Monte Carlo Modelling of Light Propagation into Three-Layered Tissues and Tissue Phantoms

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Optical Coherent Tomography

Optical Coherent Tomography (OCT) is another non-invasive diagnostic imaging modality utilizing the light scattering in tissue is. OCT is based on the coherence interferometry [59], occurring over a distance of micrometers. OCT enables for two- and three-dimensional imaging in biological tissues by detecting near infrared light distribution in the tissue, and measuring the reflected or backscattered intensity of light as a function of depth. The OCT technique provides a spatial resolution close to the cellular level. The structural tissue abnormalities influence on the scattering in the bladder wall, revealing the deviation from the normal images. Thus, at the neoplastic state, the mucosal layer can be thicker than at the normal state, and hardly distinguished from the other layers, sometimes shows complete loss of a regular structure [60]. The newest OCT technology allows for making in vivo diagnoses using optical fibers and cystoscopic biopsy channel. However, OCT cannot provide the screening of the entire bladder due to the difficulties in the navigation. For this reason it still has to be used in combination with other methods [5].

Laser Doppler Perfusion Imaging

Laser Doppler Imaging (LDI) is a non-invasive diagnostic method for optical measurement of tissue perfusion. LDI is based on the scattering contrast of moving and motionless tissue components, induced by a coherent light source. Due to the static state of the tissue surrounding the shifting blood cells, the light scattering caused by this tissue does not contribute to the Doppler shifted signal [61], [62]. The signals measured by the method are often related to flux, velocity and concentration of the moving blood cells [63]. The scanning laser Doppler perfusion imaging method acquires both single and multiple scattering from moving blood cells. The speckle phenomenon appears in the Doppler imaging in changes of the tissue optical properties in time and space, leading to the changes in the detected signal.
The major advantage of the laser Doppler techniques in general is its simple implementation in instruments, requiring only an optical probe, a source of coherent light, and a camera or a receiving fiber. This method is non-invasive and allows to measure the perfusion in real time. However, the velocity distribution of red blood cells cannot be received in absolute blood flow values, since this depends on non-considered by LDI properties of photon interaction with red blood cells. Also the technique suffers from the noise from the outside motions.

Diffuse-Reflectance Imaging and Spectroscopy

Optical spectroscopy is a diagnostic technique allowing the detection in vivo biochemical and morphological changes that occur in tissue during neoplasm progression. Modern applications in optical biophotonics rely on the use of elastic scattering parameters to characterize the underlying architectural properties of biological tissue [64]. Cellular changes in sub-surface tissue layers, caused by dysplastic progression in epithelial tissues, affect the absorption, scattering and fluorescence properties of tissue. They also lead to diagnostically significant differences in the measured back-scattered spectral signals. The absorption properties of the tissue provide information on the concentration of various chromospheres, while the scattering properties are rather important for interpretation of the form, size, and the concentration of the scattering components in tissue. Since spectral measurements do not require tissue removal, the diagnostic information can be obtained non-invasively and in real-time, providing an objective and quantitative description of the tissue.
The spectroscopic techniques are easily applicable for the endoscopic diagnosis, as they are quite simple in their instrumentation: the spectral measurements are provided by an optical probe, and analysed by a spectrometer. A laser light beam illuminates the tissue, and the back-scattered light is collected by optical fiber.
Diffuse-reflectance imaging can be used in addition to spectroscopy or as a self-dependent technique for neoplasm diagnosis. In the case of implementation of the diffuse reflectance at an imaging mode, diffuse backscattered light is detected by means of a video camera. The technique can detect both, diffuse-reflected scattered and fluorescent light. Some of the optical properties can be obtained in vivo using this imaging and spectroscopic techniques [6], which allow to describe the tissue state.
Fluorescence Techniques (imaging or spectroscopic) have high sensitivity (more than 90%), however they require photosensitive drug use, and they have low specificity [5]. Autofluorescence studies of the biological tissues are mostly based on the analysis of the endogenous fluorescing molecules [65]. Almost all biological tissues emit fluorescence when excited at appropriate wavelength in the UV or visible spectral range. A biological tissue consists of a complex matrix of fluorescing and non-fluorescing molecules. The major fluorescing tissue proteins are collagen and elastin, both present in the structural matrix of numerous tissues [65]. Due to the low autofluorescence intensity, the autofluorescence systems do not exhibit a sufficient contrast.

Conclusion and Problem Formation

Even though the disadvantages of the existing optical techniques for the bladder diagnosis are different, they can be still generalized. The main lack of all the techniques is that they are specified for a certain metamorphosis, not allowing the other tissue changes to be taken into account. Thereafter, detection of several tissue “markers” simultaneously can improve the diagnosis, by providing information on complex data of tissue state.
The DR light analysis can be used for different purposes by means of varying the type of the measurements and optical constructions. Non-polarized DR imaging or spectroscopy can be easily coupled to a WL endoscopic imaging in order to provide additional information for a precancerous and non-invasive tissue diagnosis. The principle of the non-polarized DR imaging associated to cystoscopy is to illuminate some areas on the tissue surface by means of the optical fibers, and to measure the spatial distribution of the back-scattered non-polarized light. Backscattered light, arising from illuminated tissues, is influenced by the changes in nuclear size, epithelium thickness and other tissue components, which can serve as “markers” for the neoplasm detection. Thereafter, the method of detection of non-polarized DR light has big potential to provide useful diagnostic information to specify the tissue changes of early neoplasms in vivo and non-invasively.
As modern techniques require a complex approach for soling diagnostic problems, then not only instrumentation has to be determined. Mathematical modelling is also a useful tool for modern diagnostic techniques. For multi-layered tissue-light interaction simulations, a Monte Carlo-based method can be used. Such mathematical description of tissue optical properties can provide diagnostically useful information [66]. Forward and inverse problems are commonly used for the Monte Carlo problematic solutions [67]. The complete process of solving problems of tissue-state description by the diffuse reflected light analysis can be divided into the following parts:
– Theoretical (mathematical) modelling of the DR from the surface of the medium (forward problem);
– Experimental studies of the surface DR distribution;
– Calibration (comparison of the theoretical and experimental studies);
– Solving inverse problem.
In the forward problems, for which some input parameters are known, the process of the diffuse reflectance can be mathematically described with the use of the optical parameters of the medium. In the inverse problems, on the contrary, the unknown necessary parameters can be obtained from the DR light signals. This thesis describes forward experimental and mathematical (Monte Carlo based) methods of visualization of the DR light on a surface of 3-layered models of the UB wall (Fig. 1.5).
Thus, we expect our method of DR light detection to allow to analyse scattering and absorption tissue properties, and thereafter to make it possible to differentiate in vivo different tissue states at the early cancerous and pre-cancerous stages.

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Table of contents :

CHAPTER 1 Posing a Problem: Neoplasm Formation and Optical Diagnosis
1.1 Introduction
1.2 Basic Concepts of Light Propagation into a Biological Medium
1.2.1 Reflection
1.2.2 Absorption
1.2.3 Scattering
1.2.4 Fluorescence
1.3 Urinary Bladder Carcinogenesis
1.3.1 Structure of the Urinary Bladder Wall Mucous Submucous Muscular Layer
1.3.2 Neoplasm Formation
1.4 Optical Diagnostic Methods
1.4.1 Photodynamic Diagnosis
1.4.2 Optical Coherent Tomography
1.4.3 Laser Doppler Perfusion Imaging
1.4.4 Diffuse-Reflectance Imaging and Spectroscopy
1.5 Conclusion and Problem Formation
CHAPTER 2 Experimental Investigation of Diffuse-Reflected Light
2.1 Introduction
2.2 Sensitivity Tests of the Diffuse-Reflectance Imaging Method
2.2.1 Experimental Construction
2.2.2 Three-Layered Phantoms Construction
2.2.3 Image Processing Image Subtraction Three-Dimensional Diagrams Area Measuring
2.2.4 Discussion
2.3 Five States of Urinary Bladder Epithelium
2.3.1 Phantom Construction
2.3.2 Acquired Images
2.3.3 Results and Discussion
2.4 Surface Fluorescence Signal Detection
2.4.1 Preparation of Experimental Studies Clinical Measurements of Photosensitizer concentrations Results
2.4.2 Phantom and Experimental Construction
2.4.3 Results and Discussion Fluorescence Signals Back-Scattered Laser Signals
2.5 Conclusion
CHAPTER 3 Calculation of Optical Scattering Parameters of Bladder Tissues and Tissue-Like Phantoms
3.1 Light Scattering in Biological Tissues
3.2. Small Scatterers and Mitochondrial Contribution to Light Scattering in Bladder Epithelium
3.2.1 Golgi Apparatus and Lysosomes
3.2.2 Mitochondria
3.3 Electromagnetic Wave Theory
3.4 Optical Scattering and Absorption Parameters of Biological Tissues
3.5 Mie Calculations of Optical Parameters of Bladder Tissues and Phantoms
3.5.1 Mie Scattering by Spherical Particles Modelling of Light Propagation into a Three-Layered Medium with Diffusing Particles Calculation Results:Input Parameters for Monte Carlo Modelling
3.5.2 Scattering by Nucleated Cells Modelled as “Coated” Spheres Scattering Amplitudes Definition of Input Optical Parameters for Mie Calculations Output Scattering and Absorption Parameters from Mie Calculations
3.6 Conclusion
CHAPTER 4 Monte Carlo Modelling of Light Propagation into Three-Layered Tissues and Tissue Phantoms
4.1 Introduction
4.2 Light Transport Theory
4.3 Monte Carlo Simulation Flowchart and Random Variables
4.4 Simulation Results
4.4.1 Sensitivity Tests of the Imaging Method of Surface Diffuse-Reflected Light Distribution
4.4.2 Five States of the Urothelium
4.5 Conclusion
CHAPTER 5 Clinical Studies and Mathematical Estimations of Multi-Wavelengths Light Excitation Mode
5.1 Introduction
5.2 Diffuse-Reflected Light Detection on Tissue Surface
5.3 Results and Discussion
5.4 Multi-Wavelength Mathematical Study
5.5 Conclusion
CHAPTER 6 Perspectives and Conclusion
6.1 Conclusion
6.2 Special Features of the Diffuse-Reflectance Light Diagnosis and Possible Problems
6.3 Future Prospects.


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