Maciej Kraszewski

Quantitative comparison of analysis methods for spectroscopic optical coherence tomography: comment

In a recent paper by Bosschaart et al. [Biomed. Opt. Express 4, 2570 (2013)] various algorithms of time-frequency signal analysis have been tested for their performance in blood analysis with spectroscopic optical coherence tomography (sOCT). The measurement of hemoglobin concentration and oxygen saturation based on blood absorption spectra have been considered. Short time Fourier transform (STFT) was found as the best method for the measurement of blood absorption spectra. STFT was superior to other methods, such as dual window Fourier transform. However, the algorithm proposed by Bosschaart et al. significantly underestimates values of blood oxygen saturation. In this comment we show that this problem can be solved by thorough design of STFT algorithm. It requires the usage of a non-gaussian shape of STFT window that may lead to an excellent reconstruction of blood absorption spectra from OCT interferograms. Our study shows that sOCT can be potentially used for estimating oxygen saturation of blood with the accuracy below 1% and the spatial resolution of OCT image better than 20 μm.

Spectral measurement of birefringence using particle swarm optimization analysis.

Measurement of birefringence is useful for the examination of technical and biological objects. One of the main problems, however, is that the polarization state of light in birefringent media changes periodically. Without knowledge of the period number, the birefringence of a given medium cannot be reliably determined. We propose to analyze the spectrum of light in order to determine the birefringence. We use a particle swarm optimization algorithm for an automatic processing spectra of light transmitted through birefringent material for two orthogonal states of polarization. We have tested the described algorithm on a liquid crystal cell with varying effective birefringence. The proposed method can be used for the measurement of uniaxial positive birefringence without knowing the number of retardation periods or an approximate value of the measurement result. This fact makes the proposed method useful for automatic measurements, when hundreds or thousands of spectra need to be analyzed.

Time-frequency analysis in optical coherence tomography for technical objects examination

Optical coherence tomography (OCT) is one of the most advanced optical measurement techniques for complex structure visualization. The advantages of OCT have been used for surface and subsurface defect detection in composite materials, polymers, ceramics, non-metallic protective coatings, and many more. Our research activity has been focused on timefrequency spectroscopic analysis in OCT. It is based on time resolved spectral analysis of the backscattered optical signal delivered by the OCT. The time-frequency method gives spectral characteristic of optical radiation backscattered or backreflected from the particular points inside the tested device. This provides more information about the sample, which are useful for further analysis. Nowadays, the applications of spectroscopic analysis for composite layers characterization or tissue recognition have been reported. During our studies we have found new applications of spectroscopic analysis. We have used this method for thickness estimation of thin films, which are under the resolution of OCT. Also, we have combined the spectroscopic analysis with polarization sensitive OCT (PS-OCT). This approach enables to obtain a multiorder retardation value directly and may become a breakthrough in PS-OCT measurements of highly birefringent media. In this work, we present the time-frequency spectroscopic algorithms and their applications for OCT. Also, the theoretical simulations and measurement validation of this method are shown.

Novel approach to modeling spectral-domain optical coherence tomography with Monte Carlo method

Numerical modeling Optical Coherence Tomography (OCT) systems is needed for optical setup optimization, development of new signal processing methods and assessment of impact of different physical phenomena inside the sample on OCT signal. The Monte Carlo method has been often used for modeling Optical Coherence Tomography, as it is a well established tool for simulating light propagation in scattering media. However, in this method light is modeled as a set of energy packets traveling along straight lines. This reduces accuracy of Monte Carlo calculations in case of simulating propagation of dopeds. Since such beams are commonly used in OCT systems, classical Monte Carlo algorithm need to be modified. In presented research, we have developed model of SD-OCT systems using combination of Monte Carlo and analytical methods. Our model includes properties of optical setup of OCT system, which is often omitted in other research. We present applied algorithms and comparison of simulation results with SD-OCT scans of optical phantoms. We have found that our model can be used for determination of level of OCT signal coming from scattering particles inside turbid media placed in different positions relatively to focal point of incident light beam. It may improve accuracy of simulating OCT systems.

Coherent-wave Monte Carlo method for simulating light propagation in tissue

Simulating propagation and scattering of coherent light in turbid media, such as biological tissues, is a complex problem. Numerical methods for solving Helmholtz or wave equation (e.g. finite-difference or finite-element methods) require large amount of computer memory and long computation time. This makes them impractical for simulating laser beam propagation into deep layers of tissue. Other group of methods, based on radiative transfer equation, allows to simulate only propagation of light averaged over the ensemble of turbid medium realizations. This makes them unuseful for simulating phenomena connected to coherence properties of light. We propose a new method for simulating propagation of coherent light (e.g. laser beam) in biological tissue, that we called Coherent-Wave Monte Carlo method. This method is based on direct computation of optical interaction between scatterers inside the random medium, what allows to reduce amount of memory and computation time required for simulation. We present the theoretical basis of the proposed method and its comparison with finite-difference methods for simulating light propagation in scattering media in Rayleigh approximation regime.

Optical Coherence Tomography for nanoparticles quantitative characterization

The unique features of nanocomposite materials depend on the type and size of nanoparticles, as well as their placement in the composite matrices. Therefore the nanocomposites manufacturing process requires inline control over certain parameters of nanoparticles such as dispersion and concentration. Keeping track of nanoparticles parameters inside a matrix is currently a difficult task due to lack of a fast, reliable and cost effective way of measurement that can be used for large volume samples. For this purpose the Optical Coherence Tomography (OCT) has been used. OCT is an optical measurement method, which is a non-destructive and non-invasive technique. It is capable of creating tomographic images of inner structure by gathering depth related backscattered signal from scattering particles. In addition, it can analyse, in a single shot, area of the centimetre range with resolution up to single micrometres. Still to increase OCT measurement capabilities we are using additional system extensions such as Spectroscopic OCT (SOCT). With such addition, we are able to measure depth related parameters such as scattering spectra and intensity of backscattered signal. Those parameters allow us to quantitatively estimate nanoparticles concentration. Gaining those, information allows to calculate volume concentration of nanoparticles. In addition, we analyse metallic oxides nanoparticles. To fully characterize nanoparticles it is necessary to find and differentiate those that are single particles from agglomerated ones. In this contribution we present our research results on using the LCI based measurement techniques for evaluation of materials with nanoparticles. The laboratory system and signal processing algorithms are going to be shown in order to express the usefulness of this method for inline constant monitoring of the nanocomposite material fabrication.

Parallel multithread computing for spectroscopic analysis in optical coherence tomography

Spectroscopic Optical Coherence Tomography (SOCT) is an extension of Optical Coherence Tomography (OCT). It allows gathering spectroscopic information from individual scattering points inside the sample. It is based on time-frequency analysis of interferometric signals. Such analysis requires calculating hundreds of Fourier transforms while performing a single A-scan. Additionally, further processing of acquired spectroscopic information is needed. This significantly increases the time of required computations. During last years, application of graphical processing units (GPU’s) was proposed to reduce computation time in OCT by using parallel computing algorithms. GPU technology can be also used to speed-up signal processing in SOCT. However, parallel algorithms used in classical OCT need to be revised because of different character of analyzed data. The classical OCT requires processing of long, independent interferometric signals for obtaining subsequent A-scans. The difference with SOCT is that it requires processing of multiple, shorter signals, which differ only in a small part of samples. We have developed new algorithms for parallel signal processing for usage in SOCT, implemented with NVIDIA CUDA (Compute Unified Device Architecture). We present details of the algorithms and performance tests for analyzing data from in-house SD-OCT system. We also give a brief discussion about usefulness of developed algorithm. Presented algorithms might be useful for researchers working on OCT, as they allow to reduce computation time and are step toward real-time signal processing of SOCT data.

Simulating propagation of coherent light in random media using the Fredholm type integral equation

Studying propagation of light in random scattering materials is important for both basic and applied research. Such studies often require usage of numerical method for simulating behavior of light beams in random media. However, if such simulations require consideration of coherence properties of light, they may become a complex numerical problems. There are well established methods for simulating multiple scattering of light (e.g. Radiative Transfer Theory and Monte Carlo methods) but they do not treat coherence properties of light directly. Some variations of these methods allows to predict behavior of coherent light but only for an averaged realization of the scattering medium. This limits their application in studying many physical phenomena connected to a specific distribution of scattering particles (e.g. laser speckle). In general, numerical simulation of coherent light propagation in a specific realization of random medium is a time- and memory-consuming problem. The goal of the presented research was to develop new efficient method for solving this problem. The method, presented in our earlier works, is based on solving the Fredholm type integral equation, which describes multiple light scattering process. This equation can be discretized and solved numerically using various algorithms e.g. by direct solving the corresponding linear equations system, as well as by using iterative or Monte Carlo solvers. Here we present recent development of this method including its comparison with well-known analytical results and a finite-difference type simulations. We also present extension of the method for problems of multiple scattering of a polarized light on large spherical particles that joins presented mathematical formalism with Mie theory.

Nanoparticles displacement analysis using optical coherence tomography

Optical coherence tomography (OCT) is a versatile optical method for cross-sectional and 3D imaging of biological and non-biological objects. Here we are going to present the application of polarization sensitive spectroscopic OCT system (PS-SOCT) for quantitative measurements of materials containing nanoparticles. The PS-SOCT combines the polarization sensitive analysis with time-frequency analysis. In this contribution the benefits of using the combination of timefrequency and polarization sensitive analysis are being expressed. The usefulness of PS-SOCT for nanoparticles evaluation is going to be tested on nanocomposite materials with TiO2 nanoparticles. The OCT measurements results have been compared with SEM examination of the PMMA matrix with nanoparticles. The experiment has proven that by the use of polarization sensitive and spectroscopic OCT the nanoparticles dispersion and size can be evaluated.

Efficient signal processing in spectroscopic optical coherence tomography

Spectroscopic optical coherence tomography (SOCT) is an extension of a standard OCT technique, which allows to obtain depth-resolved, spectroscopic information on the examined sample. It can be used as a source of additional contrast in OCT images e.g. by encoding certain features of the light spectrum into the hue of the image pixels. However, SOCT require computation of time-frequency distributions of each OCT A-scan, what is a very time consuming procedure. This is particularly important in a real-time OCT imaging. Here, we present a new approach to SOCT signal processing that allows for nearly tenfold reduction of a required computation time. The presented approach is based on a recursive analysis of OCT scan in time-domain without necessity of computing neither short-time Fourier transform or any other time-frequency distribution.