Optical coherence tomography (OCT) is an emerging tool for real time in-situ tissue imaging with micrometer-scale resolution. Real-time OCT systems have been integrated into clinical medical diagnostics, and functional extensions such as polarization-sensitive, Doppler, and spectroscopic OCT have recently been introduced. However, OCT imaging technology has not yet taken advantage of molecular-specific contrast mechanisms which have revolutionized other medical and biological imaging modalities such as MRI, PET, SPECT, and fluorescence microscopy. OCT is by definition insensitive to incoherent scattering processes such fluorescence or spontaneous Raman scattering. We have previously proposed the use of spectroscopic OCT for detection of light from coherent inelastic scattering processes such as stimulated emission, stimulate Raman scattering, and other four-wave mixing processes. In this report we present a new technique which we call pump probe OCT (PPOCT), which is a novel extension of OCT for enhancing the contrast of OCT images based on transient absorption in the sample induced by an external pump beam. We show preliminary data for PPOCT using methylene blue as a molecular contrast agent and show depth resolved PPOCT M-scans that localize the presence of dye in a well phantom.

Ultrahigh resolution OCT imaging is demonstrated using compact broadband light sources based on a commercially available Nd:Glass femtosecond laser with nonlinear fiber continuum generation. A tapered single mode fiber is used to generate broadband light centered at 1300 nm. Broadband light near 1064 nm can also be generated using a high numerical aperture single mode germanium doped fiber. These light sources enable ultrahigh resolution OCT imaging with 5-6 μm axial resolution at both 1064 nm and 1300 nm.

We present our progress in developing a novel technique and instrument that images specific molecular species in biological tissues using Optical Coherence Tomography (OCT). Standard OCT instruments measure only the scattering from structural features, such as refractive index changes. We utilize Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy, a nonlinear optics technique that can selectively stimulate molecular groups, to gather compositional information from the sample. Being a coherent process, our instrument will produce interference between the nonlinear anti-Stokes signal produced in the sample and a reference molecular sample to both exclude background and nonresonant signals and range features in the tissue. Because of this, we will also gain the benefits of sensitivity that interferometry can provide. By utilizing the tunability of an optical parametric oscillator, we can address a range of molecular resonances from 1500 cm^-1 to 3500 cm^-1. This frequency range offers the possibility of measuring the distributions and densities of proteins, lipids, and nuclear material that we believe will be useful for determining the early presence of epithelial carcinomas. We demonstrate the principle of this imaging method by producing interference between two separately produced CARS signals from the same probe and Stokes beams.

We report on ultrahigh-resolution Optical Coherence Tomography (OCT) using white-light interference microscopy. The experimental setup is based on a Linnik interferometer illuminated by a tungsten halogen lamp. Tomographic images in the en face orientation are calculated by combination of images recorded by a silicon CCD camera. Axial resolution of 0.8 μm is achieved due to the short coherence length of the source and the compensation of dispersion mismatch in the interferometer arms. Transverse resolution of 1.6 μm is obtained by using relatively high numerical aperture microscope objectives. A nearly shot-noise limited detection sensitivity of 90 dB is achieved with 4s acquisition time. Images of the Xenopus Laevis tadpole are presented.

Optical coherence tomography (OCT) measures and images the backscattering potential of transparent and semi-transparent samples. Recently, we reported on an extension of OCT with a differential phase contrast (DPC) technique. We have improved the technique and developed a DPC optical coherence microscope (OCM) which allows to image sub-wavelength optical path differences occurring between a narrow beam probing a sample and its surrounding. This allows the visualization of small transversal refractive index variations close to a selected interface. We report on the method and present first images of a single cell layer. The cells act as phase objects, and by imaging the phase properties the contrast is improved as compared to intensity images.

We present a characterization of high-speed wide-field coherence-gated imaging using photorefractive holography with GaAs/AlGaAs Photorefractive Multiple Quantum Well (PRWQ) devices. Results are obtained with broadband c.w. laser illumination. The limitations of photorefractive holography using PRQW devices in our non-degenerate four-wave mixing geometry are discussed in terms of typical PRQW device parameters. We discuss the effect of the sensitivity and dynamic range of the CCD camera used to record the diffracted image and how the performance of PRQW devices may be improved in the future. We also present a spatial multiplexing technique for achieving phase-stepped single-shot wide-field coherence-gated imaging using a single CCD camera.

Holographic optical coherence imaging (OCI) has been used to acquire depth resolved images in tumor spheroids. OCI is a coherence-domain imaging technique that uses dynamic holography as the coherence gate. The technique is full-frame (en face) and background free, allowing real-time acquisition to a digital camera without motional reconstruction artifacts. We describe the method of operation of the holographic OCI on highly scattering specimens of tumor spheroids. Because of the sub-resolution structure in the sample, the holograms consist primarily of speckle fields. We present two kinds of volumetric data acquisition. One is uses fly-throughs with a stepping reference delay. Another is static holograms at a fixed reference delay with the coherence gate inside the tumor spheroids. At a fixed reference delay, the holograms consist of time-dependent speckle patterns. The method can be used to study cell motility inside tumor spheroids when metabolic or cross-linking poisons are delivered to the specimens.

Optical coherence tomography (OCT) is a potentially useful 'optical biopsy' tool in medicine. To realize this potential doctors must be able to interpret OCT images with at least the same accuracy as conventional histology. It is therefore important that some accurate method of comparing OCT images with conventional histology be found. Despite numerous OCT vs. conventional histology studies in the literature the methods used for comparison have, by necessity, been approximate because it is not possible to cut a physical tissue section in the same plane as the OCT optical section (partly due to histology processing artefacts). In this paper we present a method of rendering solid tissue volumes with semi-transparency using the Virtual Reality Modelling Language (VRML) and devise a VRML script which allows any two volume data sets to be manipulated within the same region of virtual space. This allows the structure of a whole volume of tissue imaged with OCT to be directly compared with the serial section reconstruction of the conventionally stained histology. As the whole volume is visualized any corresponding tissue regions can be more easily identified despite tissue processing artefacts. We demonstrate the effectiveness of our method usng an ex vivo biopsy of human breast carcinoma.

An improved spectral Optical Coherence Tomography (OCT) technique was used to perform cross sectional ophthalmic images at the exposure time of 64 μs per A-scan. To achieve exposure times less than 1ms the fast kinetics mode of the CCD camera was used. Static and dynamic real-time in vivo images of the human macula, optic disc and iris are presented.

An ultrahigh resolution ophthalmic optical coherence tomography (OCT) system has been developed. Using a femtosecond Ti:sapphire laser light source, which generates bandwidths of ~150 nm at 800 nm, real-time, cross-sectional imaging of the retina with ~3 μm axial resolution is possible. Ultrahigh resolution OCT images of retinal morphology were obtained in normal subjects and patients with retinal disease. Intraretinal architectural morphology associated with macular diseases such as macular edema, epiretinal membranes, and macular holes can be visualized with unprecedented resolution. Ultrahigh resolution ophthalmic OCT promises to improve the early diagnosis of retinal diseases as well as enable monitoring of disease progression and the efficacy of therapeutic intervention.

This paper demonstrates the clinical application of a multiplanar imaging system, which simultaneously acquires en-face (C-scan) OCT and corresponding confocal ophthalmoscopic images along with cross-sectional (B-scan) OCT at cursor designated locations on the confocal image. Advantages of the simultaneous OCT/confocal acquisition as well as the challenges of interpreting the C-scan OCT images are discussed. Variations in tissue inclination with respect to th coherence wave surface alters the sampling of structures within the depth in the retina, producing novel slice orientations which are often challenging to interpret. We evaluate for the first time the utility of C-scan OCT for a variety of pathologies including exudative ARMD, macular hole, central serous retinopathy, diabetic retinopathy, polypoidal choroidal vasculopathy and macular pucker. Several remarkable observations of new aspects of clinical anatomy were noted. The versatility of selective capture of C-scan OCT images and B-scan OCT images at precise points on the confocal image affords the clinician a more complete and interactive tool for 3D imaging of retinal pathology.

An en face coherence gated camera equipped with adaptive optics (AO) has been constructed for imaging single cells in the living human retina. The high axial resolution of coherence gating combined with the high transverse resolution of AO provides a powerful imaging tool whose image quality can surpass either methodology performing alone. The AO system relies on a 37-actuator Xinetics mirror and a Shack-Hartmann wavefront sensor that executes up to 22 corrections per second. The coherence gate is realized with a free-space Michelson interferometer that employs a scientific-grade 12-bit CCD array for recording 2-D retinal interferograms. Images were collected of microstructures the size of single cells in the in vivo retina. Early results suggest that a coherence gated adaptive optics camera should substantially improve our ability to detect single cells in the retina over the current state-of-the-art AO retina cameras, including conventional flood illuminated and confocal scanning laser ophthalmoscopes. To our knowledge, this is the first effort to combine coherence gating and adaptive optics.

We present in vivo depth-resolved birefringence measurements of the human retinal nerve fiber layer (RNFL) by use of polarization-sensitive optical coherence tomography (PS-OCT). Because glaucoma causes nerve fiber layer damage, which may cause loss of retinal birefringence, PS-OCT is a potentially useful technique for the early detection of glaucoma. We built a fiber-based PS-OCT setup that produces real-time images of the human retina in vivo, co-registered with video images of the location of PS-OCT scans on the retina. Preliminary measurements of a healthy volunteer showed that the double-pass phase retardation per unit depth of the RNFL varies with location with values in between 0.18 and 0.37°/μm. A trend in the preliminary measurements shows that thicker nerve fiber layer tissue is more birefringent than thin nerve fiber layer tissue.

We demonstrate real time, ultrahigh resolution OCT imaging using a portable mode-locked Cr:forsterite laser. OCT imaging at 5.5 um axial resolution was performed of normal and cancerous human prostate tissue and correlated with histology.

A new approach to improving the diagnostic value of optical methods is suggested, which is based on a complementary investigation of different optical parameters of biotissues. The aim of this paper is comparative study of the feasibility of two optical methods - fluorescence spectroscopy and optical coherence tomography - for visualization of borders of neoplastic processes in the uterine cervix and vulva. Fluorescence spectroscopy is based on the detection of biochemical and optical coherence tomography on backscattering properties in norm and pathological changes of tissues. By means of these optical methods changes in biochemical and morphological properties of tissues were investigated. A parallel analysis of these two optical methods and histology from the center of tumors and their optical borders was made. Thirteen female patients with neoplastic changes in uterine cervix and vulva were enrolled in this study. The borders of the tumor determined by optical methods (fluorescence spectroscopy and optical coherence tomography) are coinciding with the biopsy proved ones. In addition, OCT and fluorescence borders of tumor in the uterine cervix and vulva exceeds colposcopically detectable borders, the averaging difference 2 mm. In future optical methods would considerably enhance diagnostic accuracy of conventional methods used in oncogynecology.

In vivo endoscopic optical coherence tomography (OCT) was performed in 154 patients with pathologic lesions, suspicious for high-grade dysplasia, intramucosal or early microinvasive cancer. One group of seven physicians and another of six, familiar with OCT, participated in the blinded recognition of benign and neoplastic conditions in different types of mucous membranes. The result for the OCT sensitivity for malignancy detection in these types of mucosa is 83 - 98%, specificity 71 - 91%. The accuracy was 81 - 87%. Recognition error rate is smaller for high-grade dysplasia and invasive cancer in urinary bladder (3.3% - 1.5%), and higher for the uterine cervix (23% - 11%) and for larynx (45.7% - 3.4%). The kappa coefficient for interobserver agreement was 0.65-0.83.

Laryngeal cancer is the most common primary head and neck malignancy and the need for early identification is very important for early treatment. Outpatient fiberoptic examination of the larynx is often unreliable in differentiating between benign, pre-malignant and malignant lesions, and therefore surgeons often have to rely on biopsies for a definite diagnosis. This is an invasive procedure requiring general anaesthesia and may have a detrimental effect on patient’s voice. The aim of our study was to investigate the feasibility of optical coherence tomography in imaging of the larynx, which will lay the foundations for investigating its ability to differentiate between benign and malignant disease. Tissue specimens from normal larynges were imaged with a polarisation sensitive OCT system at 850 nm and a second OCT system at 1300 nm. Both OCT systems were capable of providing both B-scan (longitudinal OCT) images as well as C-scan (en-face OCT or at constant depth) images. Imaged specimens were processed with standard histopathological techniques and sectioned in the plane of the B-scan OCT images. Haematoxylin-Eosin stained specimens were compared to the OCT images. Preliminary results showed good correlation between OCT images and histology sections in normal tissues.

There has been a long discussion if Fourier Domain Optical Coherence Tomography (FDOCT) is competitive to nowadays standard time domain systems. Still the main points were autocorrelation terms that obscured the object information and degraded the sensitivity and signal to noise ratio. By exploiting the phase information of the interferograms, it was possible to remove those autocorrelation terms and to double the measurement range. However standard phase retrieval algorithms need three to five interferograms. We present a novel technique that allows removing the coherent noise terms together with doubling the measurement range with only two recorded interferograms. In addition the measured system sensitivity of 94dB shows that the performance of FDOCT is comparable to that of time domain OCT (TDOCT) setups.

Projected index computed tomography (PICT) is a newly developed technique that uses the measured optical path length from multiple optical coherence tomography (OCT) scans through a semitransparent sample to computationally reconstruct tomographic images based on the spatial variations of the refractive index. Since the index of refraction of most samples is not constant, a depth-wise distortion due to the varying index of the sample is evident. Using a highly reflective reference surface placed behind the sample, optical path length measurements yield an aggregate index value for each beampath through the sample. Rotating the sample allows a data set to be formed for multiple beam angles. These data can be understood as the projections of the object index, i.e. the Radon tranform of the index of the object. Using filtered backprojection algorithms set of projection data were used to reconstruct PICT images. The resulting images are free from the spatial distortions found in standard OCT. Experimental results show that PICT images correspond well with the dimensional characteristics of specific samples.

The longitudinal resolution of an optical coherence tomography (OCT) system is conventionally defined as the full-width at half maximum (FWHM) of the interference fringe envelope, which depends on the center wavelength as well as the spectral width of the light source. One can obtain an FWHM of an interference fringe envelope larger or smaller than that resulted from a Gaussian spectrum of the same spectral FWHM when the light source spectrum is non-Gaussian distributed. In this paper, we first study the dependencies of OCT resolution on the spectral shape and dispersion mismatch with numerical simulations. We will demonstrate the capability of enhancing the longitudinal resolution of an OCT system with a proper control of spectral distribution and dispersion mismatch. Then, in experiments we built an OCT system with its light source generated from nonlinear optics effects of 12-fsec Ti:sapphire laser pulses in an optical fiber. With proper control of dispersion mismatch between the sample and reference arms, the FWHM of the interference fringe envelope was smaller than that of a Gaussian spectrum with the same spectral FWHM by a factor of two. Furthermore, the side lobes were suppressed with a process algorithm to significantly improve the longitudinal resolution.

We demonstrate phase space tomography for the measurement of the transversal spatial coherence function of light after propagation through a scattering medium. The results of this approach are compared to measurements performed with shearing-interferometry. Implications for parallel Optical Coherence Tomography will be briefly discussed.

An endoscopic system that provides simultaneous cross-sectional imaging and fluorescence spectroscopy is described. The first application of this device was the investigation of mouse colon cancer in vivo. This system combined optical coherence tomography (OCT), which provided high-resolution cross-sectional structural information in the form of a two-dimensional image, and laser induced fluorescence (LIF), which yielded histochemical information about the tissue. The design challenge and solution of packaging these two systems with widely different optical requirements are described in detail. The illumination geometry of the endoscope was similar to earlier published OCT and LIF catheter endoscope designs. However, several unique design challenges encountered in combining these two systems have been addressed. The use of a rodprism to reduce the asymmetry in the OCT beam caused by a cylindrical window is presented. Materials selection for use with wavelengths from 325nm - 1310nm presented a challenge usually avoided in OCT endoscopes. Preliminary mouse colon data collected with this endoscopic device is compared with previous experiments performed by researchers in our lab working with an earlier bulk-optic, combined OCT-LIF system.

This paper describes a compact micromirror device for use in the scanning arm of an optical coherence tomography (OCT) system using an electrostatic micromachine (MEMS) actuator. Optical deflections of these MEMS mirror devices range from 18 degrees at low frequencies to more than 140 degrees near the resonant frequencies of the structures (30-60 Hz). These devices consist of gold plated silicon mirrors resting on polyimide tables that tilt on 3 μm thick torsion hinges when pulled on by the micromachine (MEMS) actuator, the integrated force array (IFA). The IFA is a thin (2.2 μm) polyimide membrane consisting of hundreds of thousands of micron scale deformable capacitors, and contracts with strains up to 20% and forces up to 13 dynes. The support structures, hinges, and actuators are fabricated from polyimide on silicon wafers using photolithography, leading to the possibility for integrated fabrication of the devices resulting in highly repeatable and inexpensive scanning arms. These devices were inserted into the scanning arm of a high speed OCT imaging system to acquire in vitro images of porcine eye and colon tissue and in vivo images of human skin at frame rates from 4-8 Hz.

We developed an ultrahigh resolution optical coherence tomographic system utilizing broadband continuum generation from a photonic crystal fiber for high axial resolution. Longitudinal resolution of 1.3 μm has been achieved in a biological tissue by use of continuum light from 800 - 1400 nm as the light source. The system employed a dynamic focusing tracking method to maintain high lateral resolution over a large imaging depth. Subcellular imging is demonstrated.

We proposed a technique to improve OCT resolution using LPG and EDF. The proposed technique improves the resolution of OCT by resphaping the spectrum of ASE source into Gaussian-like form. The ASE source has a strong characteristic peak due to erbium that is used for doping material in EDF. To reduce the peak-induced artifact to OCT image that may degrade image quality and distort inner structure information, LPG is used as a strong ASE peak rejection filter and EDF does a function of absorber when pumping power is not applied. Both of them can do spectral tailoring function properly so that modified source shape and sidelobe suppression can be performed. With the LPG-assisted reshaping, we have enhanced the spatial resolution up to 5 times (approximately 200 μm resolution was reduced to about 40 μm). With EDF absorber, we can obtain reshaped ASE source hence resolution was enhanced from 25 μm to less than 20 μm). The spatial resolution can be further enhanced by using cascaded LPGs or control of EDF parameters such as doping material, doping concentration, and the length of used EDF.

The idea of a new low coherence interferometric system with axial resolution better than coherence length providing simultaneous measurement of the geometrical thickness and refractive index of transparent layers by sharp focusing of light on the measured object is considered. The presented interferometric system consists of two parts -- the so-called Wave Front Matching Interferometer (WFMI) and a low coherence Michelson interferometer (LCMI) as a light source for the first. The WFMI provides high separation of interference signal peaks from demarcations in layer object at high numerical aperture focusing of light on the object. The tandem optical scheme of these interferometers allows to make this system very compact and mobile.

The method of scattering media probing with the use of low-coherent light source with the controllable width of emission spectrum is considered. The contrast of partially coherent speckles is suggested as the diagnostical parameter. The additional polarization discrimination of detected speckles gives the possibility to select the components of scattered field which propagate in probed medium at different distances. Experimental results obtained for weakly ordered systems characterized by non-diffuse scattering regimes are presented.

We demonstrate real-time acquisition, processing, and display of tissue structure, birefringence, and blood flow in a multi-functional optical coherence tomography (MF-OCT) system. This is accomplished by efficient data processing of the phase-resolved inteference patterns without dedicated hardware or extensive modification to the high-speed fiber-based OCT system. The system acquires images of 2048 depth scans per second, covering an area of 5 mm in width x 1.2 mm in depth with real-time display updating images in a rolling manner 32 times each second.

Clinical application of ODT requires real-time data acquisition and signal processing. In this paper, we present a real-time signal processing ODT unit based on a custom designed digital signal processor (DSP) module. The DSP is incorporated into a conventional ODT system using a grating-based scanning optical delay line. The newly developed flow velocity algorithms are integrated into the DSP and real-time data processing can be readily achieved.

We demonstrate that spectroscopic optical coherence tomography can be used for measurement of diffusion. We measured the diffusion coefficient of a Phthalocyanine dye in an Agar gel as a first model of dye diffusing into tissue. We used a two-wavelength interferometer, with one of the wavelengths matched to the absorption peak of the dye at 675nm while the other wavelength at 805nm is not affected by the dye. The diffusion constant of the dye in Agar gel is found by fitting the measured OCT amplitude (depth and time dependent) to a mathematical model for the OCT signal. This method may be used as a tool for dosimetry in in Photo Dynamic Therapy (PDT). In PDT the therapeutic light exposure should be applied at a time when the concentration of sensitizer is optimal in the diseased tissue relative to normal tissue. By studying how the OCT signal changes with time and depth at two wavelengths differently affected by the diffusing dye, it should be possible to extract parameters determining diffusion of the sensitizer in live tissue. In comparison with fluorescence-based methods, this OCT approach has the advantage of better depth penetration and being able to account for attenuation effects due to scattering.

We studied the axial point spread function of OCT for Gaussian intensity profiles emitted from and coupled back into single mode fibers for signals from a scattering medium. The determined Rayleigh length of the axial point spread function was roughly twice the one measured from the reflection of a mirror. Using the measured point spread function in combination with the single backscatter model allowed determination of the attenuation coefficient of the suspension.

Doppler optical coherence tomography (DOCT) allows simultaneous micrometer-scale resolution cross-sectional imaging of tissue structure and blood flow. Here we demonstrate a fiber-optic, polarization diversity-based differential phase contrast DOCT system as a method to perform self-referenced velocimetry in highly scattering media. Using this strategy we reduced common-mode interferometer noise to <1Hz and improved Doppler estimates in a scattering flow phantom by a factor of 5.

We describe power optical Doppler tomography (ODT) imaging in phase-resolved optical coherence tomography (OCT) capable of providing the precise location of blood flow in human skin. The power Doppler signal is the squared amplitude of the Doppler signal. By properly setting the intensity threshold and priority displaying Doppler power in phase-resolved OCT, we obtained a Doppler power tomography image of the blood flow in human skin. Power Doppler tomography uses only the amplitude information, so it is not susceptible to aliasing and Doppler flow angle and provides more accurate and smooth imaging of the location of the blood vessels in human skin than Doppler velocimetry. We also modified the phase-resolved algorithm we published before and used it to do Doppler tomography and M-mode Doppler imaging. The dynamic of blood flow in chick chorioallantoic membrane (CAM) was studied using M-mode Doppler imaging.

Accurate flow velocity estimation requires measurement of the Doppler angle, which is not available in general clinical applications. We introduce a simple method of estimating Doppler angle and average flow velocity by using conventional single-probing-beam optical Doppler tomography. Both Doppler angle and flow velocity are estimated by combining Doppler shift and Doppler bandwidth measurements. Using two sets of intralipid experiments corresponding to fixed Doppler angle and fixed flow speed, we demonstrate that the estimated values of the Doppler angle and flow velocity are in good agreement with true values. The principle is further validated by in vivo measurements.

We propose and present TDOCT (Transversal Doppler Optical Coherence Tomography), a novel method for measurement of particle flow velocity and direction. A quadrant detector can be employed to enable separate detection of light scattered in four different angles. Transversal flow will induce different Doppler shifts in the four interferometer signals, and suitable processing of the spectra or phase changes can be used to determine the amplitude and direction of the transversal flow component.

Monitoring the spatio-temporal characteristics of blood flow (BF) is crucial for physiological studies. At present, most optical techniques used for monitoring the BF utilize either the Doppler effect or the temporal statistics of time-varying speckle to measure the blood velocity at a point. If a map of blood velocity distribution is required, some form of scanning must be introduced, thus limiting the temporal and spatial resolution. Laser speckle imaging (LSI) technique could provide real-time spatially resolved BF images without the need for scanning by utilizing the spatial statistics of time-integrated speckle. In present paper, the regional blood flows in the rat mesentery under the effect of phentolamine with incremental concentration were monitored using LSI method. Our results showed that for arterioles, the vessels expanded and BF increased under the treatment with phentolamine of 1μg/ml. However, as the concentration increased, the BF decreased and dilation only happened at the concentration of 100μg/ml; For venules, no dilation was observed except for the case of 100μg/ml while BF decreased. These suggested that compared with the conventional methods, LSI could obtain the spatio-temporal dynamic of BF in the mesentery with high resolution without scan, providing a new approach in studying the microcirculation in the mesentery.

Optical coherence tomography (OCT) is a powerful new high resolution imaging modality which can provide new insight into normal and abnormal cardiac development in animal models. Here we demonstrate for the first time the application of OCT for three-dimensional imaging of the developing cardiovascular system of the chick embryo. Using this three-dimensional data, we compared cross-sectional OCT images with histological cross-sections and we generated volumetric reconstructions of the early heart tube.

Accurate evaluation of the depth of injury in burn victims is of considerable practical value to the surgeon, both for initial determination of resuscitation fluid requirements, and in deciding whether excision and closure of the wound is necessary. Currently, burn depth is most accurately evaluated by visual inspection, though decisions concerning treatment may not be possible for a number of days post-injury. As part of our ongoing efforts to provide an objective, quantitative method for burn depth determination, we present here the results of a study using polarization-sensitive optical coherence tomography (PS-OCT) to detect and measure thermally induced changes in collagen birefringence in skin excised from burn patients. We find that PS-OCT is capable of imaging and quantifying significantly reduced birefringence in burned human skin.

The index of refraction of hemoglobin solutions with varying oxygen saturation was predicted and measured. An increase in index of refraction with increasing saturation was found, which implies a saturation dependent scattering effect in blood, which exceeds the effect of saturation dependent absorption.

A multi-channel polarization-sensitive Mueller-matrix optical coherence tomography (OCT) was built with single-mode optical fibers in both the sample and reference arms. A new rigorous algorithm was developed to eliminate dynamically the polarization distortions caused by the sampling fiber and consequently retrieve the calibrated polarization properties of the sample. The roundtrip Jones matrix of the sampling fiber used in the algorithm was acquired from the reflecting surface of the sample for each depth scan (A scan). Both this new algorithm and the algorithm used in previous fiber-based polarization-sensitive OCT (PS-OCT) were tested with simulated data, which shows that the only parameter that can be correctly retrieved by the previous algorithm is the phase retardation. The skin of a rat was imaged with this fiber-based system.

OCT has emerged in recent years to a powerful technique to measure tissue properties. Recently, OCT has been enhanced by methods providing spectroscopic information. Since water is a major constituent of most tissues, the measurement of its concentration in tissue is important for tissue diagnostics. We present measurements of water absorption in human cornea in vitro with a differential absorption optical coherence tomography technique. This technique uses two OCT images recorded simultaneously with two different light sources, one centered within (1488nm) and one centered outside (1312nm) of a water absorption band. To study influences of scattering on the absorption images and on the calculated differential absorption coefficient, the cornea was measured and imaged under three different conditions: At first, the cornea was imaged in a hydrated condition, immediately after removal from the aqueous nutrient solution. Then it was dehydrated and imaged a second time, finally it was rehydrated, however, the water was replaced by Deuterium oxide, which shows negligible absorption in the used wavelength region, but otherwise has similar optical properties as water. The cornea containing H₂O is well distinguishable from the cornea containing D₂O with our method. For quantitative determination of absorption, we performed a linear regression analysis of logarithmic backscattered intensity versus imaging depth in the cornea for each wavelength. The difference of the slopes corresponds to the difference in the absorption coefficient. If the difference in the water absorption cross section is known, the water concentration in tissue can be calculated. The results are in good agreement with those expected theoretically.

The knowledge of water content of the cornea (hydration level H) can provide crucial information for the assessment of corneal function. The correlation between the corneal thickness and its hydration enables us to estimate H indirectly by measuring changes in corneal thickness and scattering using OCT. The magnitude and axial distribution of the backscattering signal from the cornea yields additional information about the hydration gradient across the cornea. We present data on the effect of corneal hydration on its thickness and scattering in natural processes of de- and rehydration, as well as in stress tests with the use of glycerol-based dehydrating agent Ophthalgan. Our data demonstrate that scattering signal changes up to 50 times when corneal thickness varies from 60% to 200% of its normal state. The distribution of scattering intensity across the cornea also depends on the hydration level and gradient of the water distribution. Thus, OCT can provide a noninvasive and non-contact method for safe and fast measurement of thickness and optical properties of the cornea, and therefore, for estimation of corneal hydration level and corneal function.

Optical coherence tomography (OCT) and confocal laser scanning microscopy (CLSM) were applied to characterize non-invasively and in vivo the upper layers of human skin on the back of the hand. The techniques enable a detailed determination of the thickness and location of various skin layers in the epidermis and superficial dermis. Due to differences in spatial resolution and penetration depth of these methods, OCT and CLSM give complementary information on the composition and structure of skin. OCT signals of the back of the hand show three reflecting layers at different depth in the skin. A direct comparison with CLSM enables the assignment of these layers: the first one is due to the reflection at the skin surface, the second one appears to be caused by the reflection at the basal epidermal layer and the third layer can be ascribed to reflection at fibrous structure in the upper dermis. A comparison of methods reveals a consistent interpretation of the images.

We report a novel application of optical coherence tomography (OCT), to monitor post-laser irradiation collagen injury in skin model. An artificial skin model (RAFT) which closely approximates human skin, was irradiated with a Perovskite laser (λ = 1341 nm). We investigated dynamic changes in a RAFT after laser irradiation through OCT and compared the results to those of histology. OCT images clearly delineated areas of post-irradiation collagen injury and allowed non-invasive monitoring of the wound healing process. Histology was correlated well with OCT images. OCT has advantages because it is non-invasive and allows serial monitoring at the same site over time. Our study showed that OCT may be a useful tool for determination of optimal parameters for non-ablative laser skin rejuvenation (NALSR) using different devices.

The highly scattering nature of human tissue limits light penetration depth in the near infrared range, which prevents the deeper microstructures from imaging. In order to enhance the imaging depth for the current high resolution optical imaging techniques, the light scattering in tissue must be reduced. This paper demonstrates that the light scattering of tissue can be effectively reduced by the topical applications of the biocompatible chemical agents. In this study the propylene glycol and glucose solutions were chosen for the demonstrations through topical applications and intra-dermis injection, respectively. The experiments were performed in vitro and in vivo by the use of the optical coherence tomography system. The results clearly show that the OCT imaging depth and contrast are dramatically improved after the topical applications of propylene glycol solution. Such improvement was discussed on the basis of refractive index matching environment created by the chemical agents, which effectively reduces the light scattering of tissue. Rayleigh-Gans approximation of light scattering was also used to show theoretically how the increase of refractive index of background medium would have effect on the reduced scattering coefficient of tissue. The theoretical and experimental results were qualitatively consistent.

We describe a phase-resolved polarization sensitive optical coherence tomography system that can obtain the Stokes vectors, polarization diversity intensity, and birefringence images of rat-tail tendon and muscle. The Stokes vectors were obtained by processing the analytical interference fringe signals from two perpendicular polarization-detection channels for the same reference polarization state. From the four Stokes vectors, the birefringence image, which is insensitive to orientation of the optical axis in the sample, and the polarization diversity intensity image, in which speckle noise is greatly reduced, were obtained. The birefringence changes in the rat muscle caused by freezing were investigated using phase-resolved polarization sensitive optical coherence tomography. It was found that freezing degrades birefringence in rat muscle.

Optical coherence tomography (OCT) is a depth-resolved imaging modality that uses low coherence interferometry to gate the time-of-flight of backreflected sample light. The heterodyne frequency of the interferometric signal can be up- or downshifted by reflection off of a moving particle (e.g. red blood cells in an artery). This Doppler shift can be resolved with a time-frequency decomposition (TFD) and displayed as a color Doppler-OCT image. Here we compare the in virto and in vivo performance of three TFDs: the short-time Fourier transform, the Morlet wavelet transform, and the short-time MUSIC transform (STMT). The STMT is a new TFD that incorporates the MUSIC eigenfrequency estimator in a generalized short-time framework. The Morlet transform excels at identifying blood vessels, while the STMT is the most accurate predictor of Doppler shift frequency.

We report on the possibility of using a photorefractive polymer composite device as a coherence gate for optical coherence tomography. Such a system could enable high-resolution 3-dimensional images of tissue samples to be holographically stored, and read out in real-time, or at a subsequent time. A free space system based on the Michelson interferometer is used to illuminate a photorefractive polymer device with two coherent beams. Interference of these beams results in an intensity distribution, which is consequently stored as a refractive index hologram inside the polymer. The image-carrying object beam is then reproduced from the first order diffraction of a probe beam through the device, and is captured on a CCD camera. Devices used are based on a poly(N-vinylcarbazole) (PVK):2,4,7-trinitro-9-fluorenone dimalenitrile (TNFDM) charge transport network with the electro-optic organic chromophore 1-(2'-ethylhexyloxy)-2,5-dimethyl-4-(4"-nitrophenylazo)benzene (EHDNPB). Using a 1% concentration of TNFDM, whole-field 2-dimensional image planes are successfully recorded in a few seconds with a 7mW HeNe laser at 633nm. Readout of the hologram is also performed using the same source.

This paper investigates the correlation between the shape of the first derivative of a blood pressure pulse and the corresponding Doppler spectrogram, reconstructed from a Doppler signal produced by the movement of the skin above the radial artery in the human wrist. The aim is to study to what extent the arterial pulse shape can be measured using self-mixing interferometry. To obtain a point of reference, a commercial non-invasive blood pressure monitor was first used to measure both blood pressure and pulse shape. Then, a self-mixing interferometer was applied to measure the arterial pulse above the radial artery. Measurements on 10 volunteers yielded a total of 738 pulses for analysis. A cross correlation of 0.84 ± 0.05 was established between the shape of the first derivative of the pressure pulse and the Doppler spectrogram. Using an empirical constant of 0.7 as a limit for successfully detected pulses produced a detection accuracy of 95.7%. The results show that self-mixing interferometry lends itself to the measurement of the arterial pulse shape, and that the thus obtained shape is in good agreement with that produced by a commercial blood pressure monitor.

We measured second order dispersion of glucose solution using a Michelson Low Coherent Interferometer (LCI). Three different glucose concentrations: 20mg/dl (hypoglycemia), 100mg/dl (normal level), and 500mg/dl (hyperglycemia) are investigated over the wavelength range 0.5μm to 0.85μm, and the investigation shows that different concentrations are associated with different second-order dispersions. The second-order dispersions for wavelengths from 0.55μm to 0.8μm are determined by Fourier analysis of the interferogram. This approach can be applied to measure the second-order dispersion for distinguishing the different glucose concentrations. It can be considered as a potentially noninvasive method to determine glucose concentration in human eye. A brief discussion is presented in this poster as well.

We have constructed an en face coherence gated camera for optically sectioning the in vivo human retina. Coherence gating is generated by a free-space Michelson interferometer employing a superluminescent diode for illuminating the retinal tissue; voice coil and piezo-electric translators for controlling the optical path length of the reference channel; and a scientific-grade CCD camera for recording 2-D retinal interferograms. A conventional 1-D OCT is incorporated for tracking the axial motion of the retina and controlling the gating position. En face slices of test objects and retinal tissue were obtained using a four-step (λ/4) phase shift method. Ultrafast acquisition of four interferograms in less than 7 milliseconds has been achieved to mitigate eye motion blur. A 5-step reconstruction algorithm that is more robust to phase shift error and noise was compared to the 4-step. The axial width of the point spread function and the sensitivity of the camera were measured near 10µm and 76 dB, respectively, which is substantially better than current flood-illuminated and confocal scanning laser ophthalmoscopes equipped with adaptive optics.

Polarization-sensitive optical coherence tomography (PSOCT) is a powerful new optical imaging modality that is sensitive to the birefringence properties of tissues. It thus has potential applications in studying the large-scale ordering of collagen fibers within connective tisues and changes related to pathology. As a tissue for study by PSOCT, intervertebral disk respresents an interesting system as the collagen organization is believed to show pronounced variations with depth, on a spatial scale of about 100 μm. We have used a polarization-sensitive optical coherence tomography system to measure the birefringence properties of bovine caudal intervertebral disk and compared this with equine flexor tendon. The result for equine tendon, δ = (3.0 ± 0.5)x10^-3 at 1.3 μm, is in broad agreement with values reported for bovine tendon, while bovine intervertebral disk displays a birefringence of about half this, δ = 1.2 x 10^-3 at 1.3 μm. While tendon appears to show a uniform fast-axis over 0.8 mm depth, intervertebral disk shows image contrast at all orientations relative to a linearly polarized input beam, suggesting a variation in fast-axis orientation with depth. These initial results suggest that PSOCT could be a useful tool to study collagen organization within this tissue and its variation with applied load and disease.

Flow dynamics in blood vessels of a few hundred microns in diameter were investigated using Doppler optical coherence tomography (DOCT) and Doppler Amplitude optical coherence tomography (DAOCT), a novel extension of DOCT. The motivation behind this work is discussed, followed by a brief explanation of the theory underlying the motion of blood cells in small conduits and blood vessels. Preliminary results are presented and compared to the predictions expected from theory. All significant findings are analysed along with their importance to microvascular research.