Angular domain imaging of objects within highly scattering media using silicon micromachined collimating arrays

Optical imaging of objects within highly scattering media, such as tissue, requires the detection of ballistic/quasi-ballistic photons through these media. Recent works have used phase/coherence domain or time domain tomography (femtosecond laser pulses) to detect the shortest path photons through scattering media. This work explores an alternative, angular domain imaging, which uses collimation detection capabilities of small acceptance angle devices to extract photons emitted aligned closely to a laser source. It employs a high aspect ratio, micromachined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200: 1) containing 51/spl times/1000 /spl mu/m etched channels with 102-/spl mu/m spacing over a 10-mm silicon width. With precise array alignment to a laser source, unscattered light passes directly through the channels to the charge coupled device detector and the channel walls absorb the scattered light at angles >0.29/spl deg/. Objects within a scattering medium were scanned quickly with a computer-controlled Z axis table. High-resolution images of 100-/spl mu/m-wide lines and spaces were detected at scattered-to-ballistic ratios of 5/spl times/10/sup 5/: 1, with objects located near the middle of the sample seen at even higher levels. At >5/spl times/10/sup 6/: 1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction. Monte Carlo simulation programs designed to test the angular domain imaging concept showed that the collimator detects the shortest path length photons, as in other optical tomography methods. Furthermore, the collimator acts as an optical filter to remove scattered light while preserving the image resolution. Simulations suggest smaller channels and longer arrays could enhance detection by >100.     

水下主动激光扫描系统中光场记录及散射背景分离研究

传统水下主动激光扫描系统采用普通相机对反射光接收记录,反射激光点被掩埋在背景噪声中无法分离,影响成像质量。该文在水下激光主动扫描成像系统中采用光场相机记录了包括激光反射光线和各种散射背景杂光的位置和方向信息,为抑制散射背景杂光提供了可能。在后续光场记录图像处理中,首先,对记录的光场图像进行前后两次重聚焦,第一次是对激光照射到物面激光光点的重聚焦,第二次是对激光光点前景散射光的重聚焦,再对两幅图像进行差分处理;提出一种计算机自动判断流程,使得差分所得图像上激光点的衬度最大,或足够大。实验结果表明,上述方法可以达到较好的散射背景抑制作用,是对现有同步扫描主动激光成像系统空间分离散射背景抑制技术的必要补充。     

Measuring cellular structure at submicrometer scale with lightscattering spectroscopy

We present a novel instrument for imaging the angular distributions of light backscattered by biological cells and tissues. The intensities in different regions of the image are due to scatterers of different sizes. We exploit this to study scattering from particles smaller than the wavelength of light used, even when they are mixed with larger particles. We show that the scattering from subcellular structure in both normal and cancerous human cells is best fitted to inverse power-law distributions for the sizes of the scattering objects, and propose that the distribution of scattering objects may be different in normal versus cancerous cells     

Optical properties effects upon the collection efficiency of optical fibers in different probe configurations

When optical fibers are used for delivery and collection of light, two major factors affect the measurement of collected light: 1) light transport in the medium from the source to the detection fiber and 2) light coupling to the optical fiber (which depends on the angular distribution of photons entering the fiber). This paper studies the latter factor, describing how the efficiency of the coupling depends on the optical properties of the sample. The coupling dependence on optical properties is verified by comparing experimental data to a simple diffusion model and to a Monte Carlo (MC)-corrected diffusion model. Mean square errors were 7.9% and 1.4% between experiments and the diffusion, and experiments and the MC-corrected models, respectively. The efficiency of coupling was shown to be highly dependent on the numerical aperture (NA) of the optical fiber. However, for lower scattering, such as in soft tissues, the efficiency of coupling could vary two- and threefold from that predicted by fiber NA. The collection efficiency can be used as a practical guide for choosing optical fiber-based systems for biomedical applications.     

Time-gated transillumination of objects in highly scattering mediausing a subpicosecond optical amplifier

We have developed an optical system that simultaneously selects and amplifies photons in a time window of less than 10 ps. This new subpicosecond optical amplifier allowed us to realize one-dimensional images of a striped pattern with a spatial resolution of 200-μm through a 30-mm liquid scattering medium. Monte Carlo simulations were performed to study the relative importance of parameters characterizing the medium and the time-gated amplifying system in relation to the image sharpness. The potential application of this system in medical imaging is discussed     

Depth-resolved holography through turbid media usingphotorefraction

A technique based on photorefractive holography for imaging objects obscured by a scattering medium is presented. Using ultrashort pulse illumination, depth-resolved whole-field images of three dimensional objects embedded in scattering media have been obtained. Bulk photorefractive crystals and photorefractive multiple quantum-well (MQW) devices have been investigated as the hologram recording element. Images have been obtained through media of up to 16 scattering mean free paths with a system based on bulk rhodium-doped barium titanate (Rh:BaTiO₃). Using MQW devices, a real-time image acquisition (<0.4 ms) has been demonstrated when imaging through eight scattering mean free paths. The relative merits of photorefractive holography are discussed, including its potential to provide a higher dynamic range of detection than traditional photographic film based or electronic holography. This could be important for in vivo imaging through biological tissue     

Frequency-domain optical detection of subsurface blood vessels:experimental and computational studies using a scattering phantom

A simple method to localize blood vessels beneath the surface of tissue could be very useful during laparoscopic and endoscopic procedures. However, the detection of blood vessels deep within tissue using light is limited by tissue scattering. In the study, frequency-domain photon migration methods were used to detect blood vessels within a scattering medium. The depth at which blood vessels could be detected was greater than 10 mm. The experimental measurements agree well with predictions obtained from the diffusion approximation to the radiative transport equation. These studies demonstrate the potential of frequency-domain optical methods to detect subsurface blood vessels     

Sensitivity of laser opto-acoustic imaging in detection of smalldeeply embedded tumors

Current imaging modalities fail to detect small tumors in the breast. Opto-acoustic tomography is a novel technique for early cancer detection with promising diagnostic capability. The experimental limit of sensitivity and maximal depth of the laser opto-acoustic detection for small model tumors located within bulk phantom tissue were studied. Two phantoms with optical properties similar to that of breast tissue in the near infrared spectral range were used in these studies: turbid gelatin slabs with the thickness of 100 mm and chicken breast muscle slabs with the thickness of up to 80 mm. Gelatin spheres with enhanced absorption coefficient relative to the background absorption and liver tissue were used to simulate small tumors. The experiments demonstrated the capability of laser optoacoustic imaging to detect and localize phantom tumors with the diameter of 2 mm at a depth of up to 60 mm within the gelatin phantoms and 3×2×0.6-mm piece of liver tissue within 80-mm chicken breast tissue. Theoretical studies on sensitivity of opto-acoustic detection at various diameters, depths of location, and absorption coefficients of small tumors were performed using the experimental data. Our results suggest that the opto-acoustic imaging may occupy a significant niche in early detection of cancer in the breast and other organs     

Imaging of tissue microstructures using a multimodal microscope design

We investigate a microscope design that offers high signal sensitivity and hyperspectral imaging capabilities and allows for implementation of various optical imaging approaches while its operational complexity is minimized. This system uses long working distance microscope objectives that enable for off-axis illumination of the tissue, thereby allowing for excitation at any optical wavelength and nearly eliminating spectral noise from the optical elements. Preliminary studies using human and animal tissues demonstrate the feasibility of this approach for real-time imaging of intact tissue microstructures using autofluorescence and light scattering imaging methods.     

Three-dimensional computation of light scattering from cells

Using the finite-difference time-domain method, three-dimensional scattering patterns are computed for cells containing multiple organelles. The scattering cross section and average cosine of the scattering angle are computed for cells as a function of volume fraction of melanin granules and mitochondria. Results show that small organelles play a significant role in light scattering from cells, and the volume fraction of organelles affects both the total amount of scattered light and the angular distribution of scattered light     

AOI检测系统光照不均的校正方法研究

针对电子组装业自动光学检测(automatic optical inspection,AOI)系统光照不均情况,分析了背景去除法、传统同态滤波法2种校正方法,并将一种改进的基于小波变换的同态滤波方法应用于该领域。该方法对目标图像进行多层小波分解,对各层高低频系数滤波处理,减弱低频系数,增强高频系数,然后进行小波重构,从而达到光照不均校正的目的。然后取PCBA(printed circuit board assembly)图像进行了3种方法的实验对比,结果证明了改进的基于小波变换的同态滤波方法在AOI系统中具有很强的实用性。     

水下光场成像清晰度增强研究

由于水介质及水下颗粒的散射吸收作用,使得水下成像图像存在分辨率与对比度低,细节特征模糊,颜色失真等问题。针对这些问题,该文提出基于光场成像的水下图像清晰度增强算法,根据暗原色先验理论和单尺度的Retinex理论建立水下散射成像模型对成像图像的清晰度进行增强处理,并利用光场成像的多视角特性对散射成像模型的图像增强效果进行优化,进一步提高水下成像图像的质量。实验结果表明,两种理论构建的水下散射模型和多视角优化算法可以有效的提高水下成像图像的质量。     

Compact laser-diode sources for optical inspection probes

The use of laser diode sources rather than conventional tube lasers to provide the illumination for many optical probe applications is considered. The technical issues addressed include temperature control, output intensity control, and collimation of the light output. Possible solutions, as well as experimental results from a laboratory optical probe developed using a laser diode source, are presented     

Parotid gland tissues investigated by picosecond time-gated andoptical spectroscopic imaging techniques

Near-infrared (NIR) time-resolved and spectroscopic transillumination imaging techniques are used to investigate normal tissues and Warthin's tumor of human parotid glands. The time-sliced imaging arrangement uses 120-fs, 1-kHz repetition-rate, 800-nm pulses from a Ti: sapphire laser and amplifier system for sample illumination and an ultrafast gated intensified camera system (UGICS) for recording two-dimensional (2-D) images using transmitted light. Images recorded with earlier temporal slices (approximately first 100 ps) of transmitted light highlight the tumor, while those recorded with later temporal slices (later than 200 ps) accentuate normal tissues. The spectroscopic imaging arrangement uses 1210-1300 nm tunable output of a Cr: forsterite laser for sample illumination, a Fourier space gate to discriminate against multiple-scattered light, and a NIR area camera to record 2-D images. The tumor region in the specimen appears brighter than the normal region in spectroscopic images recorded with light of different wavelengths. A wavelength-dependent variation in the ratio of light transmission through the tumor to that through the normal parotid gland is observed. Differences in scattering and wavelength-dependent absorption characteristics of normal parotid gland and Warthin's tumor provide a consistent explanation of these observed features. Histopathological analysis of the specimen sheds light on the probable origin of the differences in scattering and absorption characteristics     

基于插值处理的散射介质中偏振成像实验研究

针对散射介质中光学成像质量受散射效应干扰的问题,基于Jaffe 成像模型,实验研究了基于插值处理的偏振成像降 噪方法,实现目标信号与后向散射噪声的有效分离。首先,利用退偏振特性建立信息插值处理与信号提取模型;其次,搭建偏 振成像实验光路结构进行验证,以脂肪乳溶液模拟散射环境,在偏振激光照明条件下获取散射场景的相互正交的偏振探测通 道图像;最后,对所获取偏振图像进行插值处理并利用信噪比参数评价其可靠性。实验结果表明,当脂肪乳溶液浓度依次为 0.1%、0.2%、0.3%与0.4%时,基于插值处理的偏振成像方法所对应图像的信噪比依次为112.5、9.165、13.82与11.88,能 够有效地抑制后向散射噪声对光学成像过程的影响。     

The Effect of the Phosphor Particle Sizes on the Angular Homogeneity of Phosphor-Converted High-Power White LED Light Sources

Based on optical ray tracing, we discuss the effect of the phosphor particle sizes on the angular homogeneity of the light emitted from phosphor-converted LEDs. Since the blue LED and the yellow-converted light have rather different emission characteristics, which have to be harmonized to one another by the scattering processes within the color conversion element, the phosphor particle size turns out to be an essential parameter in order to attain angular homogeneity. This can be attributed, on the one hand, to the number of scattering processes within a specific unit volume for a given phosphor concentration, and on the other hand, to the specific scattering functions, both of which depend on the phosphor particle diameter.      

Tutorial on Photoacoustic Microscopy and Computed Tomography

The field of photoacoustic tomography has experienced considerable growth in the past few years. Although several commercially available pure optical imaging modalities, including confocal microscopy, two-photon microscopy, and optical coherence tomography, have been highly successful, none of these technologies can provide penetration beyond ~1 mm into scattering biological tissues, because they are based on ballistic and quasi-ballistic photons. Heretofore, there has been a void in high-resolution optical imaging beyond this penetration limit. Photoacoustic tomography, which combines high ultrasonic resolution and strong optical contrast in a single modality, has broken through this limitation and filled this void. In this paper, the fundamentals of photoacoustics are first introduced. Then, scanning photoacoustic microscopy and reconstruction-based photoacoustic tomography (or photoacoustic computed tomography) are covered.     

实现LED台灯均匀照明的自由曲面透镜仿真设计

针对LED台灯照明区域照度均匀度不足的现状,提出了利用自由曲面透镜实现二次光学配光的方法。基于光源的朗伯光学特性,建立理想LED点光源与照明区域的余弦拓扑关系,并依据Snell定律通过MATLAB编程迭代计算自由曲面的母线。利用Tracepro光学软件仿真分析了光源实际尺寸对配光效果的影响,并探讨了透镜尺寸与配光效果的关系。经仿真优化的自由曲面透镜,实现发光半径2mm的LED光源在台灯要求的照明区域上均匀度（最小值/最大值）达到95%,同时光的利用效率提高20%以上。     

Investigation of near-infrared autofluorescence imaging for the detection of breast cancer

Detection of breast cancer in fresh tissue obtained from surgery is investigated using near-infrared autofluorescence imaging under laser excitation at 532 and 632.8 nm. The differences in intensity between the three main components of breast tissue (cancer, fibrous, and adipose) are estimated and compared to those obtained from cross-polarized light scattering images recorded under polarized illumination at 700 nm. The optical spectroscopic images for each tissue sample were subsequently compared with the histopathology slides. The experimental results indicate that the intensity of the near-infrared emission is considerably different in breast cancer compared to that of the adjacent nonneoplastic tissues (adipose and fibrous tissue). The experimental results suggest that 632.8-nm excitation offers key advantages compared to 532 nm excitation.