Confocal microscopy in turbid media

Overtone photothermal microscopy for high-resolution and high-sensitivity vibrational imaging

Photothermal microscopy is a highly sensitive pump-probe method for mapping nanostructures and molecules through the detection of local thermal gradients. While visible photothermal microscopy and mid-infrared photothermal microscopy techniques have been developed, they possess inherent limitations. These techniques either lack chemical specificity or encounter significant light attenuation caused by water absorption. Here, we present an overtone photothermal (OPT) microscopy technique that offers high chemical specificity, detection sensitivity, and spatial resolution by employing a visible probe for local heat detection in the C-H overtone region. We demonstrate its capability for high-fidelity chemical imaging of polymer nanostructures, depth-resolved intracellular chemical mapping of cancer cells, and imaging of multicellular C. elegans organisms and highly scattering brain tissues. By bridging the gap between visible and mid-infrared photothermal microscopy, OPT establishes a new modality for high-resolution and high-sensitivity chemical imaging. This advancement complements large-scale shortwave infrared imaging approaches, facilitating multiscale structural and chemical investigations of materials and biological metabolism.

Confocal Laser Scanning Microscope Imaging of Custom-Made Multi-Cylinder Phantoms: Theory and Experiment

In this work, the image formation in a confocal laser scanning microscope (CLSM) is investigated for custom-made multi-cylinder phantoms. The cylinder structures were fabricated using 3D direct laser writing and consist of parallel cylinders with radii of 5 and 10 μm for the respective multi-cylinder phantom, with overall dimensions of about 200×200×200 μm3. Measurements were performed for different refractive index differences and by varying other parameters of the measurement system, such as pinhole size or numerical aperture (NA). For theoretical comparison, the confocal setup was implemented in an in-house developed tetrahedron-based and GPU-accelerated Monte Carlo (MC) software. The simulation results for a cylindrical single scatterer were first compared with the analytical solution of Maxwell's equations in two dimensions for prior validation. Subsequently, the more complex multi-cylinder structures were simulated using the MC software and compared with the experimental results. For the largest refractive index difference, i.e., air as the surrounding medium, the simulated and measured data show a high degree of agreement, with all the key features of the CLSM image being reproduced by the simulation. Even with a significant reduction in the refractive index difference by the use of immersion oil to values as low as 0.005, a good agreement between simulation and measurement was observed, particularly with respect to the increase in penetration depth.

Improved spatial resolution using focal modulation microscopy with a Tai Chi aperture

Focal modulation microscopy (FMM) has gained significant interest in biological imaging. However, the spatial resolution and penetration depth limit the imaging quality of FMM due to the strong scattering background. Here, we introduce FMM with a Tai Chi aperture (TCFMM) based on diffraction theory to improve the spatial resolution. The results show that the transverse resolution is improved by 61.60% and 41.37% in two orthogonal directions, and the axial resolution is improved by 29.67%, compared with confocal microscopy (CM). The signal background ratio (SBR) of TCFMM is increased by 23.26% compared with CM and remains nearly the same compared with FMM using D-shape apertures (DFMM). These improvements in spatial resolution and SBR indicate that TCFMM has potential in deep tissue imaging.

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

Dual-Mode, Anisotropy-Encoded, Ratiometric Fluorescent Nanosensors: Towards Multiplexed Detection

A nanosensor with dual-mode fluorescence response to pH and an encoded identification signal, was developed by exploiting excitation energy transfer and tailored control of molecular organization in core-shell nanoparticles. Multiple signals were acquired in a simple single-excitation dual-emission channels set-up.

Accurate viscosity measurements of flowing aqueous glucose solutions with suspended scatterers using a dynamic light scattering approach with optical coherence tomography

The viscosity of turbid colloidal glucose solutions has been accurately determined from spectral domain optical coherence tomography (OCT) M-mode measurements and our recently developed OCT dynamic light scattering model. Results for various glucose concentrations, flow speeds, and flow angles are reported. The relative "combined standard uncertainty" uc(η) on the viscosity measurements was ±1% for the no-flow case and ±5% for the flow cases, a significant improvement in measurement robustness over previously published reports. The available literature data for the viscosity of pure water and our measurements differ by 1% (stagnant case) and 1.5% (flow cases), demonstrating good accuracy; similar agreement is seen across the measured glucose concentration range when compared to interpolated literature values. The developed technique may contribute toward eventual noninvasive glucose measurements in medicine.

Deep imaging in scattering media with selective plane illumination microscopy

In most biological tissues, light scattering due to small differences in refractive index limits the depth of optical imaging systems. Two-photon microscopy (2PM), which significantly reduces the scattering of the excitation light, has emerged as the most common method to image deep within scattering biological tissue. This technique, however, requires high-power pulsed lasers that are both expensive and difficult to integrate into compact portable systems. Using a combination of theoretical and experimental techniques, we show that if the excitation path length can be minimized, selective plane illumination microscopy (SPIM) can image nearly as deep as 2PM without the need for a high-powered pulsed laser. Compared to other single-photon imaging techniques like epifluorescence and confocal microscopy, SPIM can image more than twice as deep in scattering media ( ? 10 times the mean scattering length). These results suggest that SPIM has the potential to provide deep imaging in scattering media in situations in which 2PM systems would be too large or costly.

Separation of ballistic and diffusive fluorescence photons in confocal Light-Sheet Microscopy of Arabidopsis roots

Image quality in light-sheet fluorescence microscopy is strongly affected by the shape of the illuminating laser beam inside embryos, plants or tissue. While the phase of Gaussian or Bessel beams propagating through thousands of cells can be partly controlled holographically, the propagation of fluorescence light to the detector is difficult to control. With each scatter process a fluorescence photon loses information necessary for the image generation. Using Arabidopsis root tips we demonstrate that ballistic and diffusive fluorescence photons can be separated by analyzing the image spectra in each plane without a priori knowledge. We introduce a theoretical model allowing to extract typical scattering parameters of the biological material. This allows to attenuate image contributions from diffusive photons and to amplify the relevant image contributions from ballistic photons through a depth dependent deconvolution. In consequence, image contrast and resolution are significantly increased and scattering artefacts are minimized especially for Bessel beams with confocal line detection.

Numerical studies of focal modulation microscopy in high-NA system

High spatial resolution with deep imaging penetration depth is the main advantage of focal modulation microscopy (FMM). This paper investigates effects of polarization on FMM in a high-NA system based on vectorial diffraction theory. Compared with confocal microscopy, FMM shows a 20.1% improvement in axial resolution. The performance of different polarization patterns is also discussed numerically. The study on polarization modulation may provide a new way to obtain a tighter focal spot.

Comparison between optical-resolution photoacoustic microscopy and confocal laser scanning microscopy for turbid sample imaging

Optical-resolution photoacoustic microscopy (ORPAM) in theory provides lateral resolution equivalent to the optical diffraction limit. Scattering media, such as biological turbid media, attenuates the optical signal and also alters the diffraction-limited spot size of the focused beam. The ORPAM signal is generated only from a small voxel in scattering media with dimensions equivalent to the laser spot size after passing through scattering layers and is detected by an acoustic transducer, which is not affected by optical scattering. Thus, both ORPAM and confocal laser scanning microscopy (CLSM) reject scattered light. A multimodal optical microscopy platform that includes ORPAM and CLSM was constructed, and the lateral resolution of both modes was measured using patterned thin metal film with and without a scattering barrier. The effect of scattering media on the lateral resolution was studied using different scattering coefficients and was compared to computational results based on Monte Carlo simulations. It was found that degradation of lateral resolution due to optical scattering was not significant for either ORPAM or CLSM. The depth discrimination capability of ORPAM and CLSM was measured using microfiber embedded in a light scattering phantom material. ORPAM images demonstrated higher contrast compared to CLSM images partly due to reduced acoustic signal scattering.

Cone-shell Raman spectroscopy (CSRS) for depth-sensitive measurements in layered tissue

We report the development of a depth-sensitive Raman spectroscopy system using the configuration of cone-shell excitation and cone detection. The system uses a 785 nm diode laser and three identical axicons for Raman excitation of the target sample in the form of a hollow conic section. The Raman scattered light from the sample, passed through the same (but solid) conic section, is collected for detection. Apart from its ability of probing larger depths (~ few mm), an important attraction of the system is that the probing depths can be varied by simply varying the separation between axicons in the excitation arm. Furthermore, no adjustment is required in the sample arm, which is a significant advantage for noncontact, depth-sensitive measurement. Evaluation of the performance of the developed setup on nonbiological phantom and biological tissue sample demonstrated its ability to recover Raman spectra of layers located at depths of ~2-3 mm beneath the surface.

Enhancement of optical coherence microscopy in turbid media by an optical parametric amplifier

We report the enhancement in imaging performance of a spectral-domain optical coherence microscope (OCM) in turbid media by incorporating an optical parametric amplifier (OPA). The OPA provides a high level of optical gain to the sample arm, thereby improving the signal-to-noise ratio of the OCM by a factor of up to 15 dB. A unique nonlinear confocal gate is automatically formed in the OPA, which enables selective amplification of singly scattered (ballistic) photons against the multiply-scattered light background. Simultaneous enhancement in both imaging depth and spatial resolution in imaging microstructures in highly light-scattering media are demonstrated with the combined OPA-OCM setup. Typical OCM inteferograms (left) and images (right) without and with OPA.

Using Doppler shift induced by Galvanometric mirror scanning to reach shot noise limit with laser optical feedback imaging setup

This paper proposes what we believe is a new method to remove the contribution of parasitic reflections in the images of the laser optical feedback imaging (LOFI) technique. This simple method allows us to extend the LOFI technique to long-distance applications, as imaging through a fog or a smoke. The LOFI technique is an ultrasensitive imaging technique that is interesting for imaging objects through a scattering medium. However, the LOFI sensitivity can be dramatically limited by parasitic optical feedback occurring in the experimental setup. In previous papers [Appl. Opt.48, 64 (2009)10.1364/AO.48.000064APOPAI1559-128X, Opt. Lett.37, 2514 (2012)10.1364/OL.37.002514OPLEDP0146-9592], we already have proposed methods to filter a parasitic optical feedback, but they are not well suited to metric working distances. This new method uses a Doppler frequency shift induced by the moving mirror used to scan the object to be imaged. Using this Doppler frequency shift, we can distinguish the photons reflected by the target and the parasitic photons reflected by the optical components in the experimental setup. In this paper, we demonstrated theoretically and experimentally the possibility to filter the parasitic reflection in LOFI images using the Doppler frequency shift. This method significantly improves the signal-to-noise ratio by a factor 15 and we can obtain a shot noise limited image through a scattering medium of an object at 3 m from the detector.

Optical parametrically gated microscopy in scattering media

High-resolution imaging in turbid media has been limited by the intrinsic compromise between the gating efficiency (removal of multiply-scattered light background) and signal strength in the existing optical gating techniques. This leads to shallow depths due to the weak ballistic signal, and/or degraded resolution due to the strong multiply-scattering background--the well-known trade-off between resolution and imaging depth in scattering samples. In this work, we employ a nonlinear optics based optical parametric amplifier (OPA) to address this challenge. We demonstrate that both the imaging depth and the spatial resolution in turbid media can be enhanced simultaneously by the OPA, which provides a high level of signal gain as well as an inherent nonlinear optical gate. This technology shifts the nonlinear interaction to an optical crystal placed in the detection arm (image plane), rather than in the sample, which can be used to exploit the benefits given by the high-order parametric process and the use of an intense laser field. The coherent process makes the OPA potentially useful as a general-purpose optical amplifier applicable to a wide range of optical imaging techniques.

Simulating the scanning of a focused beam through scattering media using a numerical solution of Maxwell's equations

A highly efficient method based on Maxwell's theory was developed, which enables the calculation of the scanning of a focused beam through scattering media. Maxwell's equations were numerically solved in two dimensions using finite difference time domain simulations. The modeling of the focused beam was achieved by applying the angular spectrum of plane waves method. The scanning of the focused beam through the scattering medium was accomplished by saving the results of the near field obtained from one simulation set of plane waves incident at different angles and by an appropriate post processing of these data. Thus, an arbitrary number of focus positions could be simulated without the need to further solve Maxwell's equations. The presented method can be used to efficiently study the light propagation of focused beam through scattering media which is important, for example, for different kinds of scanning microscopes.

Dynamic imaging through turbid media based on digital holography

Imaging through turbid media using visible or IR light instead of harmful x ray is still a challenging problem, especially in dynamic imaging. A method of dynamic imaging through turbid media using digital holography is presented. In order to match the coherence length between the dynamic object wave and the reference wave, a cw laser is used. To solve the problem of difficult focusing in imaging through turbid media, an autofocus technology is applied. To further enhance the image contrast, a spatial filtering technique is used. A description of digital holography and experiments of imaging the objects hidden in turbid media are presented. The experimental result shows that dynamic images of the objects can be achieved by the use of digital holography.

Coherence-controlled holographic microscopy in diffuse media

Low-coherence interferometric microscopy (LCIM) enables to image through scattering media by filtration of ballistic light from diffuse light. The filtration mechanism is called coherence gating. We show that coherence-controlled holographic microscope (CCHM), which belongs to LCIM, enables to image through scattering media not only with ballistic light but also with diffuse light. The theoretical model was created which derives the point spread function of CCHM for imaging through diffuse media both with ballistic and diffuse light. The results of the theoretical model were compared to the experimental results. In the experiment the resolution chart covered by a ground glass was imaged. The experimental results are in the good agreement with the theoretical results. It was shown both by experiments and the theoretical model, that with ballistic and diffuse light we can obtain images with diffraction limited resolution.

Optimizing the performance of dual-axis confocal microscopes via Monte-Carlo scattering simulations and diffraction theory

Dual-axis confocal (DAC) microscopy has been found to exhibit superior rejection of out-of-focus and multiply scattered background light compared to conventional single-axis confocal microscopy. DAC microscopes rely on the use of separated illumination and collection beam paths that focus and intersect at a single focal volume (voxel) within tissue. While it is generally recognized that the resolution and contrast of a DAC microscope depends on both the crossing angle of the DAC beams, 2θ, and the focusing numerical aperture of the individual beams, α, a detailed study to investigate these dependencies has not been performed. Contrast and resolution are considered as two main criteria to assess the performance of a point-scanned DAC microscope (DAC-PS) and a line-scanned DAC microscope (DAC-LS) as a function of θ and α. The contrast and resolution of these designs are evaluated by Monte-Carlo scattering simulations and diffraction theory calculations, respectively. These results can be used for guiding the optimal designs of DAC-PS and DAC-LS microscopes.

A coherent model for turbid imaging with confocal microscopy

We present an engineering model of coherent imaging within a turbid volume, such as human tissues, with a confocal microscope. The model is built to analyze the statistical effect of aberrations and multiply scattered light on the resulting image. Numerical modeling of theory is compared with experimental results. We describe the construction of a stable phantom that represents the statistical effect of object turbidity on the image recorded. The model and phantom can serve as basis for system optimization in turbid imaging.

Seeing through turbidity with harmonic holography [Invited]

The ability to see inside the body noninvasively is indispensable in modern biology and medicine. Optical approaches to such abilities are of rapidly growing interest because of their nonionizing nature and low cost. However, the problem of opacity due to the optical turbidity of tissues must be addressed before optical means become practical. Harmonic holography amalgamates the capability of holographic phase conjugation with the contrast-forming mechanism of second-harmonic generation, which provides a unique opportunity for imaging through a turbid medium. In this review we give accounts of the effort of imaging through turbid media using harmonic holographic phase conjugation.

Effects of light scattering on optical-resolution photoacoustic microscopy

The imaging depth of ballistic optical imaging technologies is limited by light scattering. To study the effects of scattering on optical-resolution photoacoustic microscopy (OR-PAM), the signals were divided into target and background signals. A method to simulate the point spread function (PSF) of the PAM system considering both optical illumination and acoustic detection was proposed, then the PSF was used to calculate the contribution of each class of signal at different depths of the focal plane (zf). How image contrast is degraded when there is a uniformly absorbing background as well as when there are small targets densely packed in the acoustic resolution cell were studied. By using the hyperboloid-focusing-based Monte Carlo method, optical focusing into a scattering medium was simulated. It was found that the lateral resolution provided by optical focusing is degraded by only 14% when zf=1.1 transport mean free path (l t'), compared with the case of no scattering. When zf=1.7 lt', the fluence at 50 μm radial distance away from the focal point is 93% of that at the focal point, which shows optical focusing is very weak at this depth. The method to simulate the PSF of PAM can be used in the future to optimize parameters so as to improve the system performance.

Ptychographic transmission microscopy in three dimensions using a multi-slice approach

Generally, methods of three-dimensional imaging such as confocal microscopy and computed tomography rely on two essentials: multiple measurements (at a range of focus positions or rotations) and a weakly scattering specimen (to avoid distortion of the focal spot in the confocal microscope or to satisfy the projection approximation in tomography). Here we show that an alternative form of multi-measurement imaging, ptychography, can be extended to three dimensions and can successfully recover images in the presence of multiple scattering and when the projection approximation is not applicable. We demonstrate our technique experimentally using visible light, where it has applications in imaging thick samples such as biological tissues; however the results also have important implications for x ray and electron imaging.

Acousto-optic laser optical feedback imaging

We present a photon noise and diffraction-limited imaging method combining an imaging laser and ultrasonic waves. The laser optical feedback imaging (LOFI) technique is an ultrasensitive imaging method for imaging objects through or embedded within a scattering medium. However, LOFI performances are dramatically limited by parasitic optical feedback occurring in the experimental setup. In this Letter, we have tagged the ballistic photons by an acousto-optic effect in order to filter the parasitic feedback effect and to reach the theoretical and ultimate sensitivity of the LOFI technique. We present the principle and the experimental setup of the acousto-optic laser optical feedback imaging technique, and we demonstrate the suppression of the parasitic feedback.

HomoFRET fluorescence anisotropy imaging as a tool to study molecular self-assembly in live cells

Molecular self-assembly is a defining feature of numerous biological functions and dysfunctions, ranging from basic cell signalling to diseases mediated by protein aggregation. There is current demand for novel experimental methods to study molecular self-assembly in live cells, and thereby in its physiological context. Förster resonance energy transfer (FRET) between fluorophores of a single type, known as homoFRET, permits noninvasive detection and quantification of molecular clusters in live cells. It can thus provide powerful insights into the molecular physiology of living systems and disease. HomoFRET is detected by measuring the loss of fluorescence anisotropy upon excitation with polarised light. This article reviews recent key developments in homoFRET fluorescence anisotropy imaging for the detection and quantification of molecular self-assembly reactions in biological systems. A summary is given of the current state-of-the-art and case studies are presented of successful implementations, highlighting technical aspects which have to be mastered to bridge the gap between proof-of-concept experiments and biological discoveries.

High-speed focal modulation microscopy using acousto-optical modulators

Focal Modulation Microscopy (FMM) is a single-photon excitation fluorescence microscopy technique which effectively rejects the out-of-focus fluorescence background that arises when imaging deep inside biological tissues. Here, we report on the implementation of FMM in which laser intensity modulation at the focal plane is achieved using acousto-optic modulators (AOM). The modulation speed is greatly enhanced to the MHz range and thus enables real-time image acquisition. The capability of FMM is demonstrated by imaging fluorescence labeled vasculatures in mouse brain as well as self-made tissue phantom.

Zeeman laser scanning confocal microscope and its ability on reduction of specimen-induced spherical aberration

The spherical aberration induced by refractive-index mismatch results in the degradation on the quality of sectioning images in conventional confocal laser scanning microscope (CLSM). In this research, we have derived the theory of image formation in a Zeeman laser scanning confocal microscope (ZLSCM) and conducted experiments in order to verify the ability of reducing spherical aberration in ZLSCM. A Zeeman laser is used as the light source and produces the linearly polarized photon-pairs (LPPP) laser beam. With the features of common-path propagation of LPPP and optical heterodyne detection, ZLSCM shows the ability of reducing the specimen-induced spherical aberration and improving the axial resolution simultaneously.

Focal modulation microscopy: a theoretical study

Focal modulation microscopy is an emerging fluorescence microscopy technique for in vivo imaging of thick biological tissues. Here, we present a theoretical study to assess its performance. The scalar diffraction theory is combined with Monte Carlo simulation to evaluate the signal-to-background ratio at various depths. The performance of confocal microscopy with a similar optical setup is also evaluated for comparison.

Imaging and analysis of three-dimensional cell culture models

Three-dimensional (3D) cell cultures are important tools in cell biology research and tissue engineering because they more closely resemble the architectural microenvironment of natural tissue, compared to standard two-dimensional cultures. Microscopy techniques that function well for thin, optically transparent cultures, however, are poorly suited for imaging 3D cell cultures. Three-dimensional cultures may be thick and highly scattering, preventing light from penetrating without significant distortion. Techniques that can image thicker biological specimens at high resolution include confocal microscopy, multiphoton microscopy, and optical coherence tomography. In this chapter, these three imaging modalities are described and demonstrated in the assessment of functional and structural features of 3D chitosin scaffolds, 3D micro-topographic substrates from poly-dimethyl siloxane molds, and 3D Matrigel cultures. Using these techniques, dynamic changes to cells in 3D microenvironments can be non-destructively assessed repeatedly over time.

Monte Carlo characterization of parallelized fluorescence confocal systems imaging in turbid media

We characterize and compare the axial and lateral performance of fluorescence confocal systems imaging in turbid media. The aperture configurations studied are a single pinhole, a slit, a Nipkow disk, and a linear array of pinholes. Systems with parallelized apertures are used clinically because they enable high-speed and real-time imaging. Understanding how they perform in highly scattering tissue is important. A Monte Carlo model was developed to characterize parallelized system performance in a scattering media representative of human tissues. The results indicate that a slit aperture has degraded performance, both laterally and axially. In contrast, the analysis reveals that multipinhole apertures such as a Nipkow disk or a linear pinhole array can achieve performance nearly equivalent to a single pinhole aperture. The optimal aperture spacing for the multipinhole apertures was determined for a specific tissue model. In addition to comparing aperture configurations, the effects of tissue nonradiative absorption, scattering anisotropy, and fluorophore concentration on lateral and axial performance of confocal systems were studied.

Simple spatial phase modulator for focal modulation microscopy

Focal modulation microscopy (FMM) is an emerging microscopy technique for fluorescence imaging of thick biological tissue in vivo. A spatial phase modulator is a critical component whose characteristics have a significant impact on the performance of a FMM system. We have designed a simple spatial phase modulator based on a tilting glass plate that provides superb modulation stability. Image quality has been improved remarkably after integrating such a modulator into a FMM system.

Refinement of optical imaging spectroscopy algorithms using concurrent BOLD and CBV fMRI

We describe the use of the three dimensional characteristics of the functional magnetic resonance imaging (fMRI) blood oxygenation level dependent (BOLD) and cerebral blood volume (CBV) MRI signal changes to refine a two dimensional optical imaging spectroscopy (OIS) algorithm. The cortical depth profiles of the BOLD and CBV changes following neural activation were used to parameterise a 5-layer heterogeneous tissue model used in the Monte Carlo simulations (MCS) of light transport through tissue in the OIS analysis algorithm. To transform the fMRI BOLD and CBV measurements into deoxy-haemoglobin (Hbr) profiles we inverted an MCS of extra-vascular MR signal attenuation under the assumption that the extra-/intravascular ratio is 2:1 at a magnetic field strength of 3 T. The significant improvement in the quantitative accuracy of haemodynamic measurements using the new heterogeneous tissue model over the original homogeneous tissue model OIS algorithm was demonstrated on new concurrent OIS and fMRI data covering a range of stimulus durations.

Efficient rejection of scattered light enables deep optical sectioning in turbid media with low-numerical-aperture optics in a dual-axis confocal architecture

Miniature endoscopic microscopes, with subcellular imaging capabilities, will enable in vivo detection of molecularly-targeted fluorescent probes for early disease detection. To optimize a dual-axis confocal microscope (DACM) design for this purpose, we use a tabletop instrument to determine the ability of this technology to perform optical sectioning deep within tissue. First, we determine how tissue scattering deteriorates the diffraction-limited transverse and vertical responses in reflectance imaging. Specifically, the vertical response of a DACM to a plane reflector is measured at various depths in a scattering phantom and compared with diffraction theory and Monte Carlo scattering simulations. Similarly, transverse line scans across a knife-edge target are performed at various depths in a scattering phantom. Second, as a practical demonstration of deep-tissue fluorescence microscopy that corroborates the findings from our scattering experiments, 3-D fluorescence images are obtained in thick human gastrointestinal mucosal specimens. Our results demonstrate efficient rejection of scattered light in a DACM, which enables deep optical sectioning in tissue with subcellular resolution that can distinguish between normal and premalignant pathologies.

Applications of ultrafast lasers for optical measurements in combusting flows

Optical measurement techniques are powerful tools for the detailed study of combustion chemistry and physics. Although traditional combustion diagnostics based on continuous-wave and nanosecond-pulsed lasers continue to dominate fundamental combustion studies and applications in reacting flows, revolutionary advances in the science and engineering of ultrafast (picosecond- and femtosecond-pulsed) lasers are driving the enhancement of existing diagnostic techniques and enabling the development of new measurement approaches. The ultrashort pulses afforded by these new laser systems provide unprecedented temporal resolution for studies of chemical kinetics and dynamics, freedom from collisional-quenching effects, and tremendous peak powers for broad spectral coverage and nonlinear signal generation. The high pulse-repetition rates of ultrafast oscillators and amplifiers allow previously unachievable data-acquisition bandwidths for the study of turbulence and combustion instabilities. We review applications of ultrafast lasers for optical measurements in combusting flows and sprays, emphasizing recent achievements and future opportunities.

Simulation of a theta line-scanning confocal microscope

We describe a 2-D computational model of the optical propagation of coherent light from a laser diode within human skin to better understand the performance of a confocal reflectance theta microscope. The simulation uses finite-difference time domain (FDTD) computations to solve Maxwell's equations in a synthetic skin model that includes melanin, mitochondria, and nuclei. The theta line-scanning confocal microscope configuration experiences more localized decreases in the signal than the confocal common-path point-scanning microscope. We hypothesize that these decreases result from the bistatic imaging configuration, the imaging geometry, and the inhomogeneity of the index of refraction of the skin. All these factors result in the source path having aberrations different than those of the receiver path. The model predicts signal decreases that are somewhat greater than those seen in experiments. New details on the reflection from a spherical object show that imaging with the theta line scanner leads to somewhat different results than would be seen with a common-path point scanner. The model is used to optimize the design of the theta line-scanning confocal microscope.

Comparative study of optical sources in the near infrared for optical coherence tomography applications

Optical coherence tomography (OCT) is a powerful, noninvasive biomedical technique that uses low-coherence light sources to obtain in-depth scans of biological tissues. We report results obtained with three different sources emitting at 1570, 1330, and 810 nm, respectively. Attenuation and backscattering measurements are obtained with these sources for several in vitro biological tissues. From these measurements, we use a graphical method to make comparisons of the penetration depth and backscattering intensity of each wavelength for the studied samples. The influence of the coherence length of each source is also taken into account in order to make a more relevant comparison.

Confocal fluorescence polarization microscopy in turbid media: effects of scattering-induced depolarization

We present an experimental and theoretical study of confocal fluorescence polarization microscopy in turbid media. We have performed an experimental study using a fluorophore-embedded polymer rod immersed in aqueous suspensions of 0.1 and 0.5 microm diameter polystyrene microspheres. A Monte Carlo approach to simulate confocal fluorescence polarization imaging in scattering media is also presented. It incorporates a detailed model of polarized fluorescence generation that includes sampling of elliptical polarization, excited-state molecular rotational Brownian motion, and dipole fluorescence emission. Using both approaches, we determine the effects of the number of scattering events, target depth, photon scattering statistics, objective numerical aperture, and pinhole size on confocal anisotropy imaging. From this detailed analysis and comparison of experiment with simulation, we determine that fluorescence polarization is maintained to depths at which meaningful intensity images can be acquired.

Angular distribution of polarized photon-pairs in a scattering medium with a Zeeman laser scanning confocal microscope

A novel confocal microscope designed for use with turbid media is proposed. We use a Zeeman laser as the light source. Based on the properties of two-frequency polarized photon-pairs and the common-path feature of polarized photon-pairs with heterodyne detection employed in the proposed confocal microscope, three gatings (spatial filtering gating, polarization gating and spatial coherence gating) are thus simultaneously incorporated in the microscope. Experimental results for the angular distribution of polarized photon-pairs in a scattering medium indicate that polarization gating and spatial coherence gating preclude the detection of multiply scattered photons, whereas the pinhole selects the least scattered photon-pairs. Thus, better performance for axial resolution than can be obtained with a conventional confocal microscope is demonstrated experimentally. In addition, the proposed microscope is able to either look deeper into a turbid medium or work with a denser medium; furthermore, the axial resolution is improved.

Fluorescence spectroscopic analysis of surface and subsurface residual stress fields in alumina hip joints

We aim to establish a confocal spectroscopic technique able to study the features of fluorescence spectra arising from native Cr3+ impurity in polycrystalline alumina (Al2O3) as a biomaterial and to use their emission lines as microscopic probes for the characterization of residual stress fields stored in artificial hip prostheses during their implantation in vivo. As an application of the technique, we report for the first time concerning the evolution of microscopic (residual) stress fields stored on the surface and in the subsurface of N=7 retrieved Al2O3 hip joints after exposure in the human body from a few months to 19 yr. The micrometric diameter of the laser beam waist impinging on the joint surface (typically about 1 microm in lateral resolution) enables us to estimate the patterns and magnitude of residual stress with high spatial resolution, at least comparable with the grain size of the material. In addition, a selected confocal configuration for the optical probe enables minimization of the probe size along the in-depth direction. According to a statistical collection of data on the microscopic level for retrieved femoral heads in toto, a residual stress field arising from loading history in vivo during the lifetime of the Al2O3 femoral head can be revealed. Finally, an interpretation is given of microscopic wear mechanisms in Al2O3 artificial hip joints consistent with the observed evolution of surface residual stress fields on elapsed time in vivo.

Polarimetric imaging of subsurface tissue-numerical results

Tissue images obtained at deeper depths lack significant contrast. To enhance the contrast of these images and to increase the visibility of subsurface tissues, a method is proposed. This technique is based on the principle that photons at longer wavelengths penetrate deeper than photons at shorter wavelengths. In this technique, images in the original and orthogonal polarized states are recorded with the source illumination in linear polarized state at two different wavelengths. Image subtraction of a fraction of the copolarized image from the perpendicular polarized state is done on these images. The images obtained after the first processing are subjected to the next image subtraction where the shorter wavelength image is subtracted from the longer wavelength image. Monte Carlo simulations show that the resultant image has marked contrast up to 2.5 cm.

Sources of scattering in cervical tissue: determination of the scattering coefficient by confocal microscopy

Most models of light propagation through tissue assume that the scattering properties of various tissue layers are the same. We present evidence that the scattering coefficient of cervical epithelium varies by a factor of 3 within the epithelium owing to variations in nuclear density and to the presence of keratin. We estimated the scattering coefficient from regions of normal and precancerous cervical epithelium by fitting reflectance measurements from confocal images to an exponential function of depth based on Beer's law of attenuation. The results suggest that the normal cervix is characterized by highly variable scattering in the superficial epithelium, low scattering in the intermediate epithelium, and high scattering in the basal and stromal regions. In high-grade dysplasia, high scattering from high-density nuclei is observed throughout the entire epithelium.

Advanced modelling of optical coherence tomography systems

Analytical and numerical models for describing and understanding the light propagation in samples imaged by optical coherence tomography (OCT) systems are presented. An analytical model for calculating the OCT signal based on the extended Huygens-Fresnel principle valid both for the single and multiple scattering regimes is reviewed. An advanced Monte Carlo model for calculating the OCT signal is also reviewed, and the validity of this model is shown through a mathematical proof based on the extended Huygens-Fresnel principle. Moreover, for the first time the model is verified experimentally. From the analytical model, an algorithm for enhancing OCT images is developed: the so-called true-reflection algorithm in which the OCT signal may be corrected for the attenuation caused by scattering. For the first time, the algorithm is demonstrated by using the Monte Carlo model as a numerical tissue phantom. Such algorithm holds promise for improving OCT imagery and to extend the possibility for functional imaging.

Statistics of partially coherent beams: a numerical analysis

We show that by using Monte Carlo simulations, one can study the characteristics of beams with adjustable spatial coherence properties that propagate through highly scattering media. Moreover, we show that a single simulation is sufficient to obtain the intensity distribution at the exit surface of the scattering medium for any degree of global coherence of the input beam. The efficient numerical procedure correctly reproduces the first- and second-order statistics of the intensity distribution obtained after propagation through diffusive media.

Confocal fluorescence spectroscopy of subcutaneous cartilage expressing green fluorescent protein versus cutaneous collagen autofluorescence

Optically monitoring the expression of green fluorescent protein (GFP) in the cartilage underlying the skin of a mouse allows tracking the expression of the chondrocyte phenotype. This paper considers how confocal microscopy with spectral detection can sense GFP fluorescence in the cartilage despite light scattering and collagen autofluorescence from the overlying skin. An in vivo experiment tested the abilities of a topical optical fiber measurement and a confocal microscope measurement to detect GFP in cartilage under the skin versus the collagen autofluorescence. An ex vivo experiment tested the ability of a confocal microscope without and with its pinhole to detect a fluorescent microsphere underneath an ex vivo skin layer versus the collagen autofluorescence. In both systems, spectroscopic detection followed by linear analysis allowed spectral discrimination of collagen autofluorescence (M(C)) and the subdermal green fluorescence (M(G)) due to either GFP or the microsphere. Contrast was defined as M(G)/(M(G)+M(C)). The in vivo contrast for GFP using optical fiber and confocal measurements was 0.16 and 0.92, respectively. The ex vivo contrast for a fluorescent microsphere using a confocal system without and with a pinhole was 0.13 and 0.48, respectively. The study demonstrates that a topical optical fiber measurement is affected by collagen autofluorescence, while a confocal microscope can detect subdermal fluorescence while rejecting collagen autofluorescence.

Derivation of a Monte Carlo method for modeling heterodyne detection in optical coherence tomography systems

A Monte Carlo (MC) method for modeling optical coherence tomography (OCT) measurements of a diffusely reflecting discontinuity embedded in a scattering medium is presented. For the first time to the authors' knowledge it is shown analytically that the applicability of an MC approach to this optical geometry is firmly justified, because, as we show, in the conjugate image plane the field reflected from the sample is delta-correlated from which it follows that the heterodyne signal is calculated from the intensity distribution only. This is not a trivial result because, in general, the light from the sample will have a finite spatial coherence that cannot be accounted for by MC simulation. To estimate this intensity distribution adequately we have developed a novel method for modeling a focused Gaussian beam in MC simulation. This approach is valid for a softly as well as for a strongly focused beam, and it is shown that in free space the full three-dimensional intensity distribution of a Gaussian beam is obtained. The OCT signal and the intensity distribution in a scattering medium have been obtained for several geometries with the suggested MC method; when this model and a recently published analytical model based on the extended Huygens-Fresnel principle are compared, excellent agreement is found.

Reply to comment on "Excitation with a focused, pulsed optical beam in scattering media: diffraction effects."

We address the issues that were raised by Tycho and Jørgensen [Appl. Opt. 41, 4709 (2002)] concerning our strategy [Appl. Opt. 39, 5244 (2000)] for incorporating the wave properties of light in the description of a propagating focused excitation beam in a highly scattering medium. We explain that the strategy is consistent with the Huygens-Fresnel principle and does not violate the energy conservation principle.

Geometrical tomographic imaging of refractive indices through turbid media by a wavelength-scanning heterodyne interference confocal microscope

A wavelength-scanning heterodyne interference confocal microscope has proved to provide the tomographic image of the refractive indices of transparent and turbid media on the scale of geometrical depth when weakly reflected light with an optical power as low as of the order of 10(-14) W is used. The refractive indices of the transparent object and the turbid media were measured with accuracies of -0.5% and approximately 3%, respectively. This imaging method is advantageous for evaluating quantitative refractive indices and internal structures.

Zeeman laser-scanning confocal microscopy in turbid media

A novel Zeeman laser-scanning confocal microscope (ZLSCM) is proposed. It has the same configuration as the conventional laser-scanning confocal microscope (LSCM) in which a Zeeman laser in conjunction with a Glan-Thompson analyzer is used. In our system, the analyzer with the bandpass filter, which act simultaneously as a polarization gate and a coherence gate, enhance the collection efficiency of the weak-scattering photons and simultaneously suppress the multiple-scattering photons. The improvement in depth resolution of a ZLSCM in a scattering medium compared with that of a conventional LSCM is discussed and demonstrated experimentally.

Excitation with a focused, pulsed optical beam in scattering media: diffraction effects

To gain a better understanding of the spatiotemporal problems that are encountered in two-photon excitation fluorescence imaging through highly scattering media, we investigate how diffraction affects the three-dimensional intensity distribution of a focused, pulsed optical beam propagating inside a scattering medium. In practice, the full potential of the two-photon excitation fluorescence imaging is unrealized at long scattering depths, owing to the unwanted temporal and spatial broadening of the femtosecond excitation light pulse that reduces the energy density at the geometric focus while it increases the excitation energy density in the out-of-focus regions. To analyze the excitation intensity distribution, we modify the Monte Carlo-based photon-transport model to a semi-quantum-mechanical representation that combines the wave properties of light with the particle behavior of the propagating photons. In our model the propagating photon is represented by a plane wave with its propagation direction in the scattering medium determined by the Monte Carlo technique. The intensity distribution in the focal region is given by the square of the linear superposition of the various plane waves that arrive at different incident angles and optical path lengths. In the absence of scattering, the propagation model yields the intensity distribution that is predicted by the Huygens-Fresnel principle. We quantify the decrease of the energy density delivered at the geometric focus as a function of the optical depth to the mean-free-path ratio that yields the average number of scattering events that a photon encounters as it propagates toward the focus. Both isotropic and anisotropic scattering media are considered. Three values for the numerical aperture (NA) of the focusing lens are considered: NA = 0.25, 0.5, 0.75.