The distribution of light in tissues can be modeled using Monte Carlo simulations. Analytic expressions, however, provide both speed and the ability to derive subsequent analytic expressions. This paper attempts to distill the lessons of Monte Carlo simulations into analytic descriptions that are simple and rapid. Monte Carlo simulations in homogeneous tissue with cylindrically symmetric geometries are translated into simple expressions based on ellipses. Such an ellipsoidal approximation provides a convenient analytic expression for internal fluence rate distributions, F(r,z).

For solving the inverse problem in optical tomography, the simulation of light transport in highly scattering media under realistic conditions is a prerequisite. In this contribution we study both theoretically and experimentally the transport of photons in highly scattering media following injection of ultrashort laser pulses. The diffusion equation has been integrated by a two-dimensional finite element method (FEM). For comparison with FEM results, time- resolved transmittance was measured in such a way to effectively simulate a two-dimensional geometry. For the reconstruction of the interior structure an iterative method based on the FEM forward model is introduced. Using the full information contained in the time-resolved measurements, the number of sources and detectors necessary for reconstruction of inhomogeneities in optical properties can be reduced considerably. The effectiveness of the algorithm is demonstrated by some instructive examples.

The specific intensity arising from an anisotropically scattering illuminated slab or half space is calculated using the equation of radiative transfer. The solution is obtained in closed analytical form. The novel method used for the solution of this problem leads immediately in a straightforward- and systematical way to the known appropriate basic equations valid for the problem at hand, derived otherwise by ad-hoc methods. A new simple linear equation for the specific intensities at the boundary surfaces is also derived. This method applies equally well to similar scattering problems with other geometries.

The interference between diffusive photon density waves in turbid media was studied. A finite- difference numerical method was employed to simulate the time-dependent diffusion of photons in the red and near-infrared region. In the model the time-dispersion curves following an illumination impulse were calculated for the light transmitted through a slab geometry along a line on the distal side of the slab. The time-dispersion curves were Fourier transformed to yield the amplitude and phase as a function of modulation frequency. The interference between photon density waves was studied by having two sources, one time-delayed as compared with the other, corresponding to 180 degrees out of phase for 200 MHz modulation frequency. The diffusing waves originating from the out-of-phase sources gave as expected an amplitude null and a sharp phase transition in the mid-plane. In order to establish the model, calculated curves were compared with experimental data. Furthermore, amplitude and phase data were acquired for various objects inside the slab to study the sensitivity and robustness of the technique.

The diffusion approximation, which is often used to describe the propagation of light in biological tissues, is only good at a sufficient distance from sources and boundaries. Light- tissue interaction is however most intense in the region close to the source. It would therefore be interesting to study this region more closely. Although scattering in biological tissues is predominantly forward peaked, explicit solutions to the transport equation have only been obtained in the case of isotropic scattering. Particularly, for the case of an isotropic point source in an unbounded, isotropically scattering medium the solution is well known. We show that this problem can also be solved analytically if the scattering is no longer isotropic, while everything else remains the same.

Time-resolved backscattering from randomly scattering media is studied experimentally with the aim of human skin diagnosis. The experimental setup consists of a self-modelocked Ti:Sapphire laser and a light gating technique based on sum-frequency generation. Aqueous solutions of latex microspheres were used as scattering medium. The experimentally determined temporal profiles of the recorded backscattered photons significantly depend on the specific conditions of the whole optical system. A systematic variation of the optical parameters was performed and an optimum arrangement was determined. In a first set of experiments, relatively weak concentrations of the scatterers were investigated and scattering lengths were determined. In a second series of experiments two-layered samples of latex suspensions with scattering properties similar to human skin were studied. Under these scattering conditions penetration depths of more than 1 mm could be obtained.

We present a perturbation theory for diffusive light transport in turbid media, which allows us to model the light distribution around inhomogeneities of complex geometries. The diffusion equation for an inhomogeneous medium is transformed into an equivalent integral equation that can be solved with a fast iterative numerical algorithm. This method models three dimensional geometries considerably faster than standard methods. Furthermore, the integral formulation supports an intuitive understanding of the physical processes.

Intensity transmission measurements of biological tissue with light in the wavelength range from 600 nm to 1000 nm yields limited resolution due to strong scattering. The resolution can be improved using time gated methods. These measurements are technically complicated and require a long detection time. A different approach is the measurement of the phase shift of transmitted light using a radio frequency modulated light source. We have investigated the influence of the modulation frequency on the intensity distribution and on the phase shift both theoretically and experimentally. Further the dependence on the scattering and absorption coefficients of the illuminated medium has been examined. The theoretical calculations have been carried out using analytical and numerical solutions of the diffusion equation. Results are given for the modulation frequency range from 60 MHz to 500 MHz. They show that the modulation frequency has an effect mainly on the phase shift and only to a small degree on the intensity distribution.

In biological tissues it is possible to generate photon density waves with wavelength below 5 cm. When these kind of waves encounter optical inhomogeneities with diameters in the millimeter and centimeter range, typical scattering effects occur. These scattering effects can be compared to the well known effect of ultrasound scattering. Ultrasound gets scattered at regions with different compressibility and density, while photon density waves are scattered at regions with different absorption and scattering coefficient. In this study, analytical solutions for the time dependent photon diffusion equation are used to estimate photon density wave diffraction effects caused by spherical optical inhomogeneities. The detectability of tumors and hemorrhages in the brain, based on the diffraction pattern generated by these heterogeneities, is discussed in detail.

After an analysis of the diffraction of plane photon density waves by plane periodic objects more general diffraction problems are treated in a way analogous to Fresnel diffraction. In this way approximation formula for the Gaussian momenta of the point spread functions for transillumination processes are gained. These expressions may be further simplified by an asymptotic expansion of the wave number of the photon density waves for large values of temporal frequency. A comparison with calculations on the basis of the diffusion theory and the method of the mirror images shows that the approximation usually is accurate within 10%. The method is applied to study the effect of object position and detector diameter on the Gaussian momentum of the point spread function.

In heavily scattering media an intensity modulated light source will propagate as a diffusive wave. This technique has been adapted for dark field operation by modulating two lasers in antiphase giving destructive interference along a line equidistant to the two sources. If an object is present in the scattering medium the symmetry is broken so that a signal will be detected along this line. This paper presents a different approach to obtaining this using a single source configured as an interferometer. The fundamental properties of the interferometer are such that conservation of energy ensures that its two arms are modulated in antiphase. The application and advantages of this approach are described.

We have designed and constructed a near-infrared spectrometer for the non-invasive optical study of biological tissue. This instrument works in the frequency-domain and employs multiple source-detector distances to recover the absorption coefficient ((mu) _(alpha )) and the reduced scattering coefficient ((mu) _s') of tissue. The light sources are eight light emitting diodes (LEDs) whose intensities are modulated at a frequency of 120 MHz. Four LEDs emit light at a peak wavelength of 715 nm ((lambda) ₁), while the other four LEDs emit at a peak wavelength of 850 nm ((lambda) ₂). From the frequency-domain raw data of phase, dc intensity, and ac amplitude obtained from each one of the eight light sources, which are located at different distances from the detector fiber, we calculate (mu) _(alpha ) and (mu) _s' at the two wavelengths (lambda) ₁ and (lambda) ₂. The concentrations of oxy- and deoxy-hemoglobin, and hence hemoglobin saturation, are then derived from the known extinction coefficients of oxy- and deoxy-hemoglobin at (lambda) ₁ and (lambda) ₂. The statistical error in the measurement of the optical coefficients due to instrument noise is about 1 - 2%. The accuracy in the determination of the absolute value of the optical coefficients is within 10 - 20%. Preliminary results obtained in vivo on the forearm of a volunteer during an ischemia measurement protocol are presented.

Precise knowledge of the optical coefficients of a turbid biological medium is fundamental to the analysis of many optical therapeutic and diagnostic techniques that can now be found in medicine. It is unfortunate that the few successful techniques that have been developed for the measurement of absorption and reduced scattering coefficients have been limited to in vitro samples where the optical properties are almost certain to be significantly different to the corresponding in vivo state. The analysis of coherent backscattering peak lineshapes using an analytical solution derived from a scalar diffusion approximation has been recently reported but the results were inconsistent with the coefficients measured with an established method. The progress of this present study has resulted in the development of a Monte Carlo simulation of coherent backscatter from biological media which can cope with many of the practical limitations and theoretical simplifications that have been encountered (e.g., refractive index mismatches and a finite illumination spot size). Our initial results have highlighted the inaccuracies in the earlier study and suggest a novel strategy that may be used to determine the absorption and effective scattering coefficients of in vivo tissues with minimal invasion under clinical conditions.

In this study, the detailed dependence of the light scattering on the tissue architecture and intracellular composition was investigated. The reduced scattering coefficient ((mu) _s') of isolated rat liver mitochondria, isolated liver cells and various rat tissues was measured at 780 nm by using time-resolved spectroscopy and a sample-substitution protocol. In a first part, extrapolations of the in vitro data to the in vivo situation showed that the mitochondrial compartment contributes for 73% of the scattering of the hepatocytes and about 100% of that of the whole liver. Finally, by analyzing different normal rat tissues and tumors, we have shown that the tissue (mu) _s' is independent on the cell concentration in the tissue but is roughly proportional to the tissue mitochondrial content.

The control of light dosimetry during photodynamic therapy requires the knowledge of all the optical coefficient in situ. Therefore, a sensor based upon the backscattering phenomenon has been conceived. It is described in the first part. The second part of the paper shows how the combination of the Kubelka and Munk's theory and an ARX modeling of light gives access to the required values. In the last part the results obtained in vitro on optical phantoms and in vivo on nude mice are analyzed.

Time-resolved spectroscopy in the near-infrared wavelength range is a promising technology for the development of optical tomography to measure the profiles of oxygenation state in living tissues. Many investigators have reported the experimental results of time-resolved reflectance and transmittance of ultra-short light pulses incident on tissue samples and phantoms. However, none of them has reported the effect of the boundary reflection which takes place at the interface between the sample and air because of the difference in the refractive indices. This paper describes the effect of the boundary reflection on the time- resolved transmittance through slabs of scattering and absorbing media simulating biological tissues. Time-resolved measurement was carried out by using a pulsed diode laser of 784 nm and an optical oscilloscope. The samples were latex microsphere suspension in water with or without the addition of ink. The cells containing the suspension were equipped with uncoated or anti-reflection coated glass windows to see the effect of the boundary reflection. The measured results were compared with the Monte Carlo simulation results which incorporated the boundary reflection. It has been found that the boundary reflection broadens the transmitted pulse width, and that the neglecting of the boundary reflection leads to an overestimation of the scattering coefficients.

For successful application of laser tomography methods for earlier medical diagnostics the signal-to-noise ratio (contrast) must be increased. For this purpose it is possible to use the absorbing dyes. We have theoretically investigated optical imaging conditions in high scattering medium on a model object. In our experiments a YAG:Nd laser generating picosecond pulses was employed. Output radiation has been recorded by a high speed streak camera with 1.5 ps temporal resolution. The high stability of the laser and of measurement scheme characteristics was provided. We looked for the contrasting substances having tropism with pathologically changed tissue of the tumor. For this purpose some dyphthalocyanines were synthesized. The experiments with laboratory animals have demonstrated that saturated dye concentrations were noticeably lower than toxicologic dangerous concentration values. We have demonstrated a possibility of the contrasting for a model object. The experimental temporal profile of scattered radiation can be explained by the nonstationary two-flow theory.

In most biological tissues, absorbers such as blood in the blood vessels are localized within a low-absorbing background medium. To study the effect of distributed absorbers on the near infrared reflectance, we developed a Monte Carlo code and performed time-domain measurements on heterogeneous tissue-vessel models. The models were made of low absorbing polyester resin mixed with TiO₂ as scatters. A series of tubes with diameters of 3.2 or 6.4 mm were made in the resin sample. The volume ratio of the tubes to the total sample is about 20%. During the measurement, these tubes were filled with turbid fluids with different absorption coefficients to simulate blood in various oxygenation states. We found that the apparent absorption coefficient of the resin/tube system, determined by using the diffusion equation fit, can be approximated by a volume-weighted sum of the absorption coefficients of the different absorbing components. This approximation has to be replaced by a more complex expression if the difference in absorption between the absorbers and background is very large (approximately 20 times). The results of the tissue phantom study are supported by the Monte Carlo simulation. Possible explanations for the photon migration in this kind of heterogeneous system is also presented.

A method has been developed to evaluate the absorption and reduced scattering coefficients of slabs of turbid materials, biological tissues in particular, with a one step procedure. The biological tissue is transilluminated by a collimated source and the transmitted part of the signal is detected. The spatial intensity profile of the light is collected by a linear CCD. The temporal profile of an ultrashort laser pulse is measured with an ultrafast photodiode. The spatial or temporal profiles are compared to the results of Monte-Carlo simulations in order to predict the absorption and reduced scattering coefficients of the investigated tissue. The validity of our photon migration model has been carefully assessed by experiments on microspheres suspensions, where exact scattering coefficient and the associated phase function have been derived from Mie theory. These results have allowed us to determine the accuracy of the method. The method has been applied to determine locally the optical coefficients of several selected slabs of biological tissues, and the validity of this approach has been verified.

Diffusely scattered light models have been extensively used by research groups on their effort to predict laser light's propagation through biologic tissue. This has provided useful results in the case of homogeneous isotropic materials. Biologic tissue is a highly complex medium consisting of single cells and various scattering and absorbing multi-cellular structures. The effect of light diffraction should not be ignored when the size and shape of hidden objects in tissue is predicted via laser transmission measurements.

The total optical absorption in mammalian tissues is measured at 633 nm by using heterodyne detection which permits us to detect coherent photons after attenuation by more than 10 optical densities. In order to understand what kind of photons are coherently detected after passing through mammalian tissues of significant thickness (from 2 to 4 mm), we measured attenuation under linearly and circularly polarized illumination. Both polarizations are similarly transmitted through homogeneous tissues such as liver and muscle. Results are identical to those obtained in previous experiments without polarizer.

We describe measurements of light transmitted through a scattering sample as a function of scattering angle and polarization close to the forward direction. As scattering samples suspensions of latex with diameters between 0.3 micrometers and 1.09 micrometers and Intralipid- 10% are used. The results show that the multiple scattered light of 0.3 micrometers latex suspensions and Intralipid-10% is practically unpolarized. In contrast, the multiple scattered light of 1.09 micrometers latex suspensions is still partially polarized.

A technique is presented which allows us to resolve absorbing objects hidden in a strongly scattering depolarizing medium. The method uses incident light with modulated polarization and phase sensitive detection in dependence on polarization. Even for thick media with an average number of 29 scattering events the method yields clear images independent of the depth of the object in the medium.

Light propagation in multi-layered tissues was studied theoretically and experimentally on a rabbit thigh and on human forearms. Theoretical investigations were based on a special solution of the diffusion equation considering layered materials or on Monte Carlo simulations. The spatially resolved reflectance of a small diameter laser beam (633 or 751 nm) incident on the tissue was measured in vivo with a CCD-camera based system previously tested with homogeneous tissue simulating phantoms. The optical coefficients were determined either by non-linear regressions to a solution of the diffusion equation or to Monte Carlo simulations. We fitted up to four independent parameters to describe the spatially resolved reflectance from two- and three-layered tissues.

The observable parameters remittance, transmittance and fluorescence emission of layered systems are determined from Monte Carlo simulations and the diffusion approximation. We calculate the effect of additional absorption caused by a fluorescing dye, depending on dye- bearing layer thickness. The results of the simulations are compared to the diffusion approximation, establishing a range of accuracy of the diffusion approximation. We show that, provided the basic optical properties of the system are known, additional absorption and layer thickness can be calculated from non-invasive measurements of remittance and fluorescence emission. We give an estimation of the effect of experimental uncertainty on the results for dye absorption and layer thickness.

The temperature reversible phenomenon called phase separation is one of the mechanisms of cataract formation. At a sufficiently low temperature the protein solution separates into protein-rich and protein-poor domains. When light propagates through the medium the random refractive index fluctuations associated with the domains produce intense scattering. It is shown that a thin layer of eye lens homogenate at a sufficiently low temperature produces speckle pattern. This pattern is polarized, Gaussian and partially developed. The speckle pattern is a result of a single scattering. We have investigated the speckle pattern for the case of an induced phase separation. The specular component of scattered radiation is measured in dependence on the temperature. At 18 degree(s)C the first-order statistical properties of the speckle pattern are investigated. The pattern is investigated in the far-field region. The probability density function of speckle intensity and the average contrast of the speckle intensity is measured. Also some results related to the second order statistics are given. It is assumed that the object is phase modulation type. The standard deviations of phase variations and correlation length of the phase variations are estimated.

In order to study the behavior of laser-Doppler based tissue blood perfusion meters an experimental flow model has been developed consisting of a set of layers with dispersed scatterers and/or absorbing material which are moveable with respect to each other and to the laser-Doppler probe. As the material for the layers gelatin was used, and for the scatterers polystyrene spheres were chosen. Light (from a diode laser) scattering in the sample was measured in reflection using a photodiode array. The intensity and the Doppler spectrum were recorded as a function of the source-detector distance and the angle of laser light incidence. For comparison a number of Monte Carlo simulations of the dynamic light scattering in the sample were performed. The simulations included data regarding the Doppler spectrum, the number of scatter events, paths lengths, positions and angles of emergence and penetration depths. It is seen that in the heterodyne detection case good agreement between measurements and simulations is obtained, while in the homodyne the simulations have to be downscaled in frequency (factor 3). This may be caused by coherence effects due to the finite aperture of the detector. In the simulations the averaged Doppler frequency and averaged absolute Doppler frequency turn out to be quadratic and linear dependent on the numerical aperture. This effect was verified with an independent calculation.

The integral Doppler spectroscopy approach is used to retrieve information on the shape of the velocity profiles of scattering particles suspension flows through thin glass capillaries. Direct and inverse problems are solved for a simplified model case of slightly scattering channel walls. A Monte Carlo numerical analysis is given for the case of highly scattering tissue surrounding the flow which is related to in vivo blood perfusion and intracellular cytoplasmic streaming measurements.

Laser Doppler techniques are increasingly used in research and clinical applications to study perfusion phenomena in the skin, yet the influences of changing scattering parameters and geometry on the measure of perfusion are not well explored. To investigate these influences, a simulation program based on the Monte Carlo method was developed, which is capable of determining the Doppler spectra caused by moving red blood cells. The simulation model allows for the definition of arbitrary networks of blood vessels with individual velocities. The volume is represented by a voxel tree with adaptive spatial resolution which contains references to the optical properties and is used to store the location dependent photon fluence determined during the simulation. Two evaluation methods for Doppler spectra from biological tissue described in the literate were investigated with the simulation program. The results obtained suggest that both methods give a measure of perfusion nearly proportional to the velocity of the red blood cells. However, simulations done with different geometries of the blood vessels seem to indicate a nonlinear behavior concerning the concentration of red blood cells in the measurement volume. Nevertheless these simulation results may help in the interpretation of measurements obtained from devices using the investigated evaluation methods.

It is shown that the simultaneous measuring of the brightness coefficients and autocorrelation functions of temporary fluctuations of backscattered radiation by erythrocyte suspensions at one or several wavelengths has given information with respect to diffusion coefficients, first and second moments of size distribution of particles and their aspherical parameter. It is shown that dynamic spectral width at multiple scattering offers the information on erythrocyttes absorption and oxygenation, but not only geometric characteristics of particles. The possibility of the determination of rotation Brownian motion and flicker phenomena are analyzed in a condition of multiple scattering in the basis of the earlier received data.

When hemoglobin concentration in a beating heart muscle was determined before and after hemodilution by remission spectrometry and calculated on the basis of classical integration of the area between the hemoglobin spectrum and the dark current no meaningful changes in Hb concentration could be found. In a study performed in suspensions of scattering particles a first attempt was made to find a rational explanation for these observations.

On the basis of both homogeneous and layered skin models this paper analyzes the influences of multiple scattering of skin on the measurements of NIR spectroscopy. Emphasis is laid on the wavelengths of 660, 805, and 940 nm which are used in clinical monitoring systems. The results of Monte-Carlo simulation show that the overwhelming scattering of tissue leads to a nonlinearity in the Lambert-Beer's relation between optical density and chromophore concentration. The consequences of this effect shall be discussed using as an example the non invasive measurement of Indocyanine Green (ICG) in the blood. In this case the multiple scattering of skin causes substantial non-linear relation between the optical density at 805 nm and the concentration of an injected NIR dye in the blood if the concentration of ICG exceeds 10 mg/l. This leads to a significant distortion of the ICG clearance curve and in consequence to a systematic error in the determination of physiological parameters. For multi-wavelength spectroscopy the wavelength dependency of scattering coefficients has to be noticed. The consequence of this effect is demonstrated for blood oxygen saturation (SaO₂) measurements.

Optical properties of blood are identified by physiological ones. That's why two groups of parameters of whole blood have been investigated simultaneously (on the same sample and in the same time). The set up which allows us to register diffuse reflectance and transmissions of whole blood with hyperfine flow layers (less than 100 microns) has been created. Receiving hyperfine layers of whole blood which is compatible with capillar aperture gives us a chance to obtain experimental data in a wider spectral range with higher accuracy.

Simple approximate scattering theories such as the Rayleigh-Gans theory are not generally applicable to the case of light scattering by red blood cell (RBC) aggregates, including thrombus. This is mainly due to the extremely short distance separating erythrocytes in the aggregates (of the order of 25 nm) as well as to the substantial size of the aggregates. Therefore, in this paper a new mathematical model predicting the electromagnetic field produced by the scattering of a plane electromagnetic wave by a system of two adjacent RBCs is presented. Each RBC is modeled as a homogeneous dielectric ellipsoid of complex index of refraction surrounded by transparent plasma. The relative position and orientation of the ellipsoids are arbitrary. Scattering is formulated in terms of an integral equation which, however, contains two singular kernels. The singular equation is transformed into a pair of nonsingular integral equations for the Fourier transform of the internal field of each RBC. The latter equations are solved by reducing them by quadrature into a matrix equation. The resulting solutions are used to estimate the scattering amplitude. Convergence aspects concerning the numerical calculation of the matrix elements originating from the interaction between the RBCs are also presented.

Recent biomedical optics experiments, imaging or quantitative measurements of chemical compounds for example, are more and more sensitive to the optical characteristics of biological tissues. Artificial scattering media are used in the laboratories in order to work in reproducible, stable and well known samples. The most difficult part of the work is to obtain an adequate phase function, since the scattering and absorption coefficients can be adjusted by an appropriate concentration of the scattering particles and the addition of an absorbing dye. The most forward way to create phantoms is to use scattering spheres of equal size. If the sphere diameter which gives the desired mean cosine of the single scattering angle is not available, similarity relations may provide the necessary adjustments. However, as we show in this paper, these similarity relations may sometimes be very inaccurate and, moreover, the Mie phase function of a sphere does not match a real tissue phase function. Arridge et al, Firbank et al have suggested that a better solution would be to use a distribution of different size scattering particles in order to imitate the whole phase function, but the determination of a mixture of spheres with adequate sizes and concentrations is a difficult mathematical problem. The goal of this paper is to solve this problem. It is first shown that the extreme complexity of real biological samples can be very simply simulated by a mixture of spheres with a fractal diameter distribution. Then some simple rules, based on the knowledge of this fractal distribution, are given in order to obtain a realistic phase function with a limited number of spheres diameters.

Technology for human epidermis optical parameters determination is described. This technology includes: (1) epidermis upper layers glue stripping; (2) in vitro measurements of total transmission, diffuse reflection, and angular light scattering of strippings; (3) absorption and scattering coefficients reconstruction using inverse calculating technique based on 4-flux approximation of radiation transport theory or inverse Monte Carlo simulation method. Epidermis stripping autofluorescence spectra were acquired under different excitation wavelengths: dependence of sample autofluorescence on some external factors was followed.

Several types of tissue-like materials, animal tissues and human breast tissues in vitro, have been measured by a photospectrometer at visible and near-infrared wavelengths (between approximately 500 and 1100 nm) with the aim of correlating the dominant spectral features. In the clinical part of this work female volunteers of different age with various thicknesses of breast tissue at different sites were transilluminated spectroscopically in vivo in this diagnostic window. In addition, phantom experiments have been conducted to answer the question of how sensitively absorbing objects hidden inside a turbid medium several centimeters thick may be identified from their spectral signature. On the basis of these results it may be possible to improve the detectability of breast lesions, for example tumors, by a spectral transillumination technique.

Time-resolved in vivo measurements have been performed to determine the optical properties of the female breast. The optical parameters at 800 nm are of special interest for medical diagnosis since this wavelength is close to the isosbestic point of oxy- and deoxyhemoglobin. The measurements have been performed with a Ti:sapphire laser using a synchroscan-streak camera. The diffusion model has been used to calculate the absorption coefficient (mu) _A and the reduced scattering coefficient (mu) '_s equals (mu) _s (1 - g). Measurements at different positions on the mamma showed systematic changes of the time-resolved signals which can be explained by the composition of this heterogenous tissue. A good agreement between the measurements on the right and left mamma has been found. Additionally, time- resolved experiments on breast tissue in vitro at 532 nm, 800 nm, and 1064 nm have been performed to study the optical properties at these wavelengths.

Cornea anisotropic properties have been investigated experimentally by Mueller matrix technique. The system of the plane anisotropic layers has been used as an optical model of the cornea. In this model we consider every layer to be a closely packed system of long cylinders. Jones's transmission matrices of the above mentioned multilayer anisotropic system were calculated using transfer matrix 4 X 4 method. The multilayer systems with different ordering of layer's optical axis were analyzed. We have obtained spectral dependencies of limit values of linear and circular dichroism and birefringence from cornea fiber orientation. Mueller matrices of rabbit cornea were measured experimentally. Then we turn from the experimental Mueller matrix to Jones matrix to reveal clearly the cornea anisotropy. This transition is correct for the depolarization-free objects. Analyzing the cornea experimental Muller matrices by the depolarization criterion method we conclude that cornea has a negligible depolarization. Theoretical and experimental results appear to be in good agreement.

There are six known inequalities which connect the elements of light scattering matrix (LSM). Analysis of the inequalities shows that the region of permissible values of the elements in the upper left quadrant of LSM has a form of tetrahedron in the space of (M₁₂, M₂₁ M₂₂). In this work we have investigated regions of localization of the LSM elements corresponding to the basic types of optical objects.

We performed this work to estimate the errors of the device for measurement of light scattering matrices (LSM). The scheme of a laser polarizational nephelometer with polarization modulation by the phase plates rotating with a speed ratio of one to five was taken for analysis. For this scheme the influence of the adjustment errors on the LSM elements of different test objects was evaluated by the Monte-Carlo method, and we have found the matrix of empty space to be the most sensitive to the errors. Dependencies of the matrix error upon the different adjustment parameters have been plotted for the matrix of empty space. Also we have studied dependency of the errors from the intensity of scattered light and the distortions in the registration unit. We also analyzed the modulation of the polarization state by rotation of the phase plates. We studied the condition number [cond(C)] of the matrix of conversion of the input signal Fourier spectrum to LSM was under investigation. Dependencies of cond(C) versus the initial orientation of the phase plates axis and the phase shift values were investigated.

The coefficient of diffuse reflection and light transmission measurements in an optically thick layer of blood at atherosclerosis conditions under multiple scattering of light in the visual and nearest IR-spectra region (590 -900 nm) were measured for calculation of the absorption coefficients of the material of particles and surrounding medium K((lambda) ) and parameter Q (the latter parameter was defined by the sizes of erythrocytes and aggregates and by refraction coefficient of red cells relative to plasma at atherosclerosis). For the main quantitative spectroscopy of particles the K₁((lambda) ) for known value of K((lambda) ) and the parameter Q determinations it is necessary to have the knowledge of relative volume part H occupied by particles. In the case of a high concentration of particles H >= 0.2 as it takes place in the blood the parameters Q and K((lambda) ) are in dependence of H (H - is hematocrit ration for the case of whole blood). It should be noted that spectroscopy of multiple scattering light can give some information out of main absorption bands with the higher accuracy and higher light scattering. The latter value provides the opportunity of determination of faint absorption bands which couldn't be achieved by other methods. The method proposed is characterized by absence of probe preparations, approach to in viva conditions, expressivity, and high informativity of each experiment. A many-fold investigation of the blood of healthy men in the spectral region 650 - 810 nm shows the electron spectrum of absorption of molecular hemoglobin hem is the most optically active blood spectra component K((lambda) ). The broadening of spectral investigations, as in short wave or long wave areas of the spectrum, by the use of multiple scattering methods for calculations of K((lambda) ) and Q((lambda) ) enlarges the number of chromophores studied.

The problem of accurate determination of fluorochrome concentration in tissues from fluorescent spectra is the most important one in the spectroscopy of biological objects. Especially it is significant for the application of photodynamic therapy (PDT) when the concentration of photosensitizer (PS) in tumor and surrounding normal tissues should be known so that one could employ the appropriate treatment tactics (irradiation dose and time). It is also desirable that the screening depth be the same order as the light penetration depth used for treatment. To solve these problems we have prepared the standard samples with the optical properties close to that of tissues under investigation. The various fractions of sulphanated aluminum phtalocyanine (Al-Pc) have been used as photosensitizers. The samples with various concentration of Al-Pc have been prepared and the intensity of fluorescence excited by He-Ne laser, as well as the laser scattered line, have been measured to give a calibration curve.

A general approach for the investigation of the polarization properties of scattering mediums is presented. These properties are given by six independent parameters characterizing the amplitude and phase anisotropy of investigated mediums. The corresponding theorem is proved. This allows us to characterize suitably the physical behavior of the medium with respect to polarization phenomena and to perform a general classification of mediums based on the above mentioned approach. Furthermore, it is possible on this basis to synthesize the polarization systems with given characteristics.

The results of goniometric investigations of laser light backscattered by skull bone samples are presented. The angular dependence of the intensity of light backscattered by the samples for four wavelengths of argon laser was measured. The angular dependencies obtained from measurements are approximated with analytical functions. The diffuse reflection of light from the samples is calculated.

This paper presents the version of a Monte Carlo method for simulating optical radiation propagation in biotissue and highly scattering media allowing for three-dimensional geometry of a medium and macroinhomogeneities located in one of the layers. The simulation is based on use of Green's function of medium response to single external pulse. The process of radiation propagation is studied in the area with given boundary conditions, taking into account the processes of reflection and refraction at the boundaries of layers inside the medium under study.

Spectral reflectance is the fundamental and common parameter in satellite remote sensing analysis. A prime objective of the applied remote sensing program is to recognize scene status, without identifying or conditioning of these observable radiance measurements. The most frequently used data analysis or information extraction procedure is mapping of the scene elements, based on their radiance measurements into information classes using software techniques of pattern recognition. Generally, there are more reflectance data available than data on either emissivity or diffusivity for all terrestrial targets. The spectral responses along the electromagnetic spectrum permit the characterization of the spectral reflectance curves of the objects and permit us to distinguish them one from the other. At first, this paper presents a brief theoretical view for the spectral reflectance and measured terrain feature response parameters. For different types of targets, spectral signatures are analyzed to identify and to interpret terrain data. The reflectance of terrain elements must be considered stochastic variables in the natural field work.

This paper describes the investigation of incorporation processes of the plague lipopolysaccharide (LPS) into artificial phospholipid vesicles (PLV) on the basis of elastic laser radiation scattering. For this purpose, the angular light scattering dependencies of PLV suspensions, containing various LPS concentrations (0 - 5 mg/ml), were measured using the polarization nephelometer. The design of the polarization nephelometer and the measurement technique are described in detail. Measuring results are compared with electron microscopy data. The most pronounced variation as a result of LPS incorporation into PLV appeared to be the light scattering integral intensity (LSII) at angles exceeding 100. It is shown that the LPS adding into the PLV suspension causes the LSII to increase by a factor 2 - 6 for a LPS concentration range from 0.5 to 5 mg/ml as compared with `empty' PLV. Proceeding from the electron microscopy data it was found that the LSII increase, in general case, is conditioned by variation of the PLV membrane refraction index and formation of PLV aggregates. It was shown that the LSII measurement for the PLV suspension containing LPS can be used as a qualitative express analysis for the LPS incorporation into PLV as well as procedure for determination of the aggregate formation stage from PLV. The LPS of the plague, which as determinants being common for various gram-negative bacteria, is of great interest from the viewpoint of creating preparations for prophylactic measures against the endotoxin infections. However, the LPS toxicity due to the lipid A presence is a disadvantage of this weak antigen. Incorporation of the LPS int bilayer phospholipid membranes leads to its lower toxicity and higher immunization ability. The immunization ability and toxicity of the LPS complexes with bilayer membranes depend essentially on the LPS quantity sorbed in the membrane, as well as on the shapes and sizes of aggregates formed by the LPS and membranes in water environment.

The theoretical investigation of the processes of strongly focused Gaussian beams diffraction in blood capillaries with a diameter a bit greater than the erythrocyte size have been carried out. Spatial-temporal correlation functions of intensity fluctuations in dynamic statistically inhomogeneous speckles have been studied. Modified speckle-interferometrical method using strongly focused Gaussian beam scattering is suggested for blood flow measurements. The possibilities of this method application to blood and lymph flow velocity monitoring in narrow vessels has been analyzed.

A method for detection of absorbing objects embedded in scattering media is described. In a confocal imaging arrangement the sample is illuminated with light modulated in polarization. The modulated part of the transmitted light is detected with lock-in-technique. Under the condition that the scattering particles are smaller than the wavelength, this method is able to supply images with higher resolution and contrast than conventional transillumination.

In this work, we present a comparison of two approaches (the rigorous transfer equation and its diffusion approximation) to describe the frequency dependence of the modulation and the phase shift of scattered light for the problem of diffuse reflection form a semi-infinite medium with isotropic scattering. It is shown that both approaches lead to the same results, when the modulation frequency of the incident light is low with respect to the inverse time-of-flight between two interaction sites. At increased frequencies, however, these two models reveal differences in their predictions. Besides, the diffusion approximation fails even qualitatively to describe angular dependence of the modulation and the phase shift of back scattered radiation. The photon migration process in tissue is also influenced by the finite (non-zero) life-time of photons in the virtually absorbed state during the scattering process. Most of the existing models assume that this life-time is neglectably short. However, in dense scattering media, like in most biological tissues, this time can be comparable with the time-of-flight between two interaction sites. We also investigated this effect on the frequency dependence of the modulation and the phase shift. The results let us conclude that both -- the diffusion approximation and the assumption of short life-times in the virtually absorbed state -- should be applied with caution when using frequency domain data to determine optical properties of biological tissues especially when using high modulation frequencies. This is also true for the application of this technique in optical tomography.

In this paper the principle possibility for determining the incorporation efficiency of plague capsular antigen (PCA) into liposomes on the basis of elastic light scattering is shown. To find the most informative index, which characterizes the process of PCA sorption into liposomes, the angular light scattering dependencies for the liposome suspensions containing various PCA concentrations were measured by means of laser polarization nephelometer. Its construction and measurement technique are described in detail. For all preparations to be investigated, the incorporation efficiency was determined using traditional biochemical method and correlation between the scattering matrix element values and the incorporation efficiency was made. The most informative index, characterizing the process of PCA incorporation into the liposomes, appeared to be the light scattering integral intensity (LSII) at an angle of 90 degree(s). It was found that the PCA incorporation into liposomes results in the LSII increasing by a factor of 2 - 5 times in the incorporation efficiency range from 5% to 45%. The electron microscopy data and angular dependencies for the light scattering matrix elements have shown that LSII increase is due to the refraction index variation of the liposome membrane. The obtained LSII dependence on the incorporation efficiency enables us to determine the PCA incorporation efficiency into the liposomes by measuring the LSII.

To determine enzymatic cytolytic activities several methods, based on the change in turbidity of a suspension of microbial cells sensitive to a given enzyme, are in common use. We have developed a method based on the measurement of turbidity spectra with a high-speed turbidimeter and on the algorithms of solution of the inverse spectroturbidimetric problem. The method was evaluated using the bacterium M.lisodeicticus and the enzyme muramidase (lysozyme) as a model system. The cytolytic activity of the lysozyme was examined with a high-speed turbidimeter developed by us, which permits measuring light extinction spectra on 12 discrete wavelengths in the range of 350 to 1000 nm practically simultaneously. This enables the kinetics of change of the light extinction spectra during lysis to be investigated in more detail. In solving the inverse spectroturbidimetric problem we calculated the mean size, number and mass-volume concentrations of M. lisodeicticus in the process of lysis. The activity of the lysozyme correlated well with the rate of change of the disperse particles' mean size in the range from the second to the fifth minute of lysis. We compared the kinetics of change of the disperse particles' mean size in the process of lysis either by the pure lysozyme or by the lysozyme-containing serum. In both cases the process proceeded similarly up to the fifth minute with the activity of the serum being equivalent to the activity of the pure lysozyme in a concentration of up to 1.0 (mu) g/ml.

The purpose of this study is to evaluate the usefulness of time-resolved spectroscopy (TRS) and phase modulation spectroscopy (PMS) for the measurement of hemoglobin saturation (SO₂) in the liver. Our materials and methods were: (1) Absorption coefficient ((mu) a) and reduced scattering coefficient (microsecond(s) ') of in situ rat liver, blood-free rat liver and red blood cell (RBC) suspension were measured by TRS. (2) Changes in the light path length of blood-perfused rat liver by increasing hematocrit (3%, 12%, and 36%) and anoxia were measured by PMS. Our results show: (1) (mu) a of in situ rat liver could not be determined, because the absorption was too high. From (mu) a and microsecond(s) ' values of blood-free liver and RBC suspension, (mu) a and microsecond(s) ' of normal-hematocrit liver was extrapolated as 1.08 cm^-1 and 15.46 cm^-1 at 780 nm, respectively. (2) Although the trend of liver SO₂ by increasing hematocrit was reasonable, there was a discrepancy between liver SO₂ and output SO₂ by anoxia. Changes in the light path length of the liver were as small as 10% of the total light path length. We conclude: Quantitation of liver SO₂ by TRS and PMS was difficult because of its very high (mu) a. Since changes in the light path length were small, continuous wave spectroscopy would be an effective way to monitor the oxygenation state of the liver.

The laser light scattering technique has been used to study the aggregation process in blood platelets (P.R.P. and washed platelets used). Insulin has been used as the agonist and the fractal dimension determined. Aggregation is studied in the light of recent theories on aggregation mechanism. The structure factor for platelets has been determined.

We study theoretically the problem of sensitivity of frequency modulated reflectance to the inclusion of either a hematoma, modeled as a large slab, or a blood vessel, modeled as a straight tube, in semi-infinite human tissue systems. We use two different numerical techniques: (1) numerical integration of three-dimensional diffusion equation; and (2) Monte Carlo simulations with a biasing technique. We find that inclusion of a hematoma slab or a blood vessel cylinder gives rise to significant change in both the phase and modulation as compared to a homogeneous medium. The reflected signals from these two situations are also significantly different which may allow differentiation between the two objects. In the particular case of imbedded cylindrical blood vessel we find that up to a depth of z equals 25 mm it is possible to use the phase shift or modulation change of reflected photon flux at different locations on the sample surface to determine the direction, size, and depth of the embedded blood vessel, provided phase change sensitivity of 0.1 deg or modulation change sensitivity 0.01 are available.

In this paper, we describe the experimental evaluation of the temporally extrapolated absorbance method (TEAM) in turbid media. The TEAM is the method utilizing the extrapolated absorbance, obtained by temporally extrapolating the time-resolved absorbance difference between an object and a reference to the shortest photon flight time. Using the TEAM, we reconstructed the CT image of an absorber-containing cylinder in a strong scattering media and compared it with those obtained with the time gating and cw methods based on the same data. Among the three methods, the TEAM presented best spatial resolution in the reconstructed CT images.

We present a single-ended technique for three-dimensional imaging of objects embedded in a turbid medium using time-resolved fluorescence emission. The technique employs the earliest- arriving photons, which we show are not sensitive to the relatively long fluorescence lifetime and thus can be used to provide accurate spatial information even at a distance equivalent to 100 mean free paths (mfps). Experiments to combine fluorescence spectroscopy with time- resolved optical tomography are also presented. In addition, with the aid of the multichannel detection of a streak camera, we are able to accurately localize an embedded fluorescent object in a single measurement. Application of time-resolved Raman spectroscopy is also explored.