Experimental study of the multiple scattering effect on the flow velocity profiles measured in Intralipid phantoms by DOCT

Time domain Doppler Optical Coherence Tomography (DOCT) technique was applied to measure flow velocity profiles in highly scattering media. We analyzed the distortions of the measured velocity profiles of the 1% Intralipid solution flow embedded into the scattering medium at different embedding depths. For this purpose a tissue phantom consisting of a plain glass capillary (inner diameter 0.3 mm) embedded into a slab of Intralipid solution mimicking human skin was designed. The measured flow velocity profiles and behavior of distortions caused by multiple scattering are shown.     

Boundary segmentation based on modified random walks for vascular Doppler optical coherence tomography images

Vascular Doppler optical coherence tomography(DOCT) images with weak boundaries are usually difficult for most algorithms to segment. We propose a modified random walk(MRW) algorithm with a novel regularization for the segmentation of DOCT vessel images. Based on MRW, we perform automatic boundary detection of the vascular wall from intensity images and boundary extraction of the blood flowing region from Doppler phase images. Dice, sensitivity, and specificity coefficients were adopted to verify the segmentation performance. The experimental study on DOCT images of the mouse femoral artery showed the effectiveness of our proposed method, yielding three-dimensional visualization and quantitative evaluation of the vessel.     

Wide dynamic range detection of bidirectional flow in Doppler optical coherence tomography using a two-dimensional Kasai estimator

We demonstrate extended axial flow velocity detection range in a time-domain Doppler optical coherence tomography (DOCT) system using a modified Kasai velocity estimator with computations in both the axial and transverse directions. For a DOCT system with an 8 kHz rapid-scanning optical delay line, bidirectional flow experiments showed a maximum detectable speed of >56 cm/s using the axial Kasai estimator without the occurrence of aliasing, while the transverse Kasai estimator preserved the approximately 7 microm/s minimum detectable velocity to slow flow. By using a combination of transverse Kasai and axial Kasai estimators, the velocity detection dynamic range was over 100 dB. Through a fiber-optic endoscopic catheter, in vivoM-mode transesophageal imaging of the pulsatile blood flow in rat aorta was demonstrated, for what is for the first time to our knowledge, with measured peak systolic blood flow velocity of >1 m/s, while maintaining good sensitivity to detect aortic wall motion at <2 mm/s, using this 2D Kasai technique.     

Doppler optical coherence tomography in cardiovascular applications

The study of flow dynamics in complex geometry vessels is highly important in various biomedical applications where the knowledge of the mechanic interactions between the moving fluid and the housing media plays a key role for the determination of the parameters of interest, including the effect of blood flow on the possible rupture of atherosclerotic plaques. Doppler Optical Coherence Tomography (DOCT), as a functional extension of Optical Coherence Tomography (OCT), is an optic, non-contact, noninvasive technique able to achieve detailed analysis of the flow/vessel interactions. It allows simultaneous high resolution imaging (∼10 μm typical) of the morphology and composition of the vessel and determination of the flow velocity distribution along the measured cross-section. We applied DOCT system to image high-resolution one-dimensional and multi-dimensional velocity distribution profiles of Newtonian and non-Newtonian fluids flowing in vessels with complex geometry, including Y-shaped and T-shaped vessels, vessels with aneurism, bifurcated vessels with deployed stent and scaffolds. The phantoms were built to mimic typical shapes of human blood vessels, enabling preliminary analysis of the interaction between flow dynamics and the (complex) geometry of the vessels and also to map the related velocity profiles at several inlet volume flow rates. Feasibility studies for quantitative observation of the turbulence of flows arising within the complex geometry vessels are discussed. In addition, DOCT technique was also applied for monitoring cerebral mouse blood flow in vivo. Two-dimensional DOCT images of complex flow velocity profiles in blood vessel phantoms and in vivo sub-cranial mouse blood flow velocities distributions are presented.     

Doppler optical coherence tomography with a micro-electro-mechanical membrane mirror for high-speed dynamic focus tracking

An elliptical microelectromechanical system (MEMS) membrane mirror is electrostatically actuated to dynamically adjust the optical beam focus and track the axial scanning of the coherence gate in a Doppler optical coherence tomography (DOCT) system at 8 kHz. The MEMS mirror is designed to maintain a constant numerical aperture of approximately 0.13 and a spot size of approximately 6.7 microm over an imaging depth of 1mm in water, which improves imaging performance in resolving microspheres in gel samples and Doppler shift estimation precision in a flow phantom. The mirror's small size (1.4 mm x 1 mm) will allow integration with endoscopic MEMS-DOCT for in vivo applications.     

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media

An optical Doppler tomography (ODT) system that permits imaging of fluid flow velocity in highly scattering media is described. ODT combines Doppler velocimetry with the high spatial resolution of low-coherence optical interferometry to measure fluid flow velocity at discrete spatial locations. Tomographic imaging of particle flow velocity within a circular conduit submerged 1 mm below the surface in a highly scattering phantom of Intralipid is demonstrated.     

Scattering of sound waves by a turbulent wake

In this investigation, the Doppler shifted power spectrum of the scattering cross-section is obtained for plane acoustic waves scattered by fluid flow fluctuations appropriate to a turbulent wake. The wake considered in this paper is assumed almost homogeneous and isotropic and of low Reynolds number.It is shown that the evaluation of the Doppler scattering cross-section essentially reduces to the calculation of the wave number converted and frequency shifted energy spectrum function of the turbulent flow fluctuations. In prescribing the low Reynolds number turbulence spectrum, inertial forces are assumed negligible. Convective effects of the macro-eddies, which cause a Doppler shift in the scattered waves, are considered using a Lagrangian-type of space-time velocity correlation.After finding the spectrum of turbulent fluctuations, the Doppler shifted power spectrum of the scattering cross-section, which characterizes the scattered waves, is obtained explicitly for the far field approximation.     

Light scattering by critical fluids in the presence of a uniform shear flow

Some predictions are made on dynamic light scattering by critical fluids in the presence of a shear flow. A Doppler shift broadening is predicted to occur whenever the scattering vector has a component along the direction of flow.     

Photoacoustic Doppler effect from flowing small light-absorbing particles

From the flow of a suspension of micrometer-scale carbon particles, the photoacoustic Doppler shift is observed. As predicted theoretically, the observed Doppler shift equals half of that in Doppler ultrasound and does not depend on the direction of laser illumination. This new physical phenomenon provides a basis for developing photoacoustic Doppler flowmetry, which can potentially be used for detecting fluid flow in optically scattering media and especially low-speed blood flow of relatively deep microcirculation in biological tissue.     

Effect of wall scattering on SNR in off-axis differential-type laser Doppler velocimetry

Though various properties and applications of laser Doppler velocimetry have been extensively studied in the past decade, there is little discussion on the effect of light scattering from the surface of a cell on Doppler beat signals or on methods of reducing it. In this paper, the effect of light scattering from the surface of the cell is treated as a background noise and is studied theoretically and experimentally on the detecting process of Doppler beat signals in off-axis differential-type laser Doppler velocimetry. Laser Doppler velocimetry of an off-axis type is verified to be effective for measurement of the flow velocity in the vicinity of a scattering wall. The effect of the light scattered from the wall surface on the signal-to-noise ratio (SNR) of Doppler beat signals is discussed in detail. The minimum distance, which is close to the wall and at which good Doppler beat signals can be obtained, is defined and determined quantitatively. This minimum distance is found to be strongly affected by the off-axis angle of the detecting optical system.     

Laser Doppler perfusion imaging with a complimentary metal oxide semiconductor image sensor

We utilized a complimentary metal oxide semiconductor video camera for fast flow imaging with the laser Doppler technique. A single sensor is used for both observation of the area of interest and measurements of the interference signal caused by dynamic light scattering from moving particles inside scattering objects. In particular, we demonstrate the possibility of imaging the distribution of the moving red blood cell concentration. This is a first step toward laser Doppler imaging without scanning parts, leading to a much faster imaging procedure than with existing mechanical laser Doppler perfusion imagers.     

Description of a Doppler rayleigh LIDAR for measuring winds in the middle atmosphere

The possibility to measure winds in the middle atmosphere with a Doppler LIDAR was demonstrated in 1989. It has been used since then to study the wave-mean flow interaction, in association with the Rayleigh LIDAR providing density and temperature and their fluctuations. The Doppler LIDAR relies on Rayleigh scattering from air molecules and was originally designed to cover the height range 25–60 km, a region where radars cannot operate. The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of two narrow band-passes of a single dual Fabry-Perot interferometer tuned to either side of the emitted laser line. Its extension to lower altitudes where Mie scattering is present is under study.     

Doppler-based lateral motion tracking for optical coherence tomography

Nonuniform lateral scanning of the probe beam in optical coherence tomography produces imaging artifacts and leads to a morphologically inaccurate representation of the sample. Here, we demonstrate a solution to this problem, which is based on the Doppler shift carried by the complex-valued depth-resolved scattering amplitude. Furthermore, we demonstrate the feasibility of Doppler flow velocity measurements in underlying flow channels while laterally scanning the imaging probe over large surfaces with arbitrary and varying velocity. Finally, we performed centimeters-long hand-held B-mode imaging of skin in vivo.     

光学相干层析多普勒成像功能拓展研究

光学多普勒成像(Optical Doppler tomography,ODT)是一种结合了光学相干层析成像技术(Opticalcoherence tomography,OCT)和多普勒流速仪的非侵入、非接触的成像技术,能够实现对高散介质组织内部的血管分布和血液流速的探测。阐述了基于数字希尔伯特变换的相位分离多普勒光学相干层析成像技术的工作原理,并且通过对玻璃毛细管和生物芯片微通道管中聚苯乙烯溶液流速的实验测量,准确测量管内微粒缓慢移动时的多普勒频移量,获得了玻璃管内和生物芯片微通道管中流速分布曲线,证实了所提方法的可行性。获取的多普勒图像具有较高的空间分辨力和速度分辨力,在未来的临床应用中有潜在的应用价值。     

Pulsatile flow measurement in hydraulic systems using a laser velocimeter

Servo valves find their major application in high performance hydraulic control systems where the accurate control of position, velocity and load is essential. Because of this, it is important to determine the dynamic characteristics of the servo valve (i.e. flow rate as a function of input command signal) more precisely.Previous techniques for this evaluation depended on monitoring spool position using linear transducers, since flow measurement techniques capable of following fast transients and oscillating (or pulsating) flows were not easily applied. The introduction of the laser Doppler anemometer changes this. Steady state flow measurement, using the dual beam scatter system, is now common but since it needs a spectrum analyser to measure the Doppler frequency it is not suitable for unsteady flow. Since hydraulic systems are relatively particle free, the Doppler signals are dreceived intermittently. Because of this, a signal processing system has been developed which measures the period of one or more oscillations in the frequency burst caused by a scattering particle passing through the control volume.Using this technique steady and unsteady laminar flow profiles have been measured in order to assess its effectiveness in the performance testing of hydraulic servo systems.     

Wavelet and model-based spectral analysis of color doppler optical coherence tomography

Color doppler optical coherence tomography (CD-OCT) uses time-frequency analysis (TFA) to extract motion-induced Doppler shifted in the interferometric OCT signal. In this paper, the performance of three TFAs are compared in a scattering flow phantom and in in vivo human retina: the short-time Fourier transform, the Morlet-wavelet transform, and the short-time MUSIC transform (STMT). The STMT is a new TFA that incorporates the MUSIC eigenfrequency estimator in a generalized short-time framework. The Morlet transform excels at identifying blood vessels, while the STMT is the most accurate predictor of Doppler shift frequency.     

Phase-referenced Doppler optical coherence tomography in scattering media

We present a fiber-based, low-coherence interferometer that significantly reduces phase noise by incorporating a second, narrowband, continuous-wave light source as a phase reference. By incorporating this interferometer into a Doppler OCT system, we demonstrate significant velocity noise reduction in reflective and scattering samples using processing techniques amenable to real-time implementation. We also demonstrate 90% suppression of velocity noise in a flow phantom.     

Speckle interferometry and Doppler diagnostics of microflows of scattering liquids

A relation for the correlation function of intensity fluctuations of the speckle field formed upon scattering of focused light radiation in blood flows in microvessels is obtained. The width of the intensity-fluctuation spectrum is shown to depend not only on the velocity of the flow, but also on its scattering characteristics. Simulation of the processes of scattering of light beams in random flows is performed by the Monte Carlo method. The expression for the first spectral moment of the Doppler signal is derived both on the basis of the theory of radiative-energy transfer and within the framework of the speckle-interference approach; comparison of the obtained results is carried out.     

The effect of blood acceleration on the ultrasound power Doppler spectrum

The purpose of the present work was to study the influence of blood acceleration and time window length on the power Doppler spectrum for Gaussian ultrasound beams. The work has been carried out on the basis of continuum model of the ultrasound scattering from inhomogeneities in fluid flow. Correlation function of fluctuations has been considered for uniformly accelerated scatterers, and the resulting power Doppler spectra have been calculated. It is shown that within the initial phase of systole uniformly accelerated slow blood flow in pulmonary artery and aorta tends to make the correlation function about 4.89 and 7.83 times wider, respectively, than the sensitivity function of typical probing system. Given peak flow velocities, the sensitivity function becomes, vice versa, about 4.34 and 3.84 times wider, respectively, then the correlation function. In these limiting cases, the resulting spectra can be considered as Gaussian. The optimal time window duration decreases with increasing acceleration of blood flow and equals to 11.62 and 7.54 ms for pulmonary artery and aorta, respectively. The width of the resulting power Doppler spectrum is shown to be defined mostly by the wave vector of the incident field, the duration of signal and the acceleration of scatterers in the case of low flow velocities. In the opposite case geometrical properties of probing field and the average velocity itself are more essential. In the sense of signal–noise ratio, the optimal duration of time window can be found. Abovementioned results may contribute to the improved techniques of Doppler ultrasound diagnostics of cardiovascular system.     

Optical correlation techniques in fluid dynamics

Three flow measurement techniques make use of fast digital correlators. The most widely spread is photon correlation velocimetry using crossed laser beams, and detecting Doppler shifted light scattered by small particles in the flow. Depending on the processing of the photon correlation output, this technique yields mean velocity, turbulence level, and even the detailed probability distribution of one velocity component. An improved data processing scheme is demonstrated on laminar vortex flow in a curved channel. In the second method, rate correlation based upon threshold crossings of a high pass filtered laser Doppler signal can be used to obtain velocity correlation functions. The most powerful set-up developed in our laboratory uses a phase locked loop type tracker and a multibit correlator to analyze time-dependent Taylor vortex flow. With two optical systems and trackers, cross-correlation functions reveal phase relations between different vortices. The last method makes use of refractive index fluctuations (eg in two phase flows) instead of scattering particles. Interferometry with bidirectional counting, and digital correlation and probability analysis, constitutes a new quantitative technique related to classical Schlieren methods. Measurements on a mixing flow of heated and cold air contribute new ideas to the theory of turbulent random phase screens.