Imaging the body with diffuse optical tomography

Diffuse optical tomography (DOT) is an ongoing medical imaging modality in which tissue is illuminated by near-infrared light from an array of sources, the multiply-scattered light which emerges is observed with an array of detectors, and then a model of the propagation physics is used to infer the localized optical properties of the illuminated tissue. The three primary absorbers at these wavelengths, water and both oxygenated and deoxygenated hemoglobin, all have relatively weak absorption. This fortuitous fact provides a spectral window through which we can attempt to localize absorption (primarily by the two forms of hemoglobin) and scattering in the tissue. The most important current applications of DOT are detecting tumors in the breast and imaging the brain. We introduce the basic idea of DOT and review the history of optical methods in medicine as relevant to the development of DOT. We then detail the concept of DOT, including a review of the tissue's optical properties, modes of operation for DOT, and the challenges which the development of DOT must overcome. The basics of modelling the DOT forward problem and some critical issues among the numerous implementations that have been investigated for the DOT inverse problem, with an emphasis on signal processing. We summarize with some specific results as examples of the current state of DOT research     

生物组织光学参数的校准模型及测量

在intralipid 墨水(ink)液体模型中,通过改变intralipid溶液和墨水的浓度就可以灵活地获得不同的光学参数。制作了一种可以长期重复使用的光学特性稳定的Si树脂模型,它是利用TiO2 粉末作为散射颗粒,同时加入C粉颗粒来改变模型的吸收特。为了验证intralipid 墨水液体模型制作中基于Mie散射理论的近似计算公式,利用多波长的时间分辨(TRS)方法对不同intralipid浓度的液体模型进行了测试。实际测量结果和Mie散射理论计算的误差在5%以内,这可能是由于所作用的intralipid溶液之间的差别造成的。利用频域的相位调制系统(PMD)对Si树脂模型进行了多间距测量,斜率的线性回归拟合结果显示了良好的线性度(R2>0.99),从而保证了光学参数测量结果的准确性。     

离体猪鼻咽组织的光学特性参数测量

利用单积分球技术测定了在He Ne激光照射下离体猪鼻咽组织的反射率和透射率以及准直透射率 ,并应用反向倍加 (IAD)方法间接获得了猪鼻咽组织的光学特性参数。结果表明 ,猪鼻咽组织具有低吸收和强前向散射的特性 ,其吸收系数和散射系数分别为 0 96cm-1和 6 2 7cm-1,散射异向因子为 0 92 ,光学穿透深度为 0 2 4 1cm。这些参数为模拟计算光能流率在人体鼻咽组织中的分布提供了理论数据。     

组织体漫反射率与光学特性参数关系的研究

本文从漫射理论出发,推导出了组织体漫反射率与约化散射系数和吸收系数之间的唯一性关系,应用了新兴的无损测量方法－－光学辐出度分光术,实验研究了组织模拟液漫反射率与组织光学性质的关系,为该技术在临床医学中的应用提供了理论和实验依据。     

Discriminating optical parameters in double-layer turbid media by backscattered polarization patterns

In this letter a Monte Carlo(MC) algorithm is used to simulate the propagation of polarized light in double-layer turbid media and the 2-dimentional backscattered Stokes vectors and Mueller matrices are obtained.Relationships between backscattered Mueller matrix and optical properties,such as scattering,absorption and layered structure,are discussed in detail.Integrating the 2-dimentional Mueller matrix elements along radial and azimuthal directions,we obtain a reverse trend with respect to the optical parameters for upper and lower layers,which suggests possibilities for discriminating subtle optical properties in a double-layer structure using backscattered polarization patterns such as Mueller matrix.     

Microwave Emission and Plant Water Content: A Comparison between Field Measurements and Theory

Microwave radiation from a canopy cover depends primarily on the vegetation's thermal and dielectric properties; the latter are dependent on plant biometrical parameters and water content. Emission measurements carried out by means of ground-based X-and Ka-band radiometers have shown that crop coverage of soil can be detected through the spectral signatures of bare soil and vegetation. Moreover, measured brightness temperature and the radiative transfer theory for a scattering medium allow estimation of the scattering and absorption properties of the canopy. These parameters have been computed for corn and alfalfa using experimental data and a simple model in which anisotropic scattering is considered by means of transformed parameters w' and r'. We found that the single scattering albedo w' is always lower than 0.1, whereas the optical depth T' is very high. The latter has been correlated to plant water content by means of a logarithmic function.     

Optical Transmission and Reflection by Blood

The results of calculations which use diffusion equations for photons to calculate optical transmission and backscattering by whole blood will be compared with a variety of published data. The purpose of this work is to demonstrate the usefulness of the diffusion theory approach to the optics of blood and to present information concerning the dependence of the scattering and absorption parameters for blood on the pertinent variables such as hemoglobin concentration, hematocrit, and optical wavelength.     

超声调制光信号的调制深度与组织光学特性关系

超声调制光学成像技术融合了光学成像和超声成像的优点,已实现对浑浊介质内隐藏异体的成像,是一种很有前景的医学成像技术。通过实验探究超声调制光信号与组织光学特性的关系、超声焦区内外组织的光学特性对调制信号的影响,发现超声对光信号的调制深度与超声焦区组织的光学性质呈简单的线性关系,调制深度随样品散射系数的增大而减小,随样品吸收系数的增大而增大。超声区信号调制深度基本不受超声调制区域外复杂组织的光学特性的影响。这为超声调制光学成像提供必要的图像重构依据。     

芳香化合物生成二次有机气溶胶的光学性质研究进展

芳香化合物是二次有机气溶胶(SOA)的一类重要前体物,其在大气中的氧化可以生成具有光吸收特性的棕色碳。因此,理解芳香化合物所生成SOA的光散射、光吸收、反照率等光学性质对于定量研究其对空气质量、能见度和气候变化的影响具有重要意义。系统总结了实验室研究中单环芳香烃、含氧芳香化合物、多环芳香烃三类典型芳香化合物所生成SOA的光学性质,分析对比了文献关于光散射和光吸收参数的异同,归纳了多种环境因素对SOA光学性质的影响,并展望了该方向未来的重点研究领域。     

一种大转角SAR图像散射中心各向异性提取方法

基于各向异性特性反映了散射中心本身属性,该文提出一种大转角SAR条件下提取散射中心各向异性特性的算法。首先根据属性散射中心模型分析单个散射中心的回波构成;之后以单位矩阵作为标准正交基,将各向异性特性的估计转化为求解散射中心幅度变化的逆问题;再利用散射中心幅度变化的实数特性与连续性对该逆问题进行约束求解,实现了对散射中心各向异性特性的提取。Matlab仿真和电磁计算数据的处理结果验证了算法的准确性以及稳定性;与现有算法对比,所提算法运算效率高。     

Chrono-coherent imaging for medicine

We describe a new method for imaging with visible and near visible light inside media, such as tissues, which have strong light scattering. The chrono-coherent imaging (CCI) method is demonstrated in this paper for a transmission geometry where an absorbing object is completely hidden from normal visual observation by light scattering of the media. The resultant images are most similar to X rays, with cumulative transmission showing absorption features and refractive index differences in the media. We discuss laser coherence properties, coherence measurements, the relation of CCI to light-inflight holography, holographic film properties relevant to CCI, a particular optical setup for CCI, the results of a demonstration experiment imaging an absorbing object hidden by light scattering, and an experiment to estimate the clinical applicability of CCI.     

热作用致心肌组织在可见与近红外范围内的光学特性变化

研究了热作用对心肌组织在可见及近红外光谱范围内的光学特性改变。利用带积分球附件的分光光度计测量了心肌组织样品的漫反射率和总透射率,再用反向倍增(IAD)法从这两测量值中获取组织的吸收系数、约化散射系数和光学穿透深度。结果表明,自然的及热处理后的心肌组织的吸收系数、约化散射系数和光学穿透深度都是随着辐照波长的变化而显著变化的。自然的及热凝固后(80℃热处理)的心肌组织的吸收系数均在550 nm附近有一正峰,峰值分别为0.74 mm~(-1)和1.16 mm~(-1)。自然的心肌组织的约化散射系数在550 nm附近出现一正峰,峰值为0.25 mm~(-1);高温处理后此峰消失,而在590～625 nm出现上升坡。低温热处理对心肌组织的吸收系数没有明显的影响,高温热处理后心肌组织的吸收系数增大;心肌组织的约化散射系数随热处理的温度上升而显著地升高;心肌组织的光学穿透深度随热处理的温度上升而显著地减小。     

Skin optics

Quantitative dosimetry in the treatment of skin disorders with (laser) light requires information on propagation of light in the skin related to the optical properties of the individual skin layers. This involves the solution of the integro-differential equation of radiative transfer in a model representing skin geometry, as well as experimental methods to determine the optical properties of each skin layer. These activities are unified under the name skin optics. This paper first reviews the current status of tissue optics, distinguishing between the cases of: dominant absorption, dominant scattering, and scattering about equal to absorption. Then, previously published data as well as some current unpublished data on (human) stratum corneum, epidermis and dermis, have been collected and/or (re)analyzed in terms of absorption coefficient, scattering coefficient, and anisotropy factor of scattering. The results are that the individual skin layers show strongly forward scattering (anisotropy factors between 0.7 and 0.9). The absorption and scattering data show that for all wavelengths considered scattering is much more important than absorption. Under such circumstances, solutions to the transport equation for a multilayer skin model and finite beam laser irradiation are currently not yet available. Hence, any quantitative dosimetry for skin treated with (laser) light is currently lacking.     

激光雷达探测水体光学特征参数的方法分析

多次散射是影响激光雷达探测水体光学特征参数（包括吸收系数a,衰减系数c,漫射衰减系数K_d）的重要因素,而视场（field of view, FOV）是造成接收回波中多次散射比重差异的关键参数。研究基于Mclean-Walker海洋激光雷达模型,引入接收视场参数q_rcvr,分别针对船载、机载和星载平台激光雷达进行探测回波计算,假设其探测高度依次为10 m、300 m和7´10⁵ m,在接收视场变化情况下分析多次散射因子对反演水体光学特征参数的影响。结果表明,利用水体中激光散射回波反演水体光学特征参数的方法中,假定激光准直入射,q_rcvr与探测高度H是影响反演结果的直接因素,激光雷达衰减系数K_lidar的物理意义随之变化,当q_rcvr近似为0时K_lidar反演值为衰减系数c,随q_rcvr´H增大,K_lidar反演值逐渐接近于漫射衰减系数K_d。     

基于太阳辐射计的气溶胶光学特性反演算法研究

介绍了一种利用太阳直接辐射和天空漫射辐射来反演气溶胶光学特性的方法,研究了其算法实现过程.结合太阳辐射计CE318所测得的太阳直射辐照度和天空漫射辐亮度数据,反演获得了晴朗天气下四个波段的气溶胶光学厚度、Angstrom参数、单次散射反照率、散射相函数和偏振相函数.算法的实现为解决光学遥感应用中气溶胶散射相函数和偏振相函数的测量问题提供了一个实用的方法.     

Uncertainty and Ambiguity in Terahertz Parameter Extraction and Data Analysis

Phase sensitive measurement techniques, such as THz time-domain spectroscopy or dispersive Fourier transform spectroscopy, are very useful tools to obtain a complete set of optical material parameters. When recording the electric field as a function of time delay between THz and optical pulse, the absorption coefficient and the index of refraction can be extracted. However, the analysis shows ambiguity. Here, we describe an analysis which yields a complete set of mathematical solutions and show how the physically relevant can be deduced. We present a comprehensive mathematical survey for parameter extraction. We have recorded the THz spectra of anthracene and the fatty acid capric acid as examples for weakly absorbing solid samples, and an ionic liquid as an example for a strongly absorbing liquid sample. Finally, we discuss the uncertainty of the obtained optical parameters using error propagation of the Fourier transformation with a simple model and a rigorous mathematical procedure.     

Imaging of spatiotemporal coincident states by DC optical tomography

The utility of optical tomography as a practical imaging modality has, thus far, been limited by its intrinsically low spatial resolution and quantitative accuracy. Recently, we have argued that a broad range of physiological phenomena might be accurately studied by adopting this technology to investigate dynamic states (Schmitz et al., 2000; Barbour et al., 2000; Graber et al., 2000; Barbour et aL, 2001; and Barbour et aL, 1999). One such phenomenon holding considerable significance is the dynamics of the vasculature, which has been well characterized as being both spatially and temporally heterogeneous. In this paper, we have modeled such heterogeneity in the limiting case of spatiotemporal coincident behavior involving optical contrast features, in an effort to define the expected limits with which dynamic states can be characterized using two newly described reconstruction methods that evaluate normalized detector data: the normalized difference method (NDM) and the normalized constraint method (NCM). Influencing the design of these studies is the expectation that spatially coincident temporal variations in both the absorption and scattering properties of tissue can occur in vivo. We have also chosen to model dc illumination techniques, in recognition of their favorable performance and cost for practical systems. This choice was made with full knowledge of theoretical findings arguing that separation of the optical absorption and scattering coefficients under these conditions is not possible. Results obtained show that the NDM algorithm provides for good spatial resolution and excellent characterization of the temporal behavior of optical properties but is subject to interparameter crosstalk. The NCM algorithm, while also providing excellent characterization of temporal behavior, provides for improved spatial resolution, as well as for improved separation of absorption and scattering coefficients. A discussion is provided to reconcile these findings with theoretical expectations.     

气溶胶光学吸湿增长特性测量方法研究进展

吸湿性气溶胶会吸收环境空气中的水分,其粒径会随相对湿度的增加而发生变化,从而导致气溶胶的光学特性(如消光、散射、吸收系数与单次散射反照率等)发生显著的变化。气溶胶光学吸湿增长因子(湿状态与干状态下光学参数的比值)是衡量气溶胶光学吸湿增长能力的特征参数,是计算大气能见度和气溶胶辐射强迫的关键输入量,它的准确测量对于气溶胶环境和气候效应的评估具有重要意义。光学吸湿增长测量系统主要包括湿度调节系统和光学测量装置,通过湿度调节系统改变样品的相对湿度,再结合光学测量装置实时测量光学参数的变化,从而实现光学吸湿特性的在线测量。鉴于气溶胶吸湿性研究的重要意义,重点分析对比了现有的光学吸湿增长测量方法及应用,并对下一步气溶胶光学吸湿增长特性测量技术和研究方向做了展望。     

Theoretical Analysis of Diffuse Reflectance from a Two-Layer Tissue Model

A three-dimensional diffuse reflectance equation for a two-layer tissue model was developed using photon diffusion theory. In this model, tissue was considered to consist of two homogeneous isotropically scattering layers whose scattering and absorption constants were expressed as a linear sum of those of whole blood and a blood-free tissue component; tissue hemoglobin content and oxygen saturation were then expressed in terms of these total tissue parameters. Reflectance predictions given by the two-layer equation were used to investigate the effects of various tissue and system parameters on the partial reflectance from the second tissue layer; among such parameters significantly affecting deep-layer reflectance are the tissues scattering constants, its geometry, and the geometry of the optical transducer. When the penetration depth of the incident photons is small compared with the thickness of the first layer, reflectance contributions from the second layer are negligible, and a single-layer approximation would be adequate; resultant reflectance errors range from 6 to 8 percent of the total reflectance, for source-detector separations in the range from 1 to 4 mm. However, when the photon penetration depth is large with respect to first-layer thickness, the effects of deep layers are both important and strongly dependent on transducer geometry; partial reflectances range to 50 percent of the total when the source-detector separation is 4 mm.     

海水及模拟尾流气泡的激光背向散射特性实验研究

尾流光的背向散射特性是水下航行体跟踪、识别和定位目标的重要参数。开展海水及模拟尾流气泡的激光背向散射特性实验研究，对确定感知舰船尾流的光学参量，从散射信息中分离出光尾流特征信息，验证尾流光散射特性模型有重要意义。讨论了海水及气泡群的激光背向散射特性实验研究，给出了实验结论。