The unified Monte Carlo model of photon migration in scattering tissue-like media for the needs of Biomedical Optics
Nowadays optical and laser-based diagnostics technologies are widely used in many fields such as oncology, vascular and developmental biology, dermatology, pharmacy, materials sciences, food, and the cosmetic and health care industries. Optical techniques have a number of advantages over traditional methods and provide a broad variety of practical solutions for non-invasive diagnostics in a range of studies from single cells to biopsy of specific biological tissues and even whole organs. Conceptual design of a particular optical diagnostics system for non-invasive in vivo measurements of structural alterations in biological tissues and changes in their physiological properties requires careful selection of various technical parameters, including wavelength, coherence, polarization and intensity profile of incident optical radiation, sensitivity of the detector, size, and the geometry and mutual position of source and detector. When applied to biological tissues, multiple scattering of light overwhelmingly prevents laser-based techniques from providing high quality images of tissue structure and structural changes. Thus, comprehensive studies of optical radiation propagation and signal formation are required. Due to the complex inhomogeneous structure of biological tissues no general analytical solution exists that can describe the detected optical signal and how it is affected by structural or physiological changes. However, there is an exceptional example of a stochastic approach: the Monte Carlo method. Without extrinsic or intrinsic constraints, throughout the years the Monte Carlo method has been a gold standard for the assessment of optical radiation propagation and spacial localization of signals in biological tissues. Nonetheless, previously developed Monte Carlo models were extremely resource consuming and suffered from serious disadvantages, considerably limiting their applications. Due to the diversity of existing optical diagnostics modalities the development of a new Monte Carlo code was typically required for a particular practical application. This thesis describes the work that has gone into the development of the unified Monte Carlo model and considers its applications for the particular needs of biomedical optics. By utilizing the developed model, an intimate investigation of optical radiation propagation in highly scattering randomly inhomogeneous media such as biological tissues has been carried out. Further developments of the method and a complete theoretical background are presented. Emphasis is given to including the method the coherent properties of optical radiation. The developed model supports of a variety of configurations corresponding to the actual experimental conditions. The code of the developed model has been generalized and unified for multi-purpose use for various applications in biomedical optics utilizing object oriented programming. A dramatic speed up of simulations has been achieved using parallel programming on graphics cards. A user friendly, web-based prototype of a computational service which allows researchers worldwide to use, check and validate the developed model has been created. A hybrid peer-to-peer network has been applied for Monte Carlo simulations to deal with the limitations imposed by the multiuser nature of the online service. The developed model has undergone a number of reliability tests including a comprehensive comparison against theoretical predictions, other models and experiments. Both qualitative and quantitative agreements have been found. Aiming to understand the peculiarities of light propagation within tissue-like media, the model has been used in a number of studies. A procedure which allows estimating the distribution of the probing radiation for a particular configuration of an experimental system is presented. Pilot studies of energy transfer in various parts of human body have been performed. The influence of the optical parameters of tissue-like media such as scattering and anisotropy on the probing optical radiation has been studied and a new so called diffusing wave polarimetry optical diagnostic modality has been introduced. An opportunity to monitor the condition of biological tissues and grading cancerous stage by traversing the Stokes vector on the Poincare Sphere is presented and compared with the results of simulations. The model has been used to calculate the depolarization of light by rough surfaces of scattering phantoms. Finally, Monte Carlo modeling of the optical coherence tomography signal formation was considered, which resulted in the emergence of a novel imaging technique called the double correlation approach. The double correlation approach has been applied to observe the low frequency electric fields propagating in biological tissues, obtain the spatial distribution of superficial blood vessels in human skin and monitor the transcutaneous vaccine delivery in mouse skin. It is anticipated, that further developments of the unified Monte Carlo model presented in this thesis can potentially form a base for a comprehensive computational platform and can significantly impact the planning of experiments in the biophotonics and biomedical optics communities.
Advisor: Meglinski, Igor; Andersen, Mikkel; Eccles, Michael
Degree Name: Doctor of Philosophy
Degree Discipline: Department of Physics
Publisher: University of Otago
Keywords: Biophotonics; Biomedical Imaging; Monte Carlo method; Optical Diagnostics; Parallel programming; CUDA; Image processing; Optical coherence tomography; Diffusing-wave polarimetry; Earlier cancer diagnostics; Online Monte Carlo tool
Research Type: Thesis