Optical properties of biological tissues: a review

The terahertz (THz) region of the electromagnetic (EM) spectrum is defined as frequencies ranging from 0.1 to 10 THz. The optical properties of biological tissues have been characterized in neighboring spectral regions; however, few studies have been conducted that have examined these properties in the THz wavelength range. In this study, we used a far-infrared optically-pumped terahertz laser system, a reflection spectrometer system, and photothermal radiometric techniques to characterize the optical properties of water and biological tissues. The reflection spectrometer system performed well at lower frequencies, but proved to be unsuitable for frequencies greater than 2.52 THz. The suboptimal performance was determined to be primarily due to the higher transmission losses of the lenses, and the increased atmospheric losses that are associated with higher terahertz frequencies. The waveguide studies corroborated these findings and served to demonstrate that purging the laser beam path with nitrogen gas was an effective way to markedly reduce THz beam propagation losses. Given this finding, we have designed a temperature-controlled, nitrogen gas purged THz enclosure. The preliminary studies using photothermal radiometric techniques appeared to provide reasonable measures for the absorption coefficient (μa) of water at THz frequencies. In future studies, the tissue property measurements will made within the custom-designed enclosure using photothermal radiometric techniques.

Biomedical Applications of Integrating Sphere: A Review.

We show that optical properties of dense biological tissues can be determined from backscattered power curves measured by a low-coherence reflectometer. Our measurement approach is based on a first-order scattering theory that relates the backscattered power to the total and backscattering cross sections of scatterers in a turbid medium. As a validation of the technique, measurements were made with a commercially available reflectometer on suspensions of polystyrene microspheres having known optical properties. With this reflectometer, which employs a 1300-nm LED source that emits less than 20 µW, we found that skin tissues could be probed to a depth of nearly 1 mm. Estimates of optical coefficients of human dermis and of a variety of excised animal tissues are given.

The known optical properties (absorption, scattering, total attenuation, effective attenuation, and/or anisotropy coefficients) of various biological tissues at a variety of wavelengths are reviewed. The theoretical foundations for most experimental approaches are outlined. Relations between Kubelka-Munk parameters and transport coefficients are listed. The optical properties of aorta, liver, and muscle at 633 nm are discussed in detail. An extensive bibliography is provided.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Diabetes mellitus is a serious social and economic problem of modern society because it is widespread and fraught with numerous complications. Therefore, it is necessary to search for new methods of diabetes mellitus diagnostics and treatment and to improve the existing ones, which, in turn, requires thorough investigation of the disease development mechanisms, as well as elaboration of simple and reliable methods and criteria for detecting the complication precursors. In connection with the solution of these problems, in the paper we present an analytical review of recent publications devoted to the study of the changes of structural and optical properties of biological tissues under the conditions of diabetes mellitus development using in vitro models of glycated tissues, in vivo experimental models of diabetes in laboratory animals, and clinical studies.

Recently, non-invasive diagnostic devices using infrared light have been developed and widely use for clinical applications. To develop these devices, optical properties of biological tissue are necessary. We proposed a new optical measurement method. By using time-resolved reflectance spectroscopy and Monte Carlo simulation for the analysis of light propagation in sample, it is considered that this new method is able to measure the optical properties of small biological tissues in vivo. In this study, we investigated the possible optical property measurements of a superficial layer using this new method. As the later part of the profile of time-resolved reflectance is influenced by the optical property of the deeper layer, a time-gating technique is necessary for the measurement of the optical properties of only the superficial layer in order to use the early profile of the time-resolved reflectance measurement. The function f(t), which is described in the new method, is used for evaluation of the measurement of the superficial layer. We suggest that by using the time-gating technique for the new method and a small source-detector spacing, the optical properties of the superficial layer with a thickness is more than source-detector spacing, can be obtained.

In contemporary medical engineering industry, oneof the key problems of development of methods, apparatuses, and devices for optical spectroscopy of biologicaltissues is the compilation of effective computation algorithms providing maximal accuracy and reliability ofdetermination of initial optical properties of the object ofinterest from experimental data [14, 18]. Optical properties of biological tissues can be determined from radiationfluxes measured experimentally by solving inverse problems of scattering [11], which employ different methodsof description of radiation propagation medium. In turbidlightscattering biological media (most biological mediaare turbid [22]), numerical models of transition theoryand lightscattering in turbid media should be used [8,20]. The models have a limited number of solutions.Therefore, approximate solutions are often used for practical purposes in photometry of turbid media. For example, flux Kubelka–Munk (KM) approaches are widelyused in the practice of noninvasive spectrophotometry,because they are simple and illustrative. Moreover, theKM models allow the final calculation equations to bederived in explicit analytical form [8, 12, 17, 18, 2124].In terms of transition theory and KM models, internaloptical properties of turbid media are completely characterized by linear optical extinction and scattering coefficients. The linear optical extinction and scattering coefficients are determined coefficients of differential equations describing the model. In optics, the KM models are purely photometricand phenomenological models based on heuristic principles providing separation of radiation field into discreterectangular fluxes. The principles also support the validity of linear equation of energy balance for each flux inmedium element [2, 48, 24]. In the simplest case, twoonedimensional flux KM models are considered. Suchmodel represents onedimensional radiation propagationmedium with two oppositely directed fluxes

The study of the optical properties of biological tissues for a wide spectral range is necessary for the development and planning of noninvasive optical methods to be used in clinical practice. In this study, we propose a new method to calculate almost all optical properties of tissues as a function of wavelength directly from spectral measurements. Using this method, and with the exception of the reduced scattering coefficient, which was obtained by traditional simulation methods, all the other optical properties were calculated in a simple and fast manner for human and pathological colorectal tissues. The obtained results are in good agreement with previous published data, both in magnitude and in wavelength dependence. Since this method is based on spectral measurements and not on discrete-wavelength experimental data, the calculated optical properties contain spectral signatures that correspond to major tissue chromophores such as DNA and hemoglobin. Analysis of the absorption bands of hemoglobin in the wavelength dependence of the absorption spectra of normal and pathological colorectal mucosa allowed to identify differentiated accumulation of a pigment in these tissues. The increased content of this pigment in the pathological mucosa may be used for the future development of noninvasive diagnostic methods for colorectal cancer detection.

An integrating sphere system has been developed to non-invasively study the optical properties of biological tissues over a broad spectral range. Using the integrating sphere as both a diffuse illumination source and a detector provides a technically simple measurement apparatus with numerous advantages. A primary advantage is the reduction of the effect of spatial inhomogeneities on the determination of optical properties, afforded by the increased area of detection through the port-opening of the sphere, which challenges many fibre-based, spatially-resolved measurements. Through a single measurement of total diffuse reflectance, an estimation of the transport albedo of homogeneous, liquid phantoms can be made for those cases where scattering is greater than a determined threshold. Further estimations can be made to describe the absorption environment. The effects of the sphere geometry, particularly port-opening size, on the accuracy of the estimated optical properties will be discussed. These results will be used to modify the design of the integrating sphere as an efficient illuminator and light collector, in order to optimize its use in determining the optical properties of biological tissues.

Currently, the main aims of modern surgery are to minimize intraoperative damages and increase the effectiveness of postsurgical recovery. This is important for the automatized intraoperative diagnostics to have quantitative criteria based on knowledge of the optical properties of biological tissues and the mechanisms of interaction of optical radiation with biological tissues. Thus, due to measurements of the biotissue optical properties, the structure and composition of this biotissue can be determined and vice versa. The tasks of hardware and software development for in vitro and in situ laboratory diagnostics of the composition, structure and optical properties of biological tissues using a single methodological and metrological basis and the creation of an appropriate database are relevant.

The effect of temperature on the optical properties of biological tissue has been studied by using milk as an example. Optical properties of milk were measured by using optical coherence tomography(OCT) technology in the temperature range of 20-35°C, and we get the logarithm of the OCT signal, then the rules of change of optical properties with temperature were analyzed. The results show that obvious increase can be observed with increasing temperature in the increase attenuation coefficient. It is a novel method to study the effect of temperature on optical properties of biological tissue.

Quantitative analysis of biological tissue responding to chemical active agents (CAA) that osmotically transport across tissue poses a challenge task for modern biomedical technologies. It is known that the application of osmotically CAA to biotissue such as skin, muscle and gastrointestinal tracts can make the biological tissue transparent. Such osmotic action of agents to the biological tissue have not yet been understood or quantified in a way that degree of optical clearing to the tissue is predictive. We consider that optical properties of biological tissue are altered due to the changes of micro-structures and scattering constituents after CAA permeates into tissue. The changes of optical properties of biological tissue are due to the refractive indices matching between the particles (scatterers) with high refractive index and the ground substances leading to reduce scattering of tissue. The main reasons are that permeated CAA with higher refractive index than the ground substances of tissue makes the refractive index of ground substances of tissue higher by the enhancement of the permeated concentration. In this paper, we described a theoretical model based on the collimated transmittance changes of light penetrating fibrous tissue after the CAA administrates with different concentration.

Quantitative analysis of biological tissue responding to chemical active agents (CAA) that osmotically transport across tissues poses a challenge task for modern biomedical technologies. It is known that the application of osmotically CAA to biotissue such as skin, muscle and gastrointestinal tracts can make the biological tissue transparent. Such osmotic action of agents to the biological tissue have not yet been understood or quantified in a way that degree of optical clearing to the tissue is predictive. We consider that optical properties of biological tissue are altered due to the changes of micro-structures and scattering constituents after CAA permeates into tissue. The changes of optical properties of biological tissue are due to the refractive indices matching between the particles (scatterers) with high refractive index and the ground substances leading to reduce scattering of tissue. The main reasons are that permeated CAA with higher refractive index than the ground substances of tissue make the refractive index of ground substances of tissue higher by the enhancement of the permeated concentration. In this paper, we described a theoretical model based on the collimated transmittance changes of light penetrating fibrous tissue after the CAA administrates with different concentration.

Optical tissue properties have been widely investigated and studied in human and animal tissue due to its importance in the medical laser applications. However, published data on the optical properties of neural tissue in rodents are rare. The aim of this study was to measure the penetration depth of light into the rat brain gray mater at NIR wavelengths. Sample preparation is an important factor in measurements of tissue optical properties. Although shock and slow freezing are standard procedures, they alter the optical properties of biological tissue. In this study, intact slices of gray matter were cut in the range of 300 to 1500 mum without prior freezing to the brain samples. We found the penetration depth in the rat gray matter to be 0.39plusmn0.036 mm. Compared to the data reported in literature for the human brain, the rat brain gray matter attenuates the NIR light much more strongly.

Measurements of optical properties of biological tissues are necessary to develop optical diagnostic instruments. Although various methods to measure the optical properties of biological tissue have been proposed, very few measurements have been carried out in vivo because of their difficulties. We are developing a new method to obtain the optical properties in vivo using time-resolved measurement system. This new method determines the optical properties by comparing the experimental results of time-resolved measurement with the results of a series of Monte Carlo simulation using known optical properties.