共查询到20条相似文献,搜索用时 15 毫秒
1.
Color Doppler optical coherence tomography (CDOCT) is a recent innovation that allows spatially localized flow-velocity mapping simultaneously with microstructural imaging. We present a theoretical model for velocity-image formation in CDOCT. The proportionality between the heterodyne detector current Doppler power spectrum in CDOCT and the optical source power spectrum is established. We show that stochastic modifications of the Doppler spectrum by fluctuating scatterer distributions in the flow field give rise to unavoidable velocity-estimation inaccuracies as well as to a fundamental trade-off between image-acquisition rate and velocity precision. Novel algorithms that permit high-fidelity depth-resolved measurements of velocities in turbid media are also reported. 相似文献
2.
Color Doppler optical coherence tomography (CDOCT) is a noninvasive optical imaging technique for micrometer-scale physiological flow mapping simultaneously with morphological optical coherence tomography imaging. We have developed a novel CDOCT signal-processing strategy capable of imaging physiological flow rates at 8 frames/s. Our new strategy features hardware-implemented digital autocorrelation across subsequent scans, permitting us to measure 300-Hz-8-kHz Doppler shifts upon signals of 0.6-MHz bandwidth. The performance of the CDOCT system was demonstrated in a flow phantom and in vivo in Xenopus laevis. 相似文献
3.
Imaging and velocimetry of the human retinal circulation with color Doppler optical coherence tomography 总被引:3,自引:0,他引:3
Noninvasive monitoring of blood flow in retinal microcirculation may elucidate the progression and treatment of ocular disorders, including diabetic retinopathy, age-related macular degeneration, and glaucoma. Color Doppler optical coherence tomography (CDOCT) is a technique that allows simultaneous micrometer-scale resolution cross-sectional imaging of tissue microstructure and blood flow in living tissues. CDOCT is demonstrated for the first time in living human subjects for bidirectional blood-flow mapping of retinal vasculature. 相似文献
4.
In vivo bidirectional color Doppler flow imaging of picoliter blood volumes using optical coherence tomography 总被引:1,自引:0,他引:1
We describe a novel optical system for bidirectional color Doppler imaging of flow in biological tissues with micrometer-scale resolution and demonstrate its use for in vivo imaging of blood flow in an animal model. Our technique, color Doppler optical coherence tomography (CDOCT), performs spatially localized optical Doppler velocimetry by use of scanning low-coherence interferometry. CDOCT is an extension of optical coherence tomography (OCT), employing coherent signal-acquisition electronics and joint time-frequency analysis algorithms to perform flow imaging simultaneous with conventional OCT imaging. Cross-sectional maps of blood flow velocity with <50-microm spatial resolution and <0.6-mm/s velocity precision were obtained through intact skin in living hamster subdermal tissue. This technology has several potential medical applications. 相似文献
5.
6.
《Journal of magnetic resonance. Series B》1993,101(2):158-164
Steady-state free precession (SSFP) has a sharply defined, symmetrical response to motion which acts as a bandpass filter for magnetization over a narrow range of velocities, typically less than a few millimeters per second wide. A radiofrequency-pulse-generated moving reference frame (MORF) has been developed, which may be used within an SSFP imaging sequence. The magnetization of material moving in the MORF is suppressed by up to a factor of 10 relative to that moving at the MORF velocity. This new technique is demonstrated without slice selection for flow in a Hele-Shaw cell. 相似文献
7.
Hamel P Rachet B Werner M Grossenbacher M Vogel H Forrer M Ryser P Salathé RP 《Optics letters》2011,36(16):3182-3184
We present an optical trapping system combining individually addressable multiple laser traps with fluorescence spectroscopy. An in-line set of 64 near-IR laser diodes is used to create a line of individually addressable traps inside a microfluidic chip. This system is completed by an excitation/detection line for spectrally resolved fluorescence imaging of trapped particles. Highly parallel trapping in a constant flow (up to a few millimeters per second), fast particle handling rates (up to a few particles per second), and the possibility of recording fluorescence spectra of trapped objects lead to a performing bioanalytical platform, e.g., for highly parallel analysis and sorting. 相似文献
8.
Ultrasound Doppler is widely used for low measurement in both medicine and industry, having the advantages of being non-invasive and comparatively simple and therefore inexpensive. The technique has not however been used for capillary blood flow measurement, because of the relatively low velocities encountered and because of the presence of strong interfering signals from the encompassing tissue. An ultrasound Doppler system capable of measuring flow velocities of one millimetre per second in the presence of one thousand times stronger interfering signals is described, as well as test results using both thread and flow phantoms. 相似文献
9.
A high sensitivity fiber-optic torsion sensor, which can measure twist rate and determine twist direction simultaneously based on a novel ultra-long-period fiber grating (ULPFG) with a period of up to several millimeters, is proposed and demonstrated. Such an ULPFG is fabricated by using the high-frequency CO2 Laser pulses exposure technique. The unique torsion characteristics of the ULPFG are simply analyzed by using the mode coupling theory and the birefringence effect. The experimental results show that the high order resonant wavelengths of the ULPFG have higher torsion sensitivities, which is several times higher than that of the normal LPFG. In addition, an intensity-type demodulation approach used to realize real-time torsion measurement is proposed and demonstrated based on the edge filtering effect of the ULPFG. 相似文献
10.
Shear break-up of reversible fractal clusters is investigated by ultrasound and multiple light scattering in the low shear
regime. We consider a dense suspension of Rayleigh scatterers (particles or clusters) with acoustic properties close to those
of the surrounding liquid so that the attenuation of the ultrasonic coherent field is weak and multiple scattering is negligible.
The concept of variance in local particle volume fraction is used to derive an original expression of the ultrasound scattering
cross-section per unit volume for Rayleigh fractal clusters. On the basis of a scaling law for the shear break-up of aggregates,
then we derive the shear stress dependence of the ultrasound scattered intensity from a suspension of reversible fractal clusters.
In a second part, we present rheo-acoustical experiments to study the shear break-up of hardened red cell aggregates in plane-plane
flow geometry and we examine both the self consistent field approximation and the scaling laws used in microrheological models.
We further compare the ability of acoustical backscattering and optical reflectometry techniques to estimate the critical
disaggregation shear stress and the particle surface adhesive energy. Finally, the microrheological model from Snabre and
Mills [#!ref5!#] based on a fractal approach is shown to describe the non Newtonian behavior of a dense distribution of hardened
red cell aggregates.
Received 12 November 1998 and Received in final form 17 May 1999 相似文献
11.
12.
采用VOF模型对倾角为45°、80°、85°三种情况下倾斜上升管内弹状流的壁面传质特性进行了研究.传质特性通过其与壁面切应力的类比关系来体现.数值模拟结果表明,低混合物流速时,上管壁面切应力在液膜区有明显波动,而下管壁面切应力分布则比较光滑.随着混合流速的增大,液膜区上下壁面切应力分布趋于一致.管子下壁面切应力平均值大于管子上壁面,在Taylor气泡运动速度较低时较为突出.随着Taylor气泡速度的增大,管子上下壁面的切应力平均值趋于相同.相同的混合流速下倾斜角度越大,上下管壁的切应力分布越趋于近似.下降液膜区的壁面切应力平均值大于Taylor气泡尾迹区域.根据Chilton-Colburn的类比关系,壁面切应力的规律完全适用于壁面传质系数. 相似文献
13.
H. KOOKL. MONGEAU 《Journal of sound and vibration》2002,251(5):823-846
Subsonic flows over Helmholtz resonators often cause strong periodic pressure fluctuations inside the resonators over a range of outer flow velocities. The flow-excitation mechanism is known to be governed by both the shedding of discrete vortices within the shear layer over the orifice and the acoustic response of the cavity. This self-sustained oscillation phenomenon is often analyzed by using a feedback loop model where the flow excitation and the acoustic response of the resonator are approximately modelled as a forward gain function and as a backward gain function respectively. In the present work, a similar approach was followed and a new forward gain function was derived based on the concept of “vortex sound” to model the flow excitation. The formulation combined this forward gain function with a backward gain function from previous work, within the framework of the feedback loop analysis. The approximate method allowed the frequency and the relative amplitude of the cavity pressure fluctuations to be predicted for a range of flow velocities. In addition, the extended Nyquist stability criterion was used to estimate the onset and the termination velocities of the first two modes of the shear layer flow oscillations. Experimental data were obtained using a rigid-walled cavity in a low-speed wind tunnel. The results showed that the model predictions were in reasonably good agreement with the experimental data. 相似文献
14.
A projection MR technique for imaging the velocity profiles of moving fluids has been applied to various steady flow models designed to simulate a variety of flow conditions. From such profiles can be readily deduced peak velocities, volume flow rates, information concerning the degree of flow development, features such as flow separation, and estimates shear stresses at the vessel wall. 相似文献
15.
16.
Abstract The effect of streamwise jet-to-jet spacing on local heat transfer distribution due to an in-line rectangular array of confined multiple circular air jets impinging on a surface parallel to the jet plate are experimentally studied. The length-to-diameter ratio of nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in two opposite directions from the confined passage formed between the jet plate and target plate. Mean jet Reynolds numbers based on the nozzle exit diameter (d) covered are 3,000, 5,000, 7,500, and 10,000; jet-to-plate distances studied are d, 2d, and 3d. Streamwise jet-to-jet distances of 3d, 4d, and 5d, and a constant spanwise pitch of 4d, are considered. The jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless-steel metal foil. Local temperature distribution on a target plate is measured using a thermal infrared camera. Wall static pressures in the streamwise direction are measured midway between the spanwise jets to estimate cross-flow velocities and individual jet velocities. The streamwise distribution of the jet flow and the cross flow is found to be least influenced by the streamwise pitch variation for the range of parameters considered during the present study. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on the strip-averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, indicates deterioration for lower streamwise pitch and higher jet-to-plate distance. 相似文献
17.
An Euler solver based on locally adaptive discrete velocities 总被引:2,自引:0,他引:2
B. T. Nadiga 《Journal of statistical physics》1995,81(1-2):129-146
A new discrete-velocity model is presented to solve the three-dimensional Euler equations. The velocities in the model are of an adaptive nature—both the origin of the discrete-velocity space and the magnitudes of the discrete velocities are dependent on the local flow—and are used in a finite-volume context. The numerical implementation of the model follows the near-equilibrium flow method of Nadiga and Pullin and results in a scheme which is second order in space (in the smooth regions and between first and second order at discontinuities) and second order in time. (The three-dimensional code is included.) For one choice of the scaling between the magnitude of the discrete velocities and the local internal energy of the flow, the method reduces to a flux-splitting scheme based on characteristics. As a preliminary exercise, the result of the Sod shock-tube simulation is compared to the exact solution. 相似文献
18.
19.
Spatially resolved flow velocity measurements and projection angiography by adiabatic passage 总被引:1,自引:0,他引:1
This paper describes the basic principles of gradient modulated adiabatic passage using a CW radiofrequency excitation. The possible applications of this technique include a direct assessment of in-plane and oblique directional flow velocities, and visualization of flow velocity profiles. Flow angiography based on the time-of-flight technique is also discussed with experimental results. 相似文献
20.
利用高频CO2激光脉冲写入的周期达数毫米的超长周期光纤光栅(ULPFG),实验研究了这种新型ULPFG的扭曲特性,发现它的某些高阶谐振波长漂移与扭曲率之间具有良好的线性关系和方向相关性,其灵敏度可达0.2244nm/(rad/m),是高频CO2激光脉冲写入法写入的普通LPFG扭曲灵敏度的4倍.初步的理论分析表明,新型ULPFG横截面折变的非对称性以及导模与高阶包层模之间发生的耦合使得扭曲具有方向相关性和很高的灵敏度.基于这种ULPFG独特的扭曲特性,设计了一种可
关键词:
光纤传感
光纤光栅
2激光')" href="#">CO2激光
扭曲测量
双折射 相似文献