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Non—Destructive Testing of Industrial Materials

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Non—Destructive Testing of Industrial Materials APPLICATIONS OF GAMMA RAY TOMOGRAPHY TO NON—DESTRUCTIVE TESTING OF INDUSTRIAL MATERIALS A THESIS SUBMITTED FOR THE AWARD OF THE DEGREE OF DOCTOR OF PHILOSOPHY TO THE UNIVERSITY OF LONDON BY MOHAMED M. ENNAMI REACTOR CENTRE IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE SEPTEMBER 1988. This thesis is dedicated to my wife Mariam, my son Haroun and to our parents 1 ABSTRACT This study investigates the Possibility of applying computerised gamma—ray tomography in non—destructive testing o-f industrial materials in transmission and emission modes. Measurements -for tomography using gamma—rays and the operation of radiation detectors are based on photon interactions with matter. The theory governing these interactions is discussed. The theory o-f the mathematical reconstruction of a two dimensional distribution from its projections is shown? and reconstruction techniques and applications are reviewed. The principles of gamma—ray detection and measurements are presented in order that the physical significance of the data recorded can be assessed. Detection characteristics pertinent to imaging applications were measured for the horizontal dipstick Ge<Li> detector used in these experiments. The importance of these characteristics in determining the suitability of this detector for imaging applications is di scussed. A prototype scanning rig was designed for transmission and eiii i ssi on tamour aph y , T’.,; r i o wiii-, ‘ * • used with phantoms t o determine contrast and spatial resolution. The usefulness of line scans and contrast measurements in quantitative analysis is shown. The effect of scattering on image quality is discussed briefly in terms of the quality of the data used for reconstruction? and scatter subtraction is used for all reconstructions. The effect of attenuation in Single Photon Emission Computed Tomography is studied and the available analytical attenuation correction techniques were briefly discussed. The use of multi—energy scanning as a viable industrial tomographic technique was experimentally investigated. The application of multi—energy scanning as an experimental attenuation correction method was suggested and also verified by means of scanning an ai umi ni urn phantom containing six vials of europium nitrate solutions at four different Y~ray energies. The reconstructed data was then used to derive a semi —empirical function for attenuation correction. The results obtained from all the scanning experiments illustrate the possibility of using the tomographic system developed here at the Reactor Centre for non—destructive scanning of industrial samples and nuclear waste packages in both transmission and emission modes. They also show the use of multi—energy scanning as both a tomographic technique and an attenuation correction method. ACKNOWLEDGEMENT For his enthusiasm, advise, guidance and constant encouragement throughout the period of this research t would like to sincerely thank my supervisor Dr- Desmond MacMahon- I would like to thank Drs- Peter Gray and Bill Glauert -for their help regarding the scanning and image display software. I would like to also thank the Reactor and workshop staff for their help with collimators, Phantoms an^, source preparations I am most grateful to friends and collegues in the Reactor Centre for helpful discussions and providing a pleasant envi recent - CONTENTS PAGE NO. ABSTRACT i ACKNOWLDEGEMENT i i i CONTENTS i v CHAPTER Is Introduction 1 CHAPTER 2: Interaction of Photons with Matter 5 2.1 Introduction 5 2.2 The Photo—electric Effect 5 2.3 Gamma Ray Scattering B 2.3.1 Coherent (Thompson) Scattering by 8 a Free Electron 2.3.2 Incoherent Scattering by a Free electron 10 2.3.3 Scattering from a Bound Electron 12 2.4 Pair Production 17 2.5 Total Attenuation Coefficients 18 2.6 Attenuation Coefficients in Elemental Analysis 21 CHAPTER 3: The Principles of Gamma Ray Detection and Measurements and Applications 26 3.1 Introduction 26 3.2 General Properties of Radiation Detectors 26 3.2.1 Generalised Operation of Detectors 26 3.2.2 Pulse Height Analysis 28 3-2.3 Detector Efficiency, Energy Resolution and Dead Time 29 Gamma Ray Spectroscopy V 3.3.2 Theoretically Predicted Pulse Height Distribution 33 3.3.3 Practical Spectroscopy with Semi-conductors 3B 3.4 Application of Gamma Ray Spectroscopy in Tomography 41 3.4.1 Emmission and Transmission Tomography 41 3.4.2 Determination o-f Source Depth by Scatter to Peak Measurements 49 CHAPTER 4: The Theory of Reconstructive Computerised Tomography 53 4.1 Introduction 53 4.2 Non—Reconstruct!ve Tomography 53 4.3 Reconstruct!ve Tomography 54 4.3.1 Statement of the Problem 57 4.3.2 Reconstruction Techniques 59 4.3.2.1 The Analytical Recontruction Technniques 60 4.3.2.2 The Fourier Transform Method 61 4.3.2.3 Back Projection 64 4-3.2.4 Filtered Back Projection 67 4.3.3 Iterative Methods 73 4-3.3-1 Type of Corrections 77 4.3.3.2 Weighting Factors 78 4.4 Discussion 80 CHAPTER 5: The Scanning System 83 5.1 Introduction 83 5.2 The Scanning Rig 85 vi 5.3 The Stepping Motors and Stepper Motor Interfaces 87 5.4 Detector and Source Colli mation 90 5.5 Detector and Counting Electronics 93 5.6 Experimental Control and Data Acquisition Program 93 5.7 Image Reconstruction and Display 97 5.8 Conclusion 99 CHAPTER 6: System Characterisation 101 6.1 Introduction 101 6.2 Sensitivity of Imaging Systems 101 6.2.1 Detector Homogeniety and Size Determination 102 6.2.2 Detector Efficiency and Resolution 104 6.3 Spatial Resolution and Sensitivitya* 108 6.3.1 Definition of the Point Spr^jfi Function 110 6.3.2 Geometrical Analysis of PSF of a Single Bore Collimator 112 6.3.3 Modulation transfer Function 116 6.3.4 Sensitivity and Resolution 119 6.4 Discussion and Results 121 CHAPTER 7: Experiments in Tomography 140 7.1 Introduction 140 7.2 Scanning Geometry 140 7.2.1 Transmission Geometry 140 7.2.2 Emission Geometry 141 7.3 Tomography Experiments 143 7.3.1 Scanning Summaries 143 7.3.2 Scanning Details 144 vi i 7.4 Reconstructed Images and Analysis 148 7.4.1 Contrast In image Analysis 148 7.4.2 Analysis of Images Obtained in Tomography Experiments 151 7.5 Discussion and Conclusion 156 CHAPTER 8: liul ti —Energ y Scanning and The problem of Attenuation in SPECT 172 8.1 Introduction 172 8.2 The Effects of Attenuation On Reconstruction 172 8.2.1 Attenuation Problems for Positron ECT 173 8.2.2 Attenuation Problem for Single Photon ECT 175 8.2.3 Example of Single Photon Attenuation Effect 177 8.3 Attenuation Compensation 179 8.4 The Multi—Energy Scanning Experiment 180 8.4.1 Source and Phantom Preparation 180 8.4.2 Scanning Details 181 8.4.3 Reconstructed Images and Analysis 183 8.5’ The Use of Multi—Energy Scanning for Attenuation Correction in SPECT 187 8.6 Discussion and Conclusion 189 CHAPTER 9: Conclusions 210 9.1 Recommendations for Further Work 214 REFERENCES 216 1 CHAPTER ONE INTRODUCTION The -fundamental aim of tomographic techniques is to produce an image of a slice through an object which is free from interference effects from the underlying and overlying planes- For many years focal plane tomography in which the underlying and overlying layers are blurred whilst the slice of interest is kept in focus was employed by (BOC—1921, ANG—1968) before computerised tomography was introduced. However, the complete elimination of the effect from neighbouring layers is not possible using this non—reconstructive technique. The basic principle of computerised tomography is that the internal structure of an object can be reconstructed from the infinite set of all possible projections of the object- The mathematical theory governing this principle, that is, a two or three dimensional object can be reconstructed from the infinite set of its projections, was proved by the Austrian mathematician ,J. Radon (RAD—1917) working on gravitational theory many years before the first commercial computerised tomography scanner was introduced (HOU-1973). The first application of mathematical reconstruction was in radioastronomy in 1956 (BRA-1956) which was followed in several other fields including optics and electron microscopy (ROW—1969, De ROS-1968). However it is its medical application that has had the greatest impact and provided impetus for widespread research (HOU—1973; SHE-1974; BUD—1974; DUB—1977). The measurement of projections by means of the detection of a probe which may originate within, be transmitted through 2 dp stimulated within the object provides the basic data -for reconstruction. A projection is therefore either a measure of properties of the materials comprising the object which governs the transmission or stimulation of the probe or is the concentration of the probe itself. The number of types of probes used in imaging is large and continues to increase, including: nuclear magnetic resonance (SHE—1980, CRO—1982), electrical impedance (PRI-1979), thermal microwaves (SCH—1979), X—ray fluoroscopy (PAT—1980, BAI—1979) and pions (WIL—1980). However, the probes that have been most widely applied in both me'dica! and non—medical fields, and have therefore been the subject of a very large number of research studies, are x — and y— rays. X— and Y-rays have been extensively employed in transmission and emission tomography systems since the very start of medical imaging (KUH—1963; PHE—1977; WIL—1979; MUE—1976; HOF-1981; KUH-1976; HOU-1973) . The application of Compton scattered photons has been studied (CLA—1069; STOK-1981; HAR—1982; BAL-1986). Although the mathematical reconstruction of images was first applied in non—medical fields (RAFD—1917; BRA—1956; De ROS—1968), since then the majority of research studies and hence the major advancements have been with respect to medical imaging applications.
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