaging Der Naturwissenschaftlichen Fakult¨at der Friedrich-Alexander-Universit¨at Erlangen-Nurnberg¨ zur Erlangung des Doktorgrades Dr.rer.nat. vorgelegt von Patrick Takoukam Talla aus Yaounde/Kamerun Als Dissertation genehmigt von der Naturwissen- schaftlichen Fakult¨at der Universit¨at Erlangen-Nurnberg¨ Tag der mundlichen¨ Prufung:¨ 07. April 2011 Vorsitzender der Promotionskommission: Prof. Dr. Rainer Fink Erstberichterstatter: Prof. Dr. Gisela Anton Zweitberichterstatter: Prof. Dr. Valeria Rosso Dedication To my wife Sandrine Takoukam Talla To my parents Jean Baptiste Talla and Monique Talla To my late brother Guy Bertrand Wafeu Talla i Contents 1 Introduction 1 2 Interaction of X-rays with Matter 3 2.1 PhotoelectricEffect. .. .. .. .. .. .. .. .. .. .. 3 2.2 ComptonScattering .............................. 4 2.3 RayleighScattering. .. .. .. .. .. .. .. .. .. .. 5 2.4 PairProduction ................................ 5 2.5 Interaction of Electrons with Matter . ..... 6 2.6 Conclusion ................................... 7 3 The Medipix2 Detector 9 3.1 Description and operating Modes . ... 9 3.2 CountingPrinciple ............................... 11 3.3 ChargeSharing................................. 12 3.4 Conclusion ................................... 15 4 The Medipix3 Detector 17 4.1 Motivation ................................... 17 4.2 Description ................................... 18 4.3 Functionalities and operating Modes . ..... 20 4.4 ChargeSummingMode ............................ 20 4.5 Conclusion ................................... 21 5 Monte Carlo Simulations and Energy Responses of Medipix Detectors 23 5.1 MonteCarloToolROSI ............................ 23 5.2 Energy Response of the Medipix2 Detector . 24 5.3 Energy Response of the Medipix3 Detector . 27 5.4 Impact of Noise Contributions on the Energy Resolution of Medipix Detectors 29 5.5 Conclusion ................................... 31 6 Characterization of the Medipix3 Detector 33 6.1 Test Pulses Measurements to determine Optimal DAC Settings ...... 33 6.2 Equalisation Procedure of the Medipix3 Detector . ........ 37 6.3 Energy Calibration of the Medipix3 Detector . ...... 40 6.4 Energy Resolution of the Medipix3 Detector . ...... 42 6.5 Parameters Extraction for Monte Carlo Simulations . ........ 43 6.6 Energy Response of the Medipix3 Detector in Charge SummingMode . 43 6.7 Count Rate Linearity of the Medipix3 Detector . ...... 45 6.8 Conclusion ................................... 47 iii Contents 7 Spectrum Reconstruction with hybrid Photon counting Detectors 49 7.1 Motivation ................................... 49 7.2 Theory...................................... 50 7.3 Methods..................................... 51 7.4 Reconstruction with the Medipix2 Detector . ...... 57 7.5 Reconstruction with the Medipix3 Detector in Charge Summing Mode . 67 7.6 Conclusion ................................... 71 8 Determination of the kVp with the Medipix2 Detector 73 8.1 Determination of the kVp with multiple Filter Combinations ....... 74 8.2 Determination of the kVp with only one Filter Combination........ 80 8.3 Conclusion ................................... 81 9 Introduction to X-ray Imaging 83 9.1 ImageQualityMetrics ............................. 83 9.2 Imaging with the Medipix3 Detector . 85 9.3 Conclusion ................................... 89 10 Spatial Resolution of the Medipix3 Detector 91 10.1Methods .................................... 91 10.2 Spatial Resolution of the Medipix3 Detector in Single Pixel Mode . 93 10.3 Spatial Resolution of the Medipix3 Detector in Charge SummingMode . 96 10.4Conclusion ................................... 101 11 Material Reconstruction with Photon counting Detectors 103 11.1 Theory of Material Reconstruction . 103 11.2 CombinationMethod. 105 11.3 Minimization Algorithms . 106 11.4 Reconstruction Results with Medipix2 and Medipix3 . ......... 108 11.5 Material Reconstruction of the Solder Bumps of Medipix .......... 115 11.6Conclusion ................................... 118 12 Redesign of Charge Summing Mode of Medipix3 119 12.1 New Architecture of Charge Summing Mode of Medipix3 . 119 12.2 Simulation of the proposed architecture . ....... 120 12.3 Impact of the new Charge Summing Mode architecture on Imaging . 120 13 Summary and Outlook 123 14 Zusammenfassung und Ausblick 125 Acknowledgements 127 Literaturverzeichnis 139 iv List of Abbreviations ASIC Application Specific Integrated Circuit CMOS Complementary Metal Oxide Semiconductor MPX Medipix PCB Printed Circuit Board DAC Digital Analog Converter CSDA Continuous Slowing Down Approximation SPM Single Pixel Mode CSM ChargeSummingMode HGM High Gain Mode LGM Low Gain Mode CM Color Mode RW Read/Write Mode SNR Signal to Noise Ratio MTF Modulation Transfer Function PSF Point Spread Function LSF Line Spread Function ESF Edge Spread Function NPS Noise Power Spectrum DQE Detective Quantum Efficiency ROI Region of Interest 1 Introduction X-rays and gamma-rays were discovered respectively by W. C. R¨ontgen in 1895 and by Paul Villard in 1900. Since then, they are used in a wide range of applications e.g. in medicine or in industry for Non Destructive Testing. Nowadays, seventy percent of all medical inspections in imaging are carried out using X-rays [1]. They are electromagnetic waves like radiowaves or light but have a much smaller wave length. In medical radio- graphy the patient is irradiated using X-rays and the attenuated intensity is measured. This attenuation is material and energy dependent. This enables one to distinguish for example between soft tissues and bones. Films are usually used for the detection of X-rays in projective X-ray diagnostic. The rapid development in electronics in the last decades allows the digitization of the detec- tion signals and therefore a jump from integrating to photon counting pixel detectors like the Medipix2 detector. With this detector, we can count the incoming photons and gain at the same time information about their energy. Those properties of the detector enable for example the reconstruction of a polychromatic spectrum impinging on to the detector. A major drawback of the Medipix2 detector is that it suffers from charge sharing: the charge carriers produced by one photon can be distributed over several pixels. Therefore, an incoming photon can be detected by more than one pixel. As a consequence, the incoming and the measured spectrum are different. In order to suppress the influence of charge sharing the Medipix3 detector was developed. With its Charge Summing Mode it is able to correct for charge sharing in real time. The aim of this thesis is a detailed characterization of the detectors of the Medipix family. The first three chapters are about the basic interactions of radiation with matter and the introduction to photon counting detectors of the Medipix family. Chapter 5 intro- duces the Monte Carlo Simulation Tool ROSI and explains how the response functions of the Medipix detectors to monochromatic radiation are modelled. Characterization of the brand new Medipix3 detector in measurements is the main focus of chapter 6. Chapter 7 investigates the spectroscopic properties of the Medipix detectors and chapter 8 is about using the Medipix2 detector for quality assurance and constancy checks. Chapter 9 intro- duces metric quantities used to characterize an imaging system. The spatial resolution of the Medipix3 detector is presented in chapter 10. Chapter 11 focuses on energy resolved material reconstruction with Medipix detectors. The last chapter is about the redesign of Charge Summing Mode of Medipix3. 1 2 Interaction of X-rays with Matter Contents 2.1 PhotoelectricEffect ......................... 3 2.2 ComptonScattering......................... 4 2.3 RayleighScattering ......................... 5 2.4 PairProduction ........................... 5 2.5 InteractionofElectronswithMatter. 6 2.6 Conclusion .............................. 7 This chapter gives an overview of the interactions which occur when X-ray photons encounter matter. Photoelectric effect, Compton scattering, coherent scattering and pair production will be presented in the first part of the chapter. In fact these processes are the basis of all current photon detection devices and thus determine the sensitivity and the efficiency of a detector. The last part of the chapter focuses on the interaction of electrons with matter. 2.1 Photoelectric Effect In the photoelectric absorption process, a photon undergoes an interaction with an atom in which the photon completely disappears (see figure 2.1). An energetic photoelectron is released by the atom from one of its bound shells. For a photon of sufficient energy, the most probable origin of the photoelectron is the K-shell of the atoms since a free electron cannot absorb a photon and also conserve momentum. The recoil momentum is then absorbed by the nucleus. The energy of the ejected electron is given by [2]: E − = hν Eb (2.1) e − where Eb is the binding energy of the photo electron in its original shell, h the Planck constant, ν the frequency of the incident photon. The ejection of the photoelectron leaves an ionised atom. The vacancy is filled through rearrangement of electrons from other shells of the atom. This leads to characteristic X-ray photons or to the ejection of auger electrons. In general the cross section of the photoelectric effect increases with the atomic number Z and decreases with the energy E. For photon energies below 100 keV, a rough approximation
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