Multiple Backscattering Effects in Optical Coherence Tomography Images of Layered Turbid Media

We study the influence of multiple backscattering on optical coherence tomography (OCT) images or pulsed-sounding images of layered turbid media. A method for calculating the signal of various orders of backscattering is proposed. The retarding effects of OCT-signal dropping are analyzed. The contrast attenuation of an image of a layered medium due to multiple backscattering is examined. The blurring effect of an image of the back boundary of a scattering layer, caused by a delay of multiply scattered photons compared with single-scattered ones, is studied theoretically and experimentally using an OCT device.     

Effective point-spread function for fast image modeling and processing in microscopic imaging through turbid media

An effective point-spread function (EPSF) for microscopic imaging through turbid media is proposed and calculated. The EPSF incorporates the property of a microscope system as well as the scattering property of a turbid medium. We prove that the image of a thin object embedded in a turbid medium can be expressed by the convolution of the EPSF with an object function. With the help of the convolution relation, image modeling for 5, 000, 000 incident photons can be approximately 15 times faster than the direct Monte Carlo simulation method for a one-dimensional object and can be at least 2 orders of magnitude faster for a two-dimensional object.     

Simulation of coherent properties of photons in scattering medium for optical coherence tomography

Monte Carlo model of optical coherence tomography is developed for simulation of photon transport in half infinite homogenous media. The procedure is accelerated by scaling the baseline data from standard Monte Carlo calculation in turbid media with arbitrary optical parameters. Gaussian beam is modeled by hyperboloid of one sheet for actual condition to obtain distribution of photons on sample surface. Depth dependence coherent signal and photons distribution are calculated in this way, which is important to reconstruction of optical parameters by inverse Monte Carlo. Numerical results have verified this method in turbid medium of different optical parameters with acceptable relative errors.     

Imaging Transparent Objects in a Turbid Medium Using a Femtosecond Optical Kerr Gate

A femtosecond optical Kerr gate time-gated ballistic imaging method is demonstrated to image a transparent object in a turbid medium. The shape features of the object are obtained by time-resolved selection of the ballistic photons with different optical path lengths, the thickness distribution of the object is mapped, and the maximum is less than 3.6%. This time-resolved ballistic imaging has potential applications in studying properties of the liquid core in the near field of the fuel spray.     

Object reconstruction in scattering medium using multiple elliptical polarized speckle contrast projections and optical clearing agents

In this paper, we present a hybrid method for improving the imaging quality of objects obscured within a scattering environment by combining multiple elliptical polarized speckle contrast projections with the use of optical clearing agents (OCAs). Elliptically polarized light enables the probing of subsurface volumes, where OCAs decrease light scattering while increasing photons׳ penetration depth through the medium. Experiments were conducted on object sample and prostate cancer cells embedded within ex vivo biological samples (chicken breasts) in reflection configuration. After immersion with OCAs, the medium was irradiated with an elliptically polarized laser beam and multiple polarized speckled images obtained from a lens array were first converted to speckled contrast images and then processed using a self-deconvolution shift-and-add algorithm. The conversion to contrast images and multiple perspectives acquisition was found to emphasize contrast. Analysis of image quality indicated improvement in object visualization by the combination of elliptical polarization and OCAs. This enhanced imaging strategy may advance the development of improved methods in biomedicine field, specifically biomedical tomography.     

Three-dimensional visualization of objects in scattering medium using integral imaging and spectral analysis

This paper presents a three-dimensional visualization method of 3D objects in a scattering medium. The proposed method employs integral imaging and spectral analysis to improve the visual quality of 3D images. The images observed from 3D objects in the scattering medium such as turbid water suffer from image degradation due to scattering. The main reason is that the observed image signal is very weak compared with the scattering signal. Common image enhancement techniques including histogram equalization and contrast enhancement works improperly to overcome the problem. Thus, integral imaging that enables to integrate the weak signals from multiple images was discussed to improve image quality. In this paper, we apply spectral analysis to an integral imaging system such as the computational integral imaging reconstruction. Also, we introduce a signal model with a visibility parameter to analyze the scattering signal. The proposed method based on spectral analysis efficiently estimates the original signal and it is applied to elemental images. The visibility-enhanced elemental images are then used to reconstruct 3D images using a computational integral imaging reconstruction algorithm. To evaluate the proposed method, we perform the optical experiments for 3D objects in turbid water. The experimental results indicate that the proposed method outperforms the existing methods.     

Simulation of polarization-sensitive optical coherence tomography images by a Monte Carlo method

We introduce a new Monte Carlo (MC) method for simulating optical coherence tomography (OCT) images of complex multilayered turbid scattering media. We demonstrate, for the first time of our knowledge, the use of a MC technique to imitate two-dimensional polarization-sensitive OCT images with nonplanar boundaries of layers in the medium like a human skin. The simulation of polarized low-coherent optical radiation is based on the vector approach generalized from the iterative procedure of the solution of Bethe-Saltpeter equation. The performances of the developed method are demonstrated both for conventional and polarization-sensitive OCT modalities.     

偏振门用于对散射介质成像的蒙特卡罗模拟研究

通过蒙特卡罗模拟研究偏振光在散射介质中的传播,对通过散射介质的偏振光的强度分布进行模拟,分析了偏振门和空间滤波对成像的影响.模拟结果表明,对于Rayleigh散射体,偏振门与空间滤波可以减小散射光的影响,提高图像对比度,圆偏振光入射比线偏振光具有更高的对比度.对于Mie散射粒子,偏振门的作用有限,不同偏振光的结果差别不大.对双组分的散射介质也进行了模拟,偏振门在这种体系中仍能提高图像对比度.     

Influence of single scattering and multiple scattering on backscattered Mueller matrix in turbid media

Monte Carlo algorithm and Stokes-Mueller formalism are used to simulate the propagation behavior of polarized light in turbid media. The influence of single scattering and multiple scattering on backscattered Mueller matrix in turbid media is discussed. Single and double scattering photons form the major part of backscattered polarization patterns, while multiple scattering photons present more likely as background. Further quantitative analyses show that single scattering approximation and double scattering approximation are quite accurate when discussing the polarization patterns near the incident point.     

Polarization-based range-gated imaging in birefringent medium:Effect of size parameter

We have investigated the effect of size parameter of the scatterer on the image quality obtained with polarization-based range-gated imaging in birefringent turbid medium. Both linearly and circularly polarized light were utilized for imaging.The simulated results indicate that the improvement of visibility is more pronounced using circularly polarized light for the birefringent medium composed of smaller-sized scatterers at lower values of optical thickness and the birefringent medium comprising larger-sized scatterers. In contrast, linearly polarized light provides better image quality for the birefringent medium composed of smaller-sized scatterers at larger values of optical thickness. The evolution of the polarization characteristics of backscattered light and target light under the conditions mentioned above was measured to account for these numerical results.     

Polarization preservation in diffusive scattering from in vivo turbid biological media: effects of tissue optical absorption in the exact backscattering direction

There is considerable recent interest in using polarized light to investigate turbid biological media. Although tissue multiple scattering randomizes incident polarization states, there are circumstances when appreciable degree of polarization can be observed in diffusive scattering. In this study, we use polarization modulation and synchronous detection to examine in the exact backscattering direction the polarization properties of diffusely reflected visible light from hands of human volunteers of varying pigmentation levels. The surviving polarization fraction increases with increasing pigmentation, likely due to preferential loss of highly scattered, long-pathlength photons; this mechanism lowers the average pathlength traversed by the detected light and hence increases the measured polarization preservation. This behavior is contrasted with the overall diffuse reflectance intensity, whose magnitude decreases with increasing absorption. These experiments demonstrate the important influences of medium optical properties on the polarization characteristics of multiply scattered light, which must be further investigated to enable quantitative polarization evaluation of turbid media such as biological tissues.     

Optoacoustic imaging with synthetic aperture focusing and coherence weighting

Optoacoustic imaging takes advantage of high optical contrast and low acoustic scattering and has found several biomedical applications. In the common backward mode a laser beam illuminates the image object, and an acoustic transducer located on the same side as the laser beam detects the optoacoustic signal produced by thermoelastic effects. A cross-sectional image is formed by laterally scanning the laser beam and the transducer. Although the laser beam width is generally narrow to provide good lateral resolution, strong optical scattering in tissue broadens the optical illumination pattern and thus degrades the lateral resolution. To solve this problem, a combination of the synthetic aperture focusing technique with coherence weighting is proposed. This method synthesizes a large aperture by summing properly delayed signals received at different positions. The focusing quality is further improved by using the signal coherence as an image quality index. A phantom comprising hair threads in a 1% milk solution was imaged with an optoacoustic imaging system. The results show that the proposed technique improved lateral resolution by 400-800% and the signal-to-noise ratio by 7-23 dB over conventional techniques.     

Time-resolved backscattering of circularly and linearly polarized light in a turbid medium

Time-resolved backscattering profiles of circularly and linearly polarized light were measured from a turbid medium composed of small and large polystyrene sphere particles in water. It is shown that, based on the measurements of the time-resolved backscattered copolarized and cross-polarized components of the incident polarized light, either linearly or circularly polarized light can be used to effectively image an object that is deep inside a turbid medium composed of small particles, depending on the depolarization properties of the object itself. For large particles such as in tissue, fog, and clouds, the experimentally observed polarization memory effect on the backscattering temporal profiles suggests that a significant improvement in the image contrast can be achieved by use of circularly polarized light.     

Transient Photon Transport in Turbid Media with an Irregularly Shaped Foreign Object

A finite-volume method for the modeling of transient photon propagation in turbid media with an irregularly shaped foreign object is presented. The procedure can account for non-rectangular foreign object. Body-fitted coordinates are used to discretize the spatial domain. Absorbing, anisotropic scattering non-homogenous medium can be modeled. The procedure is validated using existing numerical results for a homogenous medium. The procedure produces physically possible trends for different optical properties of the foreign object.     

物光复合调制的数字全息散斑去噪方法

为了克服在数字全息术中激光源高相干度造成的散斑噪声降低成像质量问题,论文提出将石英退偏器与电控液晶散射片相结合用于数字全息术中。通过在物光路中加入一个石英退偏器,利用退偏器的退偏性能使物光光路中的线偏光变成随机偏振光,降低与参考光的相干程度,实现对散斑噪声的抑制;通过给液晶散射片外加电场可以引起液晶分子的运动和光轴取向改变,调整液晶驱动电压大小来改变入射光的不同散射效果,有效地降低了光源的相干性;分别将单独采用石英退偏器和电控液晶散射片的重建图像与组合装置的重建图像进行对比分析,通过实验验证了所提方法的有效性。实验结果表明：两种实验对象在组合装置下的重建像峰值信噪比分别达到16.91和18.30,结构相似度为0.47和0.43,等效视数较原始重建像提高了42%和30%,散斑指数为1.52和1.29。论文提出的方法可较好地去除散斑噪声,保留更多的图像细节部分。     

Spatially distributed two-photon excitation fluorescence in scattering media: Experiments and time-resolved Monte Carlo simulations

We report on measurements and Monte Carlo analysis of the spatial distribution of light in two-photon excitation fluorescence (TPEF) microscopy operating through scattering media. The axial profile of TPEF produced by a high numerical aperture microscope objective focusing through a tissue-like optical phantom at depths more than five photon mean-scattering lengths is studied. The measured profile is quantitatively interpreted with time-resolved Monte Carlo simulations that take into account the spatio-temporal distribution of the photons within the turbid medium under a femtosecond excitation regime. It is shown that considering only the ballistic photons leads to an under-estimation of the actual out-of-focus distribution of the fluorescence, whereas it is over-estimated when the ballistic and scattered photons are treated alike. Comparison of the in-focus signal to the overall out-of-focus background clearly pointed out that the signal-to-background ratio of TPEF microscopy images benefits from a shortening of the excitation pulses. Moreover, in homogeneously labeled specimen, the background is shown to overcome the signal at imaging depths of about five to six photon mean-scattering paths.     

天光背景下混浊大气中成像质量的分析方法

调制传递函数(MTF)定量描述混浊介质对图像质量的影响,它是混浊介质的固有光学特性,利用等效原理可以获得MTF从低频到高频的完整特征.在实际应用中,混浊介质中的图像质量不仅取决于介质的MTF,还与图像的背景辐射密切相关.本文从混浊大气中图像退化机理出发,理论分析了空间频域中天光背景下图像质量的退化过程.参考等效原理,提出了一种考虑天光背景的表观MTF,得到了表观MTF与介质MTF和天光背景的定量关系,从而得到了一种分析背景辐射下混浊介质中图像质量的有效便捷方法.针对图像质量优化方法,从空间频域的角度提出了一种评价原则.     

Correlation characteristics of optical coherence tomography images of turbid media with statistically inhomogeneous optical parameters

Noisy structure of optical coherence tomography (OCT) images of turbid medium contains information about spatial variations of its optical parameters. We propose analytical model of statistical characteristics of OCT signal fluctuations from turbid medium with spatially inhomogeneous coefficients of absorption and backscattering. Analytically predicted correlation characteristics of OCT signal from spatially inhomogeneous medium are in good agreement with the results of correlation analysis of OCT images of different biological tissues. The proposed model can be efficiently applied for quantitative evaluation of statistical properties of absorption and backscattering fluctuations basing on correlation characteristics of OCT images.     

Optical Computed Tomography Based on Extraction of Quasi-Straight forward Propagating Photons Using Sum-Frequency Generation Technique

Aiming at the realization of optical computed tomography (optical CT), a compact system comprised of two cw-lasers and a detector was proposed for the extraction of quasi-straightforward propagating photons from scattered light transmitting through a scattering medium. The extraction ability of the system based on the sum-frequency generation technique was investigated using a standard scattering medium of Intralipid-10% aqueous solution and was found to be 78 dB in dynamic range. The optical CT image of absorbers placed in the scattering medium was successfully obtained with high contrast using the proposed system.     

Multiple-scattering speckle in holographic optical coherence imaging

We investigate the effect of multiple scattering on the image quality of holographic optical coherence imaging, which is a full-field coherence-domain imaging form of optical coherence tomography. The speckle holograms from turbid media and from multicellular tumor spheroids are characterized by high-contrast speckle on a multiply-scattered background caused by channel cross-talk. We quantify the multiple-scattered light that is accepted by the holographic coherence gate, and identify a cross-over from single-scattered to multiple-scattered light beyond 15 to 20 optical thicknesses. Speckle reduction relies on vibrating diffusers and on fast adaptive holograms in photorefractive quantum well devices. The high anisotropy factor for tumor tissue reduces multiply-scattered light contributions for biomedical tumor imaging.