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Implementation and Characterization of Cone-Beam Computed Tomography Using a Cobalt-60 Gamma Ray Source for Radiation Therapy Patient Localization

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Implementation and Characterization of Cone-Beam Computed Tomography Using a Cobalt-60 Gamma Ray Source for Radiation Therapy Patient Localization IMPLEMENTATION AND CHARACTERIZATION OF CONE-BEAM COMPUTED TOMOGRAPHY USING A COBALT-60 GAMMA RAY SOURCE FOR RADIATION THERAPY PATIENT LOCALIZATION by Nicholas Jeffrey Rawluk A thesis submitted to the Department of Physics, Engineering Physics & Astronomy In conformity with the requirements for the degree of Master of Applied Science Queen’s University Kingston, Ontario, Canada (November, 2010) Copyright © Nicholas Jeffrey Rawluk, 2010 Abstract Cobalt-60 (Co-60) radiation therapy is a simple and reliable method of treating cancer by irradiating treatment volumes within the patient with high energy gamma rays. Medical linear accelerators (linacs) began to replace Co-60 units during the 1960’s in more developed countries, but Co-60 has remained the main source of radiotherapy treatment in less developed countries around the world. As a result, technological advancements made in more developed countries to deliver more precise radiation treatment that improves patient outcome have not been clinically applied to Co-60 machines. The medical physics group at the Cancer Centre of Southeastern Ontario has shown that these same technological advancements can be applied to Co-60 machines which would increase the accessibility of these modern improvements in radiotherapy treatment. However, for these modern treatments to improve patient outcome they require more precise localization of the patient prior to therapy. In more developed countries, this is currently provided by comparing computed tomography (CT) used for treatment planning with images acquired on the linac immediately before treatment. In the past decade, cone-beam CT (CBCT) has been developed to provide 3D CT images of the patient immediately prior to treatment on a linac. This imaging modality would also be ideal for patient localization when conducting modern Co-60 treatments since it would only require the addition of an imaging panel to produce CBCT images using the Co-60 source. A prototype Co-60 CBCT imaging system was implemented and characterized. Image noise, contrast, spatial resolution, and artifacts were studied. Algorithms to reduce the image artifacts were implemented and found to improve perceived image quality. The imaging system was found to have a ~1.8 mm high-contrast spatial resolution and the ability to detect 3 cm low- contrast soft-tissue structures in water. Anthropomorphic phantoms were also imaged and the observed anatomy in Co-60 CBCT images was comparable to kilovoltage CT. These results are ii comparable to clinically relevant linac-based CBCT using high energy X rays of similar energies to Co-60 gamma rays. This suggests that Co-60 CBCT should be able to provide the necessary images to localize patients for modern Co-60 radiation treatments. iii Acknowledgements First, I would like to thank my supervisors Dr. Johnson Darko and Dr. Andrew Kerr. They provided me with their constant encouragement, guidance, critique and experience while I conducted and analysed the research for my thesis. It has been a terrific experience to learn from them both. Sharing an office with Tim Olding quickly turned him into a sounding board for my ideas and I am grateful for all the discussions he provided. I would like to thank my friends and colleagues at the Cancer Centre of Southeastern Ontario for sharing their skills, experience, and time with me: Sandeep Dhanesar, Laura Drever, Tom Feuerstake, Lourdes Maria Garcia, Tracy Halsall, Chandra Joshi, Amy Macdonald, Matthew Marsh, Lynda Mowers, Chris Peters, Greg Salomons, Daxa Saparia, John Schreiner, and Kevan Welch. I also highly appreciated the patience of the rest of the CCSEO staff by allowing me to work in their clinical environment. Thanks to my friends in and outside of Kingston for their support and by providing me with the perfect amount of distraction from my research. I want to thank Jacky for being such a wonderfully supportive partner to me throughout my degree. Finally, I wish to give thanks to my parents for always supporting me while giving me the freedom to pursue my passions. iv Table of Contents Abstract............................................................................................................................................ii Acknowledgements.........................................................................................................................iv Table of Contents.............................................................................................................................v List of Figures...............................................................................................................................viii Chapter 1 Introduction .....................................................................................................................1 1.1 Motivation..............................................................................................................................1 1.2 Introduction to Radiation Therapy.........................................................................................2 1.3 Objectives ..............................................................................................................................6 1.4 Chapter Outline......................................................................................................................7 Chapter 2 Literature Review............................................................................................................9 2.1 Tumour Imaging and Treatment Planning.............................................................................9 2.2 Patient Localization and Image-Guided Radiation Therapy................................................11 2.3 Cobalt Therapy and World Context.....................................................................................13 Chapter 3 Theory ...........................................................................................................................16 3.1 Portal Image Acquisition .....................................................................................................16 3.1.1 Introductory Physics .....................................................................................................16 3.1.2 Amorphous Silicon Panel Physics ................................................................................17 3.1.3 Image Acquisition.........................................................................................................21 3.2 Cone Beam Computed Tomography....................................................................................21 3.2.1 2D Parallel-Beam Projection and Reconstruction.........................................................23 3.2.2 Filtered Back-Projection ...............................................................................................26 3.2.3 Fan-beam Projection and Back-Projection ...................................................................28 3.2.4 Half-Scan ......................................................................................................................30 v 3.2.5 Half-Beam Scan for Imaging Large Objects.................................................................31 3.2.6 Cone-Beam CT .............................................................................................................32 3.3 Co-60 CBCT Artifacts .........................................................................................................34 3.3.1 Blurring.........................................................................................................................35 3.3.2 Ring Artifact .................................................................................................................36 3.3.3 Cupping.........................................................................................................................37 3.4 Image Quality.......................................................................................................................38 3.4.1 Contrast.........................................................................................................................38 3.4.2 Resolution and the Modulation Transfer Function .......................................................39 3.4.3 Noise .............................................................................................................................41 3.5 Image Display ......................................................................................................................42 Chapter 4 Methods and Materials ..................................................................................................44 4.1 Co-60 Cone-beam Computed Tomography System ............................................................44 4.2 Image Acquisition................................................................................................................46 4.2.1 Beam and Phantom Setup .............................................................................................46 4.2.2 Acquisition....................................................................................................................46 4.2.3 Pre-Reconstruction Processing of Portal Images ..........................................................47 4.3 CBCT Image Reconstruction...............................................................................................47 4.4 Image Processing Methods for Artifact Reduction..............................................................48 4.4.1
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