A COMPUTATIONAL METHOD FOR VOXEL TO POLYGON MESH CONVERSION OF ANATOMIC COMPUTATIONAL HUMAN PHANTOMS AND AIRCREW DOSES FROM COSMIC RAY SOURCES By JUSTIN L. BROWN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2017 © 2017 Justin L. Brown To Mom and Dad ACKNOWLEDGMENTS First, I would like to thank my adviser, Dr. Wesley Bolch for providing me with so many opportunities to better myself as a researcher and securing funding for my studies. Without his support and guidance, this work wouldn’t be possible. I also thank my committee member Dr. Lynn Rill for her efforts and guidance in serving on my committee. Next, I would liketo thank my fellow ALRADS members, for getting me started and keeping me going. I would like to thank Dr. Takuya Furuta for always being available to discuss my work. I would also like to thank Dr. Emily Marshall for keeping me diligent and inspiring me to be passionate about my work. Lastly and most importantly, I thank my family, girlfriend, and friends for their constant support throughout my life and academic career. With special thanks to my mother and father for always supporting my passions in no matter what area they may be. 4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................... 4 LIST OF TABLES ...................................... 7 LIST OF FIGURES ..................................... 8 ABSTRACT ......................................... 9 CHAPTER 1 INTRODUCTION ................................... 11 1.1 Computational Phantoms ............................ 11 1.2 Polygon Meshes ................................. 12 1.3 Cosmic Radiation Sources ............................ 13 1.4 Aircrew Dosimetry ................................ 13 1.5 Research Purpose ................................ 14 2 A COMPUTATIONAL METHOD FOR VOXEL TO MESH CONVERSION ..... 15 2.1 Introduction: Computational Phantoms .................... 15 2.2 Materials and Methods ............................. 16 2.2.1 Data Preparation ............................. 16 2.2.2 Surface Generation ............................ 17 2.2.3 Surface Grouping ............................. 18 2.2.4 Surface Simplification .......................... 18 2.2.5 Line Grouping .............................. 19 2.2.6 Line Simplification ............................ 19 2.2.7 Polygon Detection ............................ 19 2.2.8 Polygon Correction ............................ 20 2.3 Results ...................................... 20 3 AIRCREW DOSIMETRY FROM COSMIC SOURCES ................ 24 3.1 Introduction: Aircrew Dosimetry ........................ 24 3.2 Materials and Methods ............................. 24 3.2.1 Source Modeling ............................. 24 3.2.2 Airplane Computational Modeling .................... 25 3.2.3 Passenger Computational Modeling ................... 26 3.3 Results ...................................... 26 4 CONCLUSION AND FUTURE WORK ........................ 34 4.1 A Computational Method for Voxel to Mesh Conversion ............ 34 4.2 Aircrew Dosimetry ................................ 34 5 REFERENCES ........................................ 35 BIOGRAPHICAL SKETCH ................................. 37 6 LIST OF TABLES Table page 2-1 Meshing time breakdown ............................... 23 2-2 Mesh element reduction data ............................. 23 3-1 Materials modeled in aircraft model .......................... 28 3-2 Ratio of neutron rates at various positions ...................... 29 3-3 Neutron rates at various positions within the aircraft ................. 30 3-4 Pilot dose due to neutrons ............................... 33 7 LIST OF FIGURES Figure page 2-1 A graphical depiction of a voxel. ........................... 21 2-2 Surface simplification process ............................. 21 2-3 Line simplification process ............................... 22 2-4 Hole detection process ................................. 22 2-5 Ear clipping process .................................. 22 2-6 Voxel and tetrahedral mesh comparison ........................ 23 3-1 Lithium-6 versus Lithium-7 cross section ....................... 27 3-2 Flight route ...................................... 27 3-3 Flight route particle spectra .............................. 28 3-4 Flight route particle spectra summary ......................... 28 3-5 Plane surface mesh .................................. 29 3-6 Surface and tetrahedral mesh plane model ...................... 29 3-7 Aircraft interior and tracking areas .......................... 30 3-8 Seated UF adult reference phantoms ......................... 30 3-9 Neutron fluence to all tracked areas in Aluminum model ............... 31 3-10 Neutron fluence to all tracked areas in Magnesium model .............. 31 3-11 Neutron fluence material comparison ......................... 32 3-12 Neutron fluence probability density functions ..................... 32 3-13 Neutron fluence cumulative probability density function ............... 33 8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science A COMPUTATIONAL METHOD FOR VOXEL TO POLYGON MESH CONVERSION OF ANATOMIC COMPUTATIONAL HUMAN PHANTOMS AND AIRCREW DOSES FROM COSMIC RAY SOURCES By Justin L. Brown December 2017 Chair: Wesley E. Bolch Major: Medical Sciences Over the past 20 years, use of computational human phantoms within existing Monte Carlo radiation transport codes required those phantoms to be in a voxelized format. Recently, however, the current generation of codes such as MCNP, PHITS, and GEANT4 now allow the transport in human computational phantoms represented by polygon mesh structures for both the outer body contour and internal organ structures. While both phantoms provide a high degree of anatomic realism compared to first-generation stylized (or mathematical phantoms), mesh-type phantoms are now considered the state-of-the-art, and permit re-sculpting of individual organs, body circumferences, body size and shape, and extremity articulation – all features not readily available to voxel-based phantoms. However, over the past two decades, a tremendous number of voxel phantoms has been developed from either CT or MR data, and thus there is a need for conversion of existing voxel phantoms to mesh-type formats to allow these additional benefits of the new phantom formats. A major goal of this work isto develop an efficient and accurate methodology to convert voxel-based phantoms to mesh-based phantoms. For this conversion, a boundary detection algorithm is implemented in conjunction with polygon detection to form high-quality meshed data suitable for radiation transport simulations and finite element analyses. This conversion can result in a reduction of required simulation time as well as allowing current voxel data to be used in modern CAD software. An additional goal of this study was to use new mesh-type phantoms to assess the radiation 9 exposure due to passengers and aircrew resulting from secondary particle irradiation following to cosmic radiation exposures of the aircraft. At present, aircrew dosimetry is performed using radiation field models in conjunction with fluence-to-dose conversion coefficients. Thisstudy utilizes a mesh-based geometry and computational human phantoms to assess effective dose as well as organ absorbed doses explicitly during a simulated aircraft flight. A novel method of reduction in secondary neutron dose to the passengers and aircrew are further explored in this study. 10 CHAPTER 1 INTRODUCTION 1.1 Computational Phantoms When assessing radiation dose to an individual whether, from a medical procedure or an occupational industrial exposure, various forms of radiation detectors are typically used to quantify the radiation dose and the energy spectra of radiation particles reaching the individual. These devices are typically placed on or near the patient (for medical exposures) or in the exposure environment (for occupational exposures). In all cases, these devices give rough estimates of radiation dose to the individual, but they do not accurately capture the true irradiation geometry of the situation. It would be impossible to determine doses to any arbitrary point in the human body with any modern dosimeter in a practical sense. A remedy for this dilemma is the application of computer simulations with detailed computational human phantoms. For radiation dosimetry, it is common to combine computational human phantoms with a general purpose Monte Carlo (MC) radiation transport code. A MC radiation transport code is able to transport particles with random sampling techniques so as to simulate a wide variety of radiation exposures and resulting organ doses. These MC simulations allow for accurate and realistic creation of scenarios in which an individual might be exposed to radiation where organ level doses can be computed. Computational phantoms have rapidly evolved over time progressing from stylized to voxel to hybrid models. Stylized phantoms were the earliest forms of human phantoms and are constructed using simple sets of 3D mathematical surface expressions such as spheres, ellipsoids, cylinders, and cones (Lee and Lee, 2006). These phantoms are limited because they cannot accurately represent the complex internal anatomical structures of the human body. The next generation of voxel-based phantoms provided a remedy to this problem. Voxel phantoms