Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

Imaging cerebral blood flow through the intact rat skull with temporal laser speckle imaging

We discovered that laser speckle temporal contrast analysis (LSTCA) is able to access the two-dimensional (2D) cerebral blood flow velocity and vessel structure through the intact rat skull. It is demonstrated that LSTCA can significantly suppress the influence of the laser speckle from the stationary structure, such as the skull, and thus reveal the blood flow and morphology of blood vessels through the laser speckle images recorded from the intact rat skull.     

Blood Flow Analyses of Skin Tissue Under the Sacrum Using Laser Speckle Flowgraphy

A new version of the Laser Speckle Flowgraphy was developed to study the dependence of the blood flow of the skin tissue under the sacrum and the supporting pressure against the sacrum. The measuring unit consists of the laser diode, imaging system, line sensor, scanning mirror, and one-board microcomputer. The speckle signal is analyzed and sent to a personal computer, where the blood flow in a tissue area of 19 cm × 25 cm is evaluated and displayed on a 2-D color map. It was found that the blood flow in the skin tissue sharply decreased under the sacrum when it was supported by a hard surface, while the decrease became negligible when the surface was replaced by a soft airbag.     

Frame-rate analysis of arterial blood flow in human and rat using laser speckle image sensing

In imaging of blood flow by means of a laser speckle technique, we have proposed so far an estimation parameter based on the spatial contrast of speckle patterns observed for the blood flow in skin tissue and a blood vessel. This parameter enable us to image a relative blood flow distribution from a single speckle pattern, thus, it analyzes the blood flow with a frame-rate of an imaging device used. In this study, we investigated availability of this parameter for detecting changes in arterial blood flow caused by medication and cold stimulation to the skin tissue. Experiments were conducted for an anesthetized rat and a human wrist to confirm the feasibility of the present parameter.     

用激光散斑跨零法监测皮肤血流变化

由皮肤散射形成的动态激光散斑携带了皮肤血流的信息.由于来自运动着的红血球的散射光两次经过皮肤表面,因而受到三次随机调制.本文根据串级散斑的快速时间涨落的特点,提出了用激光散斑的胯零率测量皮肤血流的新方法,给出了跨零率与血流速度参数之间的关系式和实验结果.     

Flow visualization and density measurement of compressible flows by a laser speckle method

Laser speckle method is a well known technique that is useful for both visualization and quantitative measurement. This technique was applied to the density measurement of Mach reflection of shock waves in the present experiment. The Object of the measurement is the density field of simple Mach reflection in relatively low shock Mach number. The non-uniform flow field is divided into three regions by incident, Mach and reflected shock waves. A shock tube was employed in the present experiment. Wedges of 20 degrees and 45 degrees were placed in the test section. YAG laser was employed as a light source. Speckle photograph was taken by a digital still camera. Simple subtraction between the reference and flow images shows a shock pattern and a degree of the correlation of speckle pattern in the flow field. Thus, we can obtain a visualized flow image showing a configuration of Mach reflection from speckle photograph. Speckle photographs which was obtained in the experiments were processed with cross-correlation method. A reconstructed density gradient vector map of Mach reflection was obtained. Comparing the experimental result with numerical one, the measured density gradient shows a good agreement with theoretical prediction.     

Non-contact blood flow determination using a laser speckle method

An electro-optical device is described which allows the non-contact determination of the skin blood flow and its temporal course. As the laser light penetrates the skin, it is not only scattered from the epidermis but also from the moving red blood cells in the capillaries. The scattered light is time dependent and can be described in terms of the dynamic laser speckle effect. Measurements at the skin demonstrate that there is a so-called ‘involuntary body movement’ which must be taken into account when the measurement of the blood flow is determined. Theoretical considerations show a way to reduce the influence of this movement. Some measurements demonstrate the response of the device to blood flow variations.     

Optical coherence Doppler tomography quantifies laser speckle contrast imaging for blood flow imaging in the rat cerebral cortex

A dual-imaging modality is demonstrated for high-resolution quantitative imaging of local cerebral blood flow in the rat cortex by combining simultaneous spectral-domain Doppler optical coherence tomography (SDOCT) and full-field laser speckle contrast imaging (LSCI). Preliminary studies in tissue flow phantom and cocaine-induced cerebral blood flow changes indicated that by correlating coregistered cortical arterial blood flow, the relative measurement of flow changes by LSCI could be accurately calibrated by the absolute flow imaging provided by SDOCT (least square fit, r(2) approximately 0.96). Quantitative LSCI of cerebral blood flow is crucial to the quantitative analyses of the spatiotemporal hemodynamics of functional brain activations and thus improved understanding of neural process.     

Comparison of laser Doppler and laser speckle contrast imaging using a concurrent processing system

Full field laser Doppler imaging (LDI) and single exposure laser speckle contrast imaging (LSCI) are directly compared using a novel instrument which can concurrently image blood flow using both LDI and LSCI signal processing. Incorporating a commercial CMOS camera chip and a field programmable gate array (FPGA) the flow images of LDI and the contrast maps of LSCI are simultaneously processed by utilizing the same detected optical signals. The comparison was carried out by imaging a rotating diffuser. LDI has a linear response to the velocity. In contrast, LSCI is exposure time dependent and does not provide a linear response in the presence of static speckle. It is also demonstrated that the relationship between LDI and LSCI can be related through a power law which depends on the exposure time of LSCI.     

Longitudinal, multimodal functional imaging of microvascular response to photothermal therapy

Although studies have shown that photothermal therapy can coagulate selectively abnormal vasculature, the ability of this method to achieve consistent, complete removal of the vasculature is questionable. We present the use of multimodal, wide-field functional imaging to study, in greater detail, the biological response to selective laser injury. Specifically, a single-platform instrument capable of coregistered fluorescence imaging and laser speckle imaging was utilized to monitor vascular endothelial growth factor gene expression and blood flow, respectively, in a transgenic rodent model. Collectively, the longitudinal, in vivo data collected with our instrument suggest that the biological response to selective laser injury involves early-stage redistribution of blood flow, followed by increased vascular endothelial growth factor promoter activity to stimulate pro-angiogenic events.     

Simultaneous imaging of total cerebral hemoglobin concentration,oxygenation, and blood flow during functional activation

A simple instrument is demonstrated for high-resolution simultaneous imaging of total hemoglobin concentration and oxygenation and blood flow in the brain by combining rapid multiwavelength imaging with laser speckle contrast imaging. The instrument was used to image changes in oxyhemoglobin and deoxyhemoglobin and blood flow during cortical spreading depression and single whisker stimulation in rats through a thinned skull. The ability to image blood flow and hemoglobin concentration changes simultaneously with high resolution will permit detailed quantitative analysis of the spatiotemporal hemodynamics of functional brain activation, including imaging of oxygen metabolism. This is of significance to the neuroscience community and will lead to a better understanding of the interrelationship of neural, metabolic, and hemodynamic processes in normal and diseased brains.     

Flow visualization by means of single-exposure speckle photography

A photograph of a flow field is taken under laser illumination. If the exposure is short enough, the velocity distribution in the field will be mapped on the photograph as variations in speckle contrast. These contrast variations can be converted to intensity variations by means of a simple spatial filtering technique, to give a direct picture of the velocity distribution in the flow field. A potential application of this technique to the mapping of retinal blood flow is described.     

Velocity contours by speckle photography

We describe a method of generating velocity contours of an object using speckle photography. The object is illuminated by a series of laser pulses following each other at equal intervals. A speckle record of the object is observed in a spatial filtering setup with a small aperture in the frequency plane. A fringe pattern representing a map of velocity contours is obtained in the output plane.     

半导体激光器的自混合散斑干涉测量流体速度

提出了一种简便的激光自混合散斑干涉测量流体速度的方法。根据散斑和法布里-珀罗腔的理论,提出了半导体激光器的自混合散斑干涉模型。研究了流体运动时在半导体激光器内产生的自混合散斑干涉效应(SMPI),给出了激光器输出增益的变化及其概率密度分布。得到了激光自混合散斑干涉平均频率与流体速度之间的关系。模拟计算和实验结果验证了这个关系。利用散斑干涉的平均频率与流体速度的关系测量了高分子材料溶液的速度,并在溶液浓度和背景光变化时,对流体速度进行了测量、比较和分析。实验表明。在溶液浓度不太低时,测量误差小于8％。     

Speckle variance detection of microvasculature using swept-source optical coherence tomography

We report on imaging of microcirculation by calculating the speckle variance of optical coherence tomography (OCT) structural images acquired using a Fourier domain mode-locked swept-wavelength laser. The algorithm calculates interframe speckle variance in two-dimensional and three-dimensional OCT data sets and shows little dependence to the Doppler angle ranging from 75 degrees to 90 degrees . We demonstrate in vivo detection of blood flow in vessels as small as 25 microm in diameter in a dorsal skinfold window chamber model with direct comparison with intravital fluorescence confocal microscopy. This technique can visualize vessel-size-dependent vascular shutdown and transient vascular occlusion during Visudyne photodynamic therapy and may provide opportunities for studying therapeutic effects of antivascular treatments without on exogenous contrast agent.     

Enhancement in sensitivity of a blood perfusion parameter for frame-rate analysis of bio-speckle image

We have previously presented a blood perfusion parameter by means of the spatial contrast of speckle patterns observed for blood perfusion in skin tissue and a blood vessel. This parameter permits us to visualize a relative blood perfusion distribution from a single speckle pattern. Therefore, it can estimate the blood perfusion with a frame rate of an imaging device employed. This parameter is, however, dependent on the speckle size and the condition of image processing of speckle patterns. In the present study, we estimated this condition with relation to the speckle size for obtaining high sensitivity and good linearity of the parameter to the blood perfusion. Experiments were conducted for ground-glass model, flow model and human wrist to investigate perfusion sensing characteristics of the present parameter.

     

Improvement of estimation parameters in bio-speckle blood flow imaging

Two novel estimation parameters are proposed for bio-speckle blood flow imaging in skin tissue with reducing effects of a speckle contrast and random noises. Estimation parameters are introduced to reduce the effect of the speckle contrast on the sensitivity of the blood flow sensing and to eliminate an offset component due to random noises superimposed on the speckle images. Experiments were conducted for a sample model using ground-glass plates, human vein and finger to confirm the feasibility of the proposed parameters.     

Numerical study on trapping and guiding of nanoparticles in a flow using scattering field of laser light

Optical trapping and guiding using laser have been proven to be useful for non-contact and non-invasive manipulation of small objects such as biological cells, organelles within cells, and dielectric particles. We have numerically investigated so far the motion of a Brownian particle suspended in still water under the illumination of a speckle pattern generated by the interference of coherent light scattered by a rough object. In the present study, we investigate numerically the motion of a particle in a water flow under the illumination of a speckle pattern that is at rest or in motion. Trajectory of the particle is simulated in relation with its size, flow velocity, maximum irradiance, and moving velocity of the speckle pattern to confirm the feasibility of the present method for performing optical trapping and guiding of the particle in the flow.     

Whole field spatial frequency analysis of double-or multiple-exposed PIV images

Speckle Velocimetry or high-image density PIV gives a velocity vector map of a twodimensional flow field by point-by-point spatial frequency analysis of local pattern at an interrogation spot in a double- or a multiple-exposed image of laser speckle or pseudo-speckle pattern generated by pulsed laser-light-sheet illuminations of a plane in the flow densely seeded with fine particles. The whole field spatial frequency analysis of the double- or multiple-exposed PIV image gives more useful information of the flow field. Optical Fourier transform is a conventional technique not only for the local spatial frequency analysis but for the whole field analysis. Filtering of spatial frequency is one of the typical techniques for the latter which can reconstruct a velocity contour or a component velocity contour map of the flow. Fundamentally this technique is a simple and efficient analogue method to get more information in the velocity field analysis of the flow than a digital image processing, but in practical applications the measurement is restricted to simple flows because of optical noise and low spatial resolution. In order to improve the technique, the fundamental characteristics of the filtering and the noise yielded in the filtering process must be investigated. Meanwhile, wavelet analysis can also be applied to the whole field spatial frequency analysis of PIV image. In this paper the filtering technique is examined by numerical convolution integral, and the results obtained are compared with ones obtained by the wavelet analysis.     

激光光斑形状和尺寸对扫描显示中散斑对比度的影响

计算了圆形、矩形激光光斑对于激光扫描显示系统中散斑对比度的影响.对于圆形光斑,当光斑尺寸大于散射表面相关长度时,得到的散斑对比度随着光斑尺寸变小而下降;当光斑尺寸接近表面相关长度大小时,由于光斑中包含的散射颗粒变少,得到的是非高斯散斑,散斑的对比度反而会变大;随着激光斑进一步变小,由于镜面反射效果散斑的对比度会很快下降.对圆形光斑部分结果给出了实验验证.为了保证光斑中有足够的散射颗粒,只在一个方向压窄光斑,分别计算了平行于扫描方向和垂直于扫描方向压窄光斑得到的散斑对比度.平行于扫描方向压窄光斑,散斑的时间相关性会下降,平均效果变明显,散斑的对比度变小;垂直于扫描方向压窄,散斑对比度变化不大.