Extraction of optical scattering parameters and attenuation compensation in optical coherence tomography images of multilayered tissue structures

A recently developed analytical optical coherence tomography (OCT) model [Thrane et al., J. Opt. Soc. Am. A 17, 484 (2000)] allows the extraction of optical scattering parameters from OCT images, thereby permitting attenuation compensation in those images. By expanding this theoretical model, we have developed a new method for extracting optical scattering parameters from multilayered tissue structures in vivo. To verify this, we used a Monte Carlo (MC) OCT model as a numerical phantom to simulate the OCT signal for heterogeneous multilayered tissue. Excellent agreement between the extracted values of the optical scattering properties of the different layers and the corresponding input reference values of the MC simulation was obtained, which demonstrates the feasibility of the method for in vivo applications. This is to our knowledge the first time such verification has been obtained, and the results hold promise for expanding the functional imaging capabilities of OCT.     

小鼠卵母细胞染色体三维双光子荧光图像的轴向衰减

双光子荧光显微镜作为一种高分辨光学仪器,已经被广泛应用于生物样品的非侵入式三维光学成像中。相比共聚焦显微镜,双光子荧光显微镜拥有更深的探测深度。然而,即便如此,在对较厚的生物样品进行非侵入式光学三维成像时,样品的成像质量也往往会随着探测深度的增加而下降。在临床和生物学领域对研究母性遗传起重要作用的小鼠卵母细胞拥有较大的直径(80～100 μm),吸收和散射效应较为明显。本文研究小鼠卵母细胞染色体的三维双光子荧光图像随探测深度增加图像质量的衰减程度。通过对所得图像进行轴向衰减矫正,利用体积作为参数,将矫正前后小鼠卵母细胞内染色体三维双光子荧光图像进行对比。结果表明,由于吸收和散射效应,卵母细胞存在较严重的光学轴向衰减问题,因此,对用双光子荧光三维成像手段获得的小鼠卵母细胞图像进行衰减矫正是有必要的。这为进一步精确定量的研究卵母细胞内染色体的三维构像打下良好的基础。     

光学相干层析成像系统的蒙特卡罗模拟

将光学相干层析成像系统采样臂的光纤与聚焦系统和被测样品作为一个整体来模拟光学相干层析成像信号的形成机理.通过追迹采样光束中每一光子在聚焦系统和被测样品中的随机传输轨迹,来决定该光子对光学相干层析成像信号是否有贡献及贡献大小.对样品IntralipidTM的模拟结果与实验结果表明：1）高散射系数和弱前向散射是引起光学相干层析成像信号随测量深度增加迅速减弱的主要原因;2）对于确定的聚焦系统,光纤的数值孔径有一最佳取值范围.数值孔径太小,则光学相干层析成像信号很弱;数值孔径超出这一范围继续增大,光学相干层析成像信号变化很小.3）当光纤的光轴偏离聚焦系统的光轴h距离时,光学相干层析成像信号随h的增加而减弱.     

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.     

光学相干层析术用于鲜红斑痣诊断

光学相干层析术(optical coherence tomography,简称OCT)具有非侵入性,高分辨及高速成像的优点,特别适合于生物医学领域。但由于大部分生物组织具高散射系数,通常仅能对表层组织下数毫米深度内进行成像。穿透深度不足限制了OCT在皮肤科等领域应用。作为常见多发病的鲜红斑痣具有病变组织浅,血管增生明显等特点,所以OCT非常适于鲜红斑痣的检测。通过选择皮肤穿透性好的中心波长为1310nm超辐射二极管,合理优化样品臂和参考臂光强比例及偏振控制,实现了对鲜红斑痣在体成像研究,采集了清晰的OCT图像,得到其关键特征参数,如表皮层厚度,血管直径等,对鲜红斑痣的诊断及制定合理治疗方案具有重要意义。     

In vivo biomicroscopy with ultrasound 2

In the first article of this series it was shown that the use of inverse scattering theory to analyse ultrasound reflections could provide high resolution images of the acoustic impedance profile of the retina. Unlike the retina, most tissue structures of interest, like small tumours and arterial plaque deposits, are shielded from view by intervening layers of tissue of appreciable acoustic impedance and attenuation. By analysing a one-dimensional model for a plaque deposit on the wall of a carotid artery embedded in a 5 cm thick layer of tissue, we demonstrate that a relatively high quality image can be recovered when compensation for the attenuation of the intervening tissue is made. We observe that because of the dearth of low frequency power in the recovered signal of ultrasound transducers, it is important that the field of view imaged is not taken to be too large. We compare the exact iterative distorted wave Born approximation inverse scattering method with the approximate but computationally faster plane wave Born approximation method and find that they give images of comparable quality for this model.     

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.     

Determination of optical properties of oxidative bleaching human dental tissue samples using optical coherence tomography

Oxidative bleaching changes of human teeth induced changes in the optical properties of dental tissue. We introduced 1310 nm wavelengths of optical coherence tomography (OCT) attenuation coefficient method which is a relatively novel and rarely reported methodology to measure the correlation coefficient during the teeth oxidative bleaching procedure. And the quantitative parameters of enamel optical thickness and disruption of the entrance signal (DES) were extracted from the OCT images. The attenuation coefficient of the bleached tissue is 6.2 mm⁻¹ which is significant (p < 0.001) higher than that unbleached sample is 1.4 mm⁻¹. But attenuation coefficient varied significantly (p < 0.001) between 5.9 and 1.5 mm⁻¹ in dentine which is downtrend. Furthermore, the persistence of bleaching oxidation in 35% hydrogen peroxide-induced optical thickness of enamel is similar with unbleached tissue which may indicate the refractive index of enamel is unchanged. Moreover, disruption of the entrance signal (DES) analysis showed that remarkable difference was appeared at enamel surface. The results indicate that optical properties of oxidative bleaching human dental tissue can be determined by attenuation coefficient using OCT system.     

Molecularly sensitive optical coherence tomography

Molecular contrast in optical coherence tomography (OCT) is demonstrated by use of coherent anti-Stokes Raman scattering (CARS) for molecular sensitivity. Femtosecond laser pulses are focused into a sample by use of a low-numerical-aperture lens to generate CARS photons, and the backreflected CARS signal is interferometrically measured. With the chemical selectivity provided by CARS and the advanced imaging capabilities of OCT, this technique may be useful for molecular contrast imaging in biological tissues. CARS can be generated and interferometrically measured over at least 600 microm of the depth of field of a low-numerical-aperture objective.     

基于赝热光照明的单发光学散斑成像

散射介质对光的散射是当前限制光学成像深度或距离的一个严重的问题.本文首先数值模拟比较了光透过随机散射介质成像研究中常用的基于光学记忆效应(memory effect, ME)和自相关(autocorrelation, AC)方法的HIOER算法和乒乓(Ping-Pang, PP)算法的优缺点.通过对HIOER算法和PP算法的恢复效果和迭代次数进行比较,发现PP算法在保持较高恢复效果的前提下拥有更快的运行速度.实验中,利用连续HeNe激光器和旋转毛玻璃产生赝热光源,通过物镜对随机散射介质后数毫米距离内的不同形状物体进行了单帧成像,并采用PP算法成功地恢复出微米量级物体的实际图像.这一研究结果将进一步促进ME和AC方法在深层生物组织医学成像研究上的应用.最后,实验研究了不同的物镜和散射介质的间距对成像恢复的放大率、分辨率和图像强度的影响特性,并进行了详细研究.     

基于维纳滤波的同轴电缆脉冲信号传输畸变补偿研究

由于同轴电缆的低通传输特性,脉冲信号在同轴电缆中传输时不可避免地会出现畸变,并且畸变程度会随着脉冲信号频率、带宽以及传输距离的增加而增大。创新性地将图像处理领域中的一种图像复原方法——维纳滤波法应用于脉冲信号同轴电缆传输畸变补偿,仅利用同轴电缆的S参数和输出信号即可完成输入信号的重构。并以10 m同轴电缆为对象,采用该方法分别对双指数脉冲信号、高斯调制脉冲信号、调制方波信号进行传输畸变补偿。结果表明:对于不同样式的信号,该方法均具有优异的补偿性能;并且,与工程上常用的衰减补偿法相比,该方法不仅补偿精度高,还具有高的计算效率,在同轴电缆脉冲信号传输畸变补偿中具有很好的实用价值。     

聚吡咯修饰的金纳米棒对肿瘤的光学相干层析成像的影响

聚吡咯(PPy)制备简单、生物相容性好,且在近红外(NIR)光谱范围内有很强的吸收,可作为一种良好的光热治疗试剂;同时,其NIR光吸收性质也可用于增强光学相干层析成像(OCT)的对比效果。因此,采用PPy对传统的OCT对比试剂——金纳米棒(GNR)进行表面修饰,有望获得对比效果更好且生物毒性较小的新型OCT对比试剂。选用吡咯为起始原料,在GNR表面进行一步简单的氧化聚合反应即可制备得到PPy修饰的金纳米棒(GNR-PPy)。利用紫外-可见吸收光谱,拉曼光谱和透射电子显微镜对制备的样品进行了分析和表征。构建小鼠荷瘤模型,以研究GNR-PPy对肿瘤OCT图像对比度的增强效果。采用中心波长为840 nm的OCT系统对注射了纳米粒子的肿瘤区域进行OCT成像。结果表明,肿瘤组织注射了GNR-PPy后,OCT信号衰减非常明显;与注射了GNR的OCT图像相比,840 nm光在GNR-PPy的OCT图像中的穿透深度明显更低。从OCT图像中抽提出一维的衰减曲线对OCT图像进行定量分析,发现注射有GNR-PPy肿瘤组织的OCT信号衰减系数明显高于注射了GNR的组织。表明,相对于GNR,GNR-PPy具有更好的OCT信号对比效果,这在增强肿瘤成像效果方面具有潜在应用价值。     

适合于内窥成像的共路型光学相干层析成像系统

提出了一种采用光纤型迈克耳孙干涉仪进行光程补偿的菲佐型光学相干层析成像(OCT)系统.该系统的传感探头为共路干涉结构,以解决现有内窥光学相干层析成像系统中存在的探头运动导致图像失真、以及更换使用不同探头时需进行色散和偏振态调节等问题.光程补偿和振动干扰实验结果表明,光程补偿方法正确可行,系统对环境干扰不敏感.利用研制的系统对反射镜和近红外卡进行了成像实验,验证了系统的有效性.提出的方法非常适合于内窥成像,并给出了把系统扩展为内窥光学相干层析成像系统的具体实现过程.     

基于受激辐射信号的谱域光学相干层析分子成像方法

鉴于不同生理病理状态下组织复折射率实部的变化不大,传统光学相干层析(OCT)成像技术在分子特异性识别上存在先天不足.为此,本文提出了基于受激辐射信号的OCT成像方法,可在实现传统散射成像的同时,实现基于受激辐射信号的分子成像.在超高分辨率谱域OCT系统的基础上,通过增设光谱分光与调制抽运光支路,建立了基于单宽谱光源的抽运探测谱域OCT系统,详细推导了调制抽运下受激辐射信号的获取与成像公式.利用搭建的抽运探测谱域OCT系统,实现了瞬态受激辐射信号的相干探测.基于同时获取的受激辐射OCT信号和传统OCT信号,成功重构了氮化物粉末构建样品的基于受激辐射信号的分子对比OCT图像.     

用于频域光学相干层析成像的深度分辨色散补偿方法

提出一种深度分辨的色散补偿方法,用于增强频域光学相干层析成像的纵向分辨率。将频域光学相干层析成像采集的干涉谱信号傅里叶变换到空域,获取对应不同深度的干涉谱条纹相位,通过对其进行调整实现对被测样品不同深度处的色散的精确补偿。避免传统方法中采用统一色散系数进行色散补偿所带来的过补偿与欠补偿误差,可以有效消除色散引起的频域光学相干层析成像系统点扩展函数的展宽和扭曲。模拟和实验结果表明,基于深度分辨的色散补偿方法在样品的全深度探测范围内可以达到较佳的补偿效果,可有效提高光学相干层析成像系统的纵向分辨率。     

Interference effects in low coherence interferometry and optical coherence tomography

Optical coherence tomography (OCT) relies on the construction of two-dimensional images from a series of depth scans. These depth scans are made up of a series of interferograms relating to reflections from individual interfaces within a sample. The theoretical resolution of an OCT system is given as half the coherence length of the light source used and therefore interfaces between layers with different refractive indices, which are separated by less than this distance, cannot be resolved. We consider the occurrence of interference between adjacent interferograms and its consequence in signal, and image interpretation. Computational simulations were created to model two or three interfaces in close proximity such that interference would occur between component interferograms. Further to this, images were acquired of an air wedge between two glass slides that corresponded to the simulations. The results of both the simulated OCT signals and those from the air wedge showed the presence of interference effects that could influence the interpretation of the final (OCT) image.     

频域OCT的背向散射率补偿

针对频域OCT中背向散射光的强度随探测深度加深而急剧减弱,从而导致最后的组织成像模糊不清的现象,提出了一种对不同探测深度图像灰度补偿的方法。该方法通过补偿样品每层析面的入射光功率与背向功率散射率,来得到样品每层析面图像的真实灰度,提高了OCT图像质量。以理想的、由多层均匀介质组成的组织模型为基础,深入剖析了影响深度图像灰度的主要因素。通过建立以多层盖玻片为被测物体的频域OCT系统,得到多层盖玻片的深度图像。在对以盖玻片为被测物体的前提下,得出逐层深度图像灰度补偿的公式。并将理论补偿和实验系统的复杂情况相结合,对深度灰度图像进行入射和出射光强的补偿。实验结果证明这种补偿法是一种有效可行的补偿方法。     

PMD compensation using a fuzzy automatic polarization controller

A new first-order polarization mode dispersion (PMD) compensation technique is experimentally demonstrated in a 2.5 Gbit/s optical transmission system. It achieves adaptive compensation over a wide range of differential group delay (DGD) values, since it does not try to compensate it, but to eliminate one of the principal states of polarization by means of a liquid crystal polarization controller and a polarizer. The PMD parameters of the received signal are measured in real time through its spectrum analysis, and used as the feedback signal in a fuzzy logic algorithm controlling the liquid crystal device. The algorithm achieves a response time faster than the PMD variations in real links, also featuring endless operation. Moreover, a deterministic compact PMD emulator has been implemented in order to test the PMD compensator system.     

多尺度条件卷积的OCT视网膜图像降噪研究

散斑噪声存在于光学相干层析成像(OCT)中,影响OCT图像质量.在使用OCT设备诊断各种常见眼科疾病时,高质量的OCT图像是极为重要的.利用深度神经网络对OCT图像进行降噪处理,使图像在保留空间结构细节的基础上能展示更多的信息.提出了一种基于残差学习网络的新型OCT图像降噪网络-CMCNN,其具有多尺度、多权重和多层次...     

基于蒙特卡罗模拟的超声辅助OCT成像研究

通过蒙特卡罗模拟方法研究了超声波提高OCT成像质量的机理,以宏观经典理论和微观量子理论分别阐述了光和超声的相互作用,建立了存在超声辅助时光在生物组织中传播的模型,模拟了有超声辅助和无超声辅助两种情况下生物组织多次散射光的分布.模拟结果表明,与光平行射入生物组织的超声波能够使OCT成像系统中的多次散射光降低80%,携带组织信息的单次散射光在组织返回光中所占比重增大为无超声辅助时的2倍,从而增大了OCT成像的探测深度及图像对比度.