We investigate tissue and instrument parameters affecting the penetration depth in two-photon microscopy. We show that the temporal redistribution of the same average power into fewer pulses of higher peak energy by means of a regenerative amplifier results in an increase in excitation depth by approximately 2-3 scattering mean free paths. We then measure the excitation scattering mean free path in vitro, using rat brain slices, as a function of the excitation wavelength and tissue age. We find that young-animal tissue (< P18) is two-fold less scattering than adult tissue (P90). We quantify the fall-off of the collected fraction of generated fluorescence in a backward detection geometry, in vivo. At large depths, we observe that the collected fraction scales as the angular acceptance squared (related to the effective field-of-view) of the detection optics. Matching the angular acceptance of the detection optics to that of the objective (63X NA-0.90) results in a factor 3-4 of the collected fluorescence. The collection efficiency can be further increased (10X) by using an objective with large field-of-view and high numerical aperture (20X NA-0.95). These gains translate into approximately 120 micrometers additional depth penetration when working in the rat brain in vivo with a standard Ti:sapphire source.

The penetration and diffusion capabilities of fluorophores inside microbial biofilms of heterogeneous three-dimensional structure were studied by fluorescence correlation microscopy under two-photon excitation. The influence of the size and the surface charge of the fluorophores were investigated in order to understand the mechanisms involved in molecular diffusion inhibition occuring in biofilms such as electrostatic and steric interactions.

In this paper a numerical a posteriori depth-variant dispersion compensation technique for PCI and OCT depth-scan signals is presented. This technique is based on numerical correlation of the depth-scan interferometer signal with a depth-variant kernel. Examples of dispersion compensated depth-scan signals obtained from microscope cover glasses are presented. First results show substantial resolution improvements.

The feasibility of using Optical Coherence Tomography (OCT) for oxygenation determination of whole blood was investigated on porcine blood samples. Our data show a sensitivity in the OCT spectral content to changes in oxygenation that qualitatively correspond to expectations based on the absorption spectra of oxidized hemoglobin and hemoglobin.

We demonstrate ultrahigh resolution optical coherence tomography using the continuum generation in an air-silica microstructure fiber. A broadband OCT system was developed, supporting a bandwidth of 370 nm at 1.3 micrometers center wavelength. We achieved longitudinal resolutions of 2.5 micrometers in air, or approximately 2 micrometers in tissue. This is to our knowledge the highest longitudinal OCT resolution demonstrated at this wavelength range and the first application of this new light source for OCT. We will also describe the application of this technique for imaging biological tissue in vivo.

A two-dimensional optical coherence tomography technique has been developed in order to obtain multiple longitudinal slices of a biological sample directly, in a single Z-scan. The system is based on a femtosecond Cr⁴⁺:Forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 9 micrometers . With this imaging apparatus, we investigated biological tissues such as human skin, human tooth and a mouse ear to observe the different constitutive tissues of the samples.

A very simple OCT system has been developed, based on a Linnik interferometric microscope with its reference mirror mounted on a piezoelectric translator. The geometrical extension of the optics allows efficient illumination of this device with a low power (3 W) light bulb, yielding full field interferometric images at 50 Hz acquisition rate with a fast CCD camera. Due to the very broad spectral width of the light source and camera response, an axial resolution better than 2 micrometers is easily achieved. Tomographic images of cell smears are shown.

Real-time optical coherence tomography (OCT) was used to visualize and quantify structures in the anterior segment of the eye. Current results of ongoing clinical trials are presented. Preliminary data indicates strong potential for the use of real time OCT as a tool for noninvasive characterization of the anterior chamber angle and for anterior segment biometry.

Two improved spectral Optical Coherence Tomography (OCT)techniques: a differential and complex methods are compared with classical spectral OCT. Both methods enable to eliminate parasitical terms. Additionally the complex spectral OCT doubles the useful depth range. Opthalmic applications of above-mentioned techniques are presented.

The optical imaging properties of the eye are both determined by optical aberrations and deviations of the eye length. We report on a device for simultaneous measurement of aberrations and eye length combining a Shack Hartmann sensor and a short coherence interferometer. The short measurement duration minimizes disturbing influences like tear film or eye length changes. The low irradiance of the atient's eye minimizes exposure time.

Visualization of the complex structure composed of oriented axons leaving the retina in a loose connective tissue matrix has been carried out in living tissue using white light confocal microscopy. Optical sections were acquired at normal and elevated intraocular pressures. A 3D model of the lamina was produced and techniques were developed to study the effects of intraocular pressure on the structural integrity of the lamina cribrosa,

3-D optical fluorescence microscopy becomes now an efficient tool for volumic investigation of living biological samples. However, acquired raw data suffer from different distortions. In order to carry out biological analysis, restoration of raw data by deconvolution is mandatory. The system identification is useful to obtain the knowledge of the actual system and to quantify the influence of experimental parameters. High order centered moments are used as PSF descriptors. Oil immersion index, numerical aperture and specimen thickness are critical parameters for data quality. Furthermore, PSF identification is helpful to precise the experimental protocol. Application to 3-D anthracycline distribution in breast cancer cells is presented.

The Photonic Force Microscope is a novel scanning probe microscope based on optical tweezers to hold a probe, which fluctuates in its position due to thermal noise. The three-dimensional position of the probe is detected with high temporal resolution and spatial precision by analyzing the interference of unscattered and forward scattered light. We present the theoretical framework of the optical forces acting on the probe, as well as of the detection signal due to coherent scattering and describe thermal noise position fluctuations by a Brownian dynamics simulation. As an application we simulate the temporal and spatial behavior of a probe tethered to the coverslip (according to the molecular kinesin/microtubule system) at different laser powers and different anchor positions.

We report a whole-field fluorescence imaging microscope that combines 3-D spatial resolution by optical sectioning, using structured illumination, with fluorescence lifetime imaging and spectrally-resolved imaging. We show the potential of this technique in the elimination of common artefacts in fluorescence lifetime imaging and apply it to study the dependence of the lifetime on the emission wavelength in biological tissue.

We use a two-photon laser scanning microscope with a new Time-Correlated Single Photon Counting (TCSPC) imaging technique to obtain combined intensity-lifetime images for FRET measurements in living cells. Single photon pulses from a photomultiplier and signals from the scanning head are used to record the three-dimensional photon density over the time- and image coordinates. Double exponential decay analysis delivers the lifetime components of the quenched and the unquenched molecules in all pixels of the image. We use the ratio of the intensity coefficients of the fast and slow decay component to create images that show the size of the FRET effects in different parts of the cell.

The fluorescence decay in fluorescence lifetime imaging (FLIM) is typically fitted to a multi-exponential model with discrete lifetimes. The interaction between fluorophores in heterogeneous samples (e.g. biological tissue) can, however, produce complex decay characteristics that do not correspond to such models. Although they appear to provide a better fit to fluorescence decay data than the assumption of a mono-exponential decay, the assumption of multiple discrete components is essentially arbitrary and often erroneous. The stretched exponential function (StrEF) describes fluorescence decay profiles using a continuous lifetime distribution as has been reported for tryptophan, being one of the main fluorophores in tissue. We have demonstrated that this model represents our time-domain FLIM data better than multi-exponential discrete decay components, yielding excellent contrast in tissue discrimination without compromising the goodness of fit, and it significantly decreases the required processing time. In addition, the stretched exponential decay model can provide a direct measure of the sample heterogeneity and the resulting heterogeneity map can reveal subtle tissue differences that other models fail to show.

This article describes a setup for subcellular time-resolved fluorescence spectroscopy and fluorescence lifetime measurements using a confocal laser scanning microscope in combination with a short pulsed diode laser for fluorescence excitation and specimen illumination. The diode laser emits pulses at 398 nm wavelength with 70 ps full width at half maximum (FWHM) duration. The diode laser can be run at a pulse repetition rate of 40 MHz down to single shot mode. For time resolved spectroscopy a spectrometer setup consisting of an Czerny Turner spectrometer and a MCP-gated and -intensified CCD camera was used. Subcellular fluorescence lifetime measurements were achieved using a time-correlated single photon counting (TCSPC) module instead of the spectrometer setup. The capability of the short pulsed diode laser for fluorescence imaging, fluorescence lifetime measurements and time-resolved spectroscopy in combination with laser scanning microscopy is demonstrated by fluorescence analysis of several photosensitizers on a single cell level.

We demonstrate accurate and efficient three-dimensional optical diffusion imaging using simulated noisy data from a set of measurements at a single modulation frequency. A Bayesian framework provides for prior model conditioning, and a dual-step cost function optimization allows sequential estimation of the data noise variance and the image.

Apoptosis is the effector of regulated cell death and plays a role in many physiologic and pathologic processes. It is characterized by a highly regulated condensation and fragmentation of the cell nucleus, and breakup of the entire cell into vesicles, (apoptotic bodies) containing cell organelles and fragments of the nucleus. Previous experiments indicate that changes in optical properties after induction of apoptosis might be detected using optical imaging systems, such as optical coherence tomography (OCT), due to an increase in scattering of apoptotic cells. The previous in vitro experiments are extended to ex vivo and in vivo experiments. A nearly two-fold increase in attenuation coefficient is observed in a tissue culture of porcine carotid artery, in which apoptosis is induced by balloon dilation, and a significant 20 % increase in an in vivo setup. The preliminary results of this study indicate that the apoptotic process may also be detected ex vivo and in vivo using optical imaging systems, such as optical coherence tomography (OCT), due to an increase in scattering by the typical disintegration of cellular material.

In recent years the interest in the determination of optical properties of layered tissue structure has resurfaced. Applications include, for example, studies on layered skin tissue and underlying muscles, imaging of the brain underneath layers of skin, skull, and meninges, and imaging of the fetal head in utero beneath the layered structures of the maternal abdomen. In this work we approach the problem of layered structures in the framework of model-based iterative image reconstruction schemes. These schemes are currently developed to determine the optical properties inside tissue from measurement on the surface. If applied to layered structure these techniques yield substantial improvements over currently available semi-analytical approaches.

Due to the fact that the Kirchhoff Approximation (KA) does not involve matrix inversion for solving the forward problem, it is a very useful tool for quickly solving 3D geometries of arbitrary size and shape. Its potential mainly lies in the rapid generation of Green?s functions for arbitrary geometries, which is key to tomography techniques. We here apply it to light diffusion and study its limits of validity, proving that it is a very useful approximation for diffuse optical tomography (DOT). Its use can improve the existing imaging techinques for real time diagnostics in medicine.

In order to provide depth resolution for bulk tissue imaging experiments using absorption signals, we have designed an internal laser point spread technique. A laser light source has been imbedded in different depths into cardiac tissue and tissue phantoms, the signal on the tissue surface detected by a CCD detector. These measurements in combination with an analytic solution of the diffusion equation allow us to estimate optical properties of the investigated tissue. We show how this information provides the core of depth quantification of fluorescence and absorption measurements in bulk tissue and investigate experimentally the transition from single scattering to diffuse photon transport in cardiac tissue and suspensions of microscopic spherical particles that serve as model systems.

We present the Radiosity-Diffusion model in three dimensions(3D), as an extension to previous work in 2D. It is a method for handling non-scattering spaces in optically participating media. We present the extension of the model to 3D including an extension to the model to cope with increased complexity of the 3D domain. We show that in 3D more careful consideration must be given to the issues of meshing and visibility to model the transport of light within reasonable computational bounds. We demonstrate the model to be comparable to Monte-Carlo simulations for selected geometries, and show preliminary results of comparisons to measured time-resolved data acquired on resin phantoms.

We discuss how a spectral-domain method in combination with a split-operator technique can be used to calculate exact solutions of the time-dependent Maxwell equations. We apply this technique to study the propagation of a light pulse through an inhomogeneous medium consisting of practically arbitrarily shaped dielectric and metallic materials.

Near infrared spectroscopy is increasingly being used for monitoring cerebral oxygenation and haemodynamics. Since the light in the head is strongly scattered, it is necessary to modelling the light propagation in the head to obtain the volume of tissue sampled and partial optical path length in the brain. The serious problem to calculate the light propagation in the head is the heterogeneity of tissue especially the presence of low scattering CSF layer. Since the diffusion equation no longer holds in the low scattering layer, the light propagation in the head model with low scattering layer should be analysed by Monte Carlo method or light transport equation. In this study, we propose a new approach Hybrid Monte Carlo-Diffusion Method to calculate the light propagation in the adult head model with a low scattering CSF layer. The light propagation in a high scattering medium is calculated by the diffusion theory and that in a low scattering CSF layer is predicted by Monte Carlo method. The results of detected intensity and mean time of flight in a simplified adult head model by the Hybrid Monte Carlo-Diffusion method agree well with those predicted by Monte Carlo method.

Near infrared topographic imaging is an effective instrument to image brain-cortex activity. The image is reconstructed by changes in light intensity detected with multi-channel source-detector pairs. However, light scattering in tissue prevents us from improving the spatial resolution of the reconstructed image, hence it is important to evaluate the effect of scattering on the spatial resolution of the reconstructed image. In this study, separation of two absorbers in topographic image is investigated by Monte Carlo simulation to evaluate the spatial resolution of topographic imaging. Because of heterogeneity of tissue, especially presence of low scattering CSF layer affects the light propagation in the adult brain. The adult head model consists of three layers including a low scattering medium. In case where the separation of two absorbers is greater than the distance between adjacent measurment points, the two absorbers can be separated in the topographic image.

The light propagation in a simple layered ellipse model and more complex neonatal head model are calculated by the finite-difference method. The finite-difference method has advantage of simple algorithm and fast calculation time however has been successful under restricted condition for a heterogeneous medium. The light propagation in the both models is predicted by Monte Carlo simulation to validate the results of the finite-difference method. The detected intensity and partial optical path length calculated by the finite-difference method agree with those by Monte Carlo simulation. The boundary of the grey and white matter in the neonatal head model is more complex than the simple ellipse model. However, the tendency of spatial sensitivity profiles in the neonatal head model is scarcely affected by the effect of heterogeneity of the brain tissue.

Absorption and reduced scattering coefficients calculated from spatially resolved diffuse reflectance measurements are usually assumed to be unreliable for wavelengths lower than approximately 600 nm. A correction factor was developed for non-homogenous distribution of absorbers concentrated in discrete cylindrical blood vessels. The concept of an effective absorption coefficient extends the applicability of diffusion theory too much lower wavelength regions.

An instrument for time resolved optical mammography working in the compressed breast geometry was constructed. The system operates with pulsed laser sources emitting simultaneously at two different wavelengths. Time-correlated single-photon counting is used for acquisition. The breast is slightly compressed between two plates and scanned along the X and Y directions. Continuous acquisition and on line display of the processed images permit optimal measurement efficiency and control of the operation. Phantom tests and preliminary in vivo measurements on healthy volunteers demonstrate capability to distinguish between scattering and absorption contributions, as well as discrimination of key breast structures. The equipment is about to be tested in a clinical study on time-resolved optical mammography.

We have developed a new four-wavelength optical imager to evaluate the oxygen supply in human skin for peripheral circulation. It measures the reflected light from human skin by a CCD and presents the maps of hemoglobin distribution. This paper describes the principle, system configuration and performance test for human foot during and after venous and arterial occlusion. The obtained images showed distinct local difference in skin oxygenation due to blood flow.

We report on the first three-dimensional, volumetric, tomographic localization of changes in the concentration of oxyhemoglobin and deoxyhemoglobin in the brain. To this end we have developed a model-based iterative image reconstruction scheme that employs adjoint differentiation methods to minimize the difference between measured and predicted data. To illustrate the performance of the technique, the three-dimensional distribution of changes in the concentration of oxyhemoglobin and deoxyhemoglobin during a Valsalva maneuver are visualized. The observed results are consistent with previously reported effects concerning optical responses to hemodynamic perturbations.

The time-resolved spatial sensitivity profiles on the brain surface and in the plane perpendicular to the brain surface are predicted by Monte Carlo simulation to discuss the volume of tissue sampled by multi-channel near infrared instruments. The adult head model consists of five types of tissue. The temporal point spread function of the detected light is divided into five parts and the trajectories of photons detected during each gate are accumulated to obtain the time-resolved spatial sensitivity profiles. Early photons only graze the cortex surface around the middle of the source and detector whilst late photons tend to penetrate into white matter. The spatial sensitivity profiles for the late photons widely spread on the cortex surface and these results suggest that the detected signal mainly reflects the absorption change in the grey matter.

In this paper, image formation under single-photon (1-p), two-photon (2-p) and three-photon (3-p) fluorescence imaging through turbid media which consist of different sized scatterers has been investigated in detail. It has been demonstrated that the size of scattering particles plays an important role in determining whether to use 1-p, 2-p, or 3-p excitation. For small scatterers, where Rayleigh scattering is dominant, multi-photon excitation provides significantly better resolution. Such improvement reduces dramatically for large scatterers, where Mie scattering becomes dominant. Another disadvantage of using multi-photon fluorescence excitation in highly scattered media is that penetration depth is limited by fast dropping of signal strength in deep tissue imaging. In this paper, we introduce a deconvolution method with a novel concept of the effective point spread function, which is effective in restoring the loss of imaging resolution caused by multiple scattering in a tissue medium.

Quantitative measurements of diffusive media, in spectroscopic or imaging mode, rely on the generation of appropriate forward solutions, independently on the inversion scheme employed. For complicated boundaries, the use of numerical methods is usually pursued due to implementation simplicity, but this results in great computational needs. Even though some analytical expressions are available, an analytical solution to the diffusion equation that deals with arbitrary volumes and boundaries is needed. We use here an analytical approximation, the Kirchhoff Approximation or the tangent-plane method, and put it to test with experimental data in a cylindrical geometry. We examine the experimental performance of the technique, as a function of the optical properties of the medium and demonstrate how it greatly speeds up the computation time when performing 3D reconstructions.

Random lasing and second harmonic generation after two-photon excitation was observed for the first time, in both scattering and amplifying polymer samples. Pumping was performed with sub-picosecond laser pulses at 830 nm and the emission, was observed with a spectrograph streak camera detection system for simultaneous recording of spectral and temporal features. SHG was detected at 415 nm and random lasing at 470 nm. The advantages of two photons absorption are discussed because of its enhanced detection efficiency and minimized effects on the irradiated sample (e.g. healthy tissue surrounding tumors).

An improved Time-Correlated Single Photon Counting (TCSPC) technique features high count rate, low differential nonlinearity and multi-detector capability. The system has four completely parallel TCSPC channels and achieves an effective overall count rate of 20 MHz. By an active routing technique, up to eight detectors can be connected to each of the TCSPC channels. We used the system to record optical mammograms after pulsed laser illumination at different wavelengths and projection angles.

A new instrumentation based on an intensified CCD camera with picosecond time resolution has been developed for optical imaging in turbid media. Scattering inclusions in a homogeneous sample have been detected by fitting the experimental data with a theoretical expression derived from the random walk theory, while absorption inclusions have been localized using time-gated images taken at suitable delays with respect to the expected arrival time of non-scattered photons.

We developed a portable dual-wavelength 8-channel system for time-resolved reflectance measurements based on pulsed laser diodes, a multianode photomultiplier and a time-correlated single-photon counting board. The performances of the system were tested on phantoms in terms of stability, reproducibility among channels, and accuracy in the determination of absolute values of the absorption and transport scattering coefficients. Preliminary in vivo measurements (cuff occlusion protocols) were performed on healthy volunteers to monitor spatial changes in tissue oxygenation and blood volume.

The applicability of backprojection algorithms of filtered shadows that have been earlier developed for computer tomography is shown for the case of optical tomography of strongly scattering media. This opportunity is based on the presence of a long rectilinear part in the approximation of the statistical Photon Average Trajectories of photons propagating through the scattering medium. The results of numerical experiments showed that the quality of reconstruction using filtered backprojection algorithms do not surrender to that for multi-iterative algorithms, at much shorter reconstruction time.

Theoretical analysis and numerical experiments show a significant difference in a temporal dynamics of shadows caused by absorbing and scattering macroinhomogeneities. This difference is especially noticeable at the leading front of the pulse passed through the scattering medium. This makes it possible to image absorbing and scattering inhomogeneities separately using shadows obtained at subsequent time moments.

Presented are the operating characteristics of an integrated CW-near infrared tomographic imaging system capable of fast data collection and producing 2D/3D images of optical contrast features that exhibit dynamic behavior in tissue and other highly scattering media in real time. Results of preliminary in vivo studies on healthy and cancerous breast tissue are shown.

The resolution power of photon tomography is estimated by calculating the root-mean-square deviation of the photon trajectory from its mean path. Statistical simulation is performed using exact values of the migration probability density. To estimate this quantity we calculate the root- mean-square deviation of photon trajectories from the mean path, the initial and final space-time points of trajectory being fixed. We apply the Monte-Carlo procedure in order to calculate the photon-travel deviation variance. In this paper we confine ourselves to infinite isotropically scattering media.

The dependence of the optical diffusion coefficient D on the absorption coefficient (mu) _a was investigated. The spatial intensity distribution of light emitted by a low frequency intensity-modulated diode laser was studied in aqueous suspensions of pmma latex beads. The absorption coefficient of the suspensions was gradually increased by adding the dye isosulfan blue (isb) up to a ratio between the absorption and the effective scattering coefficient of the mixture of 3.6. Conventional transmission measurements showed that the absorption coefficient of the mixture increases linearly in the entire concentration range employed.The measurements were evaluated in the framework of photon diffusion theory.The results obtained indicated that for the suspensions investigated the optical diffusion coefficient does depend on absorption.

Redox absorbance changes in living cells (monolayer of HeLa) under laser irradiation at 633, 670, and 820 nm have been studied by the method of multichannel registration in spectral range 530-890 nm. It has been found that the irradiation causes changes in the absorption spectram of the cells in two regions, near 754-795 nm (maxima at 757, 775, and 795 nm) and near 812-873 nm (maxima at 819, 837, 858, and 873 nm). Changes occur in band parameters (peak positions, width, and integral intensity). Virtually no changes occur in the red spectral region and a few changes are recorded in the green region near 556-565 nm. The results obtained evidence that cytochrome c oxidase becomes more oxidized (which means that the oxidative metabolism is increased) due to irradiation at all wavelengths used. The results of present experiment support the suggestion (Karu, Lasers Life Sci., 2:53, 1988) that the mechanism of low- power laser therapy on cellular level is based on the electronic excitation of chromophores in cytochrome c oxidase which modulates redox status of the molecule and enhances its functional activity.

The multi-channel NIR system can obtain the topographic image of brain function in the cortex. Since the light is strongly scattered in biological tissue, the spatial sensitivity of the source-detector pairs is broadly distributed in the brain cortex. This phenomenon causes poor spatial resolution of topographic imaging. In this study, the spatial sensitivity distribution of the source-detector pairs is incorporated into the reconstruction algorithm of the topographic image to improve the spatial resolution.

First order perturbation approach to the diffusion equation provides a more realistic model to describe inhomogeneous structures. To this regard, we obtained a general expression of the time-resolved transmittance relatively to a homogeneous turbid medium with an embedded inclusion. This expression was further approximated in the case of a small volume object. Then, we studied the limit and accuracy of this approach concerning on a cubic absorber placed in the centre of the source-detector line of a turbid slab.

The authors have proposed and tested the new type of the microscope with superimposed interference and fluorescence images of the living cell. The fluorescence image gives the information about the distribution of drugs in different parts of the cell. Interference microscopy technique allows implementing the quantitative analysis of a living transparent cell, for example, to measure dry cell weight and cell density spatial distribution. Interference microscopy makes possibility the observation cell vital activity at dynamics. Computer-aided processing of interferograms using phase shift technique, allows carrying out continuous (over 12 hours and more) monitoring of living cells quantitative parameters with time interval between measurements less than one minute. RMS error for dry weight of non-living fixed test object using this microscope is about 1%.

Dictyosphaerium puichellum Wood, a typical a fresh water alga, is known for photosynthesis and expected to have photoluminescence also. We have, therefore, carried out a confocal fluorescence microscopic and spectroscopic study of this species. 3-D image constructed from 50 optical sections clearly reveals that fluorescence is originated from only the outer layer and not from the central core. Spectral investigation shows that a strong and sharp peak is located at 684 nm. At this peak value, the power measurement was carried out and the number of photons emitted by alga in the solid angle 48.59 f was calculated and found to be 3.4*1010/ sec. The other significant feature of the responsible fluorophore is that the present photoluminescence has a very low quenching rate and hence the emitted radiation is observed for a very long time (5 12 sec.). This directly suggests that this fluorophore could have direct applications in dye related industries.

A recently developed multiple beam interference confocal microscopy and conventional confocal microscopic techniques were employed to obtain 3-D images and morphological details of chondrocytes obtained from a postmortem of 1 day old baby for the first time. Unprecedented images clearly showed that they were circular and internal structures were noticeably visible. The nucleus shows two or three nucleoids. Peripheral nuclear organelles were also perceptible. These techniques permit to examine with precision the thickness of the organelles.

Parallel optical coherence tomography in scattering samples is demonstrated using a 58 by 58 smart-pixel detector array. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve 4micrometers longitudinal resolution and 9mm transverse resolution on a 260x260 micrometers ² field of view. We imaged a resolution target covered by an intralipid solution with different scattering coefficients as well as onion cells.

Expressions for radiation-induced forces are presented for the case of a Rayleigh particle at the focus of a Gaussian laser beam. Classical electromagnetic theory was used to obtain the dependence of the scattering and gradient forces on the incident laser frequency, beam convergence angle, and spatial position of the particle in the focus. Numerical analysis performed for particles with a single resonant absorption peak demonstrates the occurrence of enhanced trapping forces at near-resonant frequencies. An indication of the breakdown of the paraxial Gaussian approximation for large convergence angles is also shown by calculation of the scattering force. Use of this technique of gradient force enhancement may provide optical tweezers with enhanced trapping strengths and a degree of specificity.

We have been developing and applying a new type of polarized light microscope, the new Pol-Scope, which dramatically enhances the unique capabilities of the traditional polarizing microscope. In living cells, without applying exogenous dyes or florescent labels, we have studied the dynamic organization of filamentous actin in neuronal growth cones and improved the efficiency of spindle imaging for in-vitro fertilization and enucleation procedures.