Ensemble tracking for 2D vector velocity measurement: Experimental and initial clinical results

We describe a new method, called ensemble tracking, for estimating two-dimensional velocities with ultrasound. Compared to previous speckle tracking techniques, ensemble tracking measures motion over smaller times and distances, increasing maximum velocities and reducing errors due to echo decorrelation. Ensemble tracking uses parallel receive processing, 2D pattern matching, and interpolation of the resulting tracking grid to estimate sub-pixel speckle translations between successive ultrasonic acquisitions. In this study, small translations of a tissue mimicking phantom were quantified at transducer angles of 0 degrees , 45 degrees , and 90 degrees . Measurements over three parallel beam spacings and all transducer angles had mean errors from -4% to +11%, when parallel beam amplitudes were normalized. Such amplitude normalization substantially improved results at 45 degrees and 90 degrees . The amplitude, spacing, and correlation between the parallel beams were quantified, and their effects on the accuracy and precision of estimates are discussed. Finally, initial clinical results demonstrate the ability to track and display blood flow in the carotid artery.     

Ultrasound three-dimensional velocity measurements by featuretracking

This article describes a new angle-independent method suitable for three-dimensional (3-D) blood flow velocity measurement that tracks features of the ultrasonic speckle produced by a pulse echo system. In this method, a feature is identified and followed over time to detect motion. Other blood flow velocity measurement methods typically estimate velocity using one- (1-D) or two-dimensional (2-D) spatial and time information. Speckle decorrelation due to motion in the elevation dimension may hinder this estimate of the true 3-D blood flow velocity vector. Feature tracking is a 3-D method with the ability to measure the true blood velocity vector rather than a projection onto a line or plane. Off-line experiments using a tissue phantom and a real-time volumetric ultrasound imaging system have shown that the local maximum detected value of the speckle signal may be identified and tracked for measuring velocities typical of human blood flow. The limitations of feature tracking, including the uncertainty of the peak location and the duration of the local maxima are discussed. An analysis of the expected error using this method is given     

Effect of the beam-vessel angle on the received acoustic signalfrom blood

In order to explore the feasibility of algorithms to determine the three dimensional (3D) velocity magnitude from the received ultrasonic blood echo from a single line of sight, the signal from small sample volumes is studied as a function of beam-vessel angle. As opposed to previous treatments of the effect of the beam-vessel angle on the received acoustic signal, a wideband signal is transmitted and the returned signal in each sample volume is analyzed. High-resolution experimental M-mode images of radio-frequency (rf) echo signals are used to visualize the flow in individual regions of interest. These experiments confirm the predictions of a theoretical model for the signal and its second moment. It is shown that the two major effects limiting the correlated signal interval are the spread of axial velocities within the sample volume and the transit time across the lateral beam width. Particularly for small beam-vessel angles, the spread of velocities limits the correlated signal interval. In addition, the experimental results demonstrate that accurate velocity estimation for low volume flow rates and particularly for large beam-vessel angles may involve detection of changes in the correlation magnitude. For low volume flow rates, the shape of the correlation surface can be affected by small regions of blood with a strong scattering intensity located near the initial region of interest     

PSF dedicated to estimation of displacement vectors for tissue elasticity imaging with ultrasound

This paper investigates a new approach devoted to displacement vector estimation in ultrasound imaging. The main idea is to adapt the image formation to a given displacement estimation method to increase the precision of the estimation. The displacement is identified as the zero crossing of the phase of the complex cross-correlation between signals extracted from the lateral direction of the ultrasound RF image. For precise displacement estimation, a linearity of the phase slope is needed as well as a high phase slope. Consequently, a particular point spread function (PSF) dedicated to this estimator is designed. This PSF, showing oscillations in the lateral direction, leads to synthesis of lateral RF signals. The estimation is included in a 2-D displacement vector estimation method. The improvement of this approach is evaluated quantitatively by simulation studies. A comparison with a speckle tracking technique is also presented. The lateral oscillations improve both the speckle tracking estimation and our 2-D estimation method. Using our dedicated images, the precision of the estimation is improved by reducing the standard deviation of the lateral displacement error by a factor of 2 for speckle tracking and more than 3 with our method compared to using conventional images. Our method performs 7 times better than speckle tracking. Experimentally, the improvement in the case of a pure lateral translation reaches a factor of 7. Finally, the experimental feasibility of the 2-D displacement vector estimation is demonstrated on data acquired from a Cryogel phantom.     

Two bit correlation-an adaptive time delay estimation

Time delay estimation is a very important operation in ultrasound time-domain flow mapping and correction of phase aberration of an array transducer. As the interest increases in the application of one and a half-dimensional (1.5-D) and two-dimensional (2-D) array transducers to improving image quality and three-dimensional (3-D) imaging, the need of simple, fast, and sufficiently accurate algorithms for real-time time delay estimation becomes exceedingly crucial. In this paper, we present an adaptive time-delay estimation algorithm which minimizes the problem of noise sensitivity associated with the one bit correlation while retaining simplicity in implementation. This algorithm converts each sample datum into a two bit representation including the sign of the sample and an adaptively selected threshold. A bit pattern correlation operation is applied to find the time delay between two engaged signals. By using the criterion of misregistration as an indicator, we are able to show that the proposed algorithm is better than one bit correlation in susceptibility to noise level. Analytical results show that the improvement in reducing misregistration of the two bit correlation over its counterpart is consistent over a wide range of noise level. This is achieved by an adaptive adjustment of the threshold to accommodate signal corruption due to noise. The analytical results are corroborated by results from simulating the blood as a random distribution of red blood cells. Finally, we also present a memory-based architecture to implement the two bit correlation algorithm whose computation time does not depend upon the time delay of the signals to be correlated     

An extended autocorrelation method for estimation of blood velocity

The conventional autocorrelation method (AM) to estimate the blood velocity for color flow imaging (CFI) is based on the phase estimation of the autocorrelation function. In this paper, a new extended autocorrelation method (EAM) that uses both phase and magnitude of the two dimensional (depth and temporal direction) autocorrelation function for estimating the blood velocity is presented. It is shown that the EAM has similar performance as the cross-correlation method (CCM). Both of them have smaller estimation variance than the AM and have the ability to estimate velocities beyond the Nyquist velocity, but the EAM is more computationally efficient than the CCM. A 2-D blood flow signal with rectilinear velocity including the transit time effect has also been simulated and the results are presented in this paper. For comparison, the EAM and the CCM have been applied to the simulated signals in which the flow velocities are up to four times the Nyquist velocity. The EAM has been further verified by experimental RF data from the subclavian artery     

2-D motion estimation using two parallel receive beams

We describe a method for estimating 2-D target motion using ultrasound. The method is based on previous ensemble tracking techniques, which required at least four parallel receive beams and 2-D pattern matching. In contrast, the method described requires only two parallel receive beams and 1-D pattern matching. Two 1-D searches are performed, one in each lateral direction. The direction yielding the best match indicates the lateral direction of motion. Interpolation provides sub-pixel magnitude resolution. We compared the two beam method with the four beam method using a translating speckle target at three different parallel beam steering angles and transducer angles of 0, 45, and 90 degrees. The largest differences were found at 90 degrees, where the two beam method was generally more accurate and precise than the four beam method and also less prone to directional errors at small translations. We also examined the performance of both methods in a laminar flow phantom. Results indicated that the two beam method was more accurate in measuring the flow angle when the flow velocity was small. Computer simulations supported the experimental findings. The poorer performance of the four beam method was attributed to differences in correlation among the parallel beams. Specifically, center beams 2 and 3 correlated better with each other than with the outer beams. Because the four beam method used a comparison of a kernel region in beam pair 2-3 with two different beam pairs 1-2 and 3-4, the 2-to-1 and 3-to-4 components of this comparison increased the incidence of directional errors, especially at small translations. The two beam method used a comparison between only two beams and so was not subject to this source of error. Finally, the two beam method did not require amplitude normalization, as was necessary for the four beam method, when the two beams were chosen symmetric to the transmit axis. We conclude that two beam ensemble tracking can accurately estimate motion using only two parallel receive beams.     

Correlation distance of dynamic speckles

The translational and boiling motions of dynamic speckles produced in the Fresnel diffraction field under illumination of a Gaussian beam are investigated in detail. The speckle motion is analyzed from the space-time cross-correlation function of speckle intensity fluctuations detected at the two points in the receiving plane. The correlation distance of time-varying speckles is compared with the translation distance of the spatial speckle pattern. The optical conditions for the translational and boiling motions of dynamic speckles are examined and expressed in a diagram. The characteristics for the correlation distance of time-varying speckle intensity fluctuations are finally verified by several experiments.     

Lateral blood flow velocity estimation based on ultrasound speckle size change with scan velocity

Conventional (Doppler-based) blood flow velocity measurement methods using ultrasound are capable of resolving the axial component (i.e., that aligned with the ultrasound propagation direction) of the blood flow velocity vector. However, these methods are incapable of detecting blood flow in the direction normal to the ultrasound beam. In addition, these methods require repeated pulse-echo interrogation at the same spatial location. A new method has been introduced which estimates the lateral component of blood flow within a single image frame using the observation that the speckle pattern corresponding to blood reflectors (typically red blood cells) stretches (i.e., is smeared) if the blood is moving in the same direction as the electronically-controlled transducer line selection in a 2-D image. The situation is analogous to the observed distortion of a subject photographed with a moving camera. The results of previous research showed a linear relationship between the stretch factor (increase in lateral speckle size) and blood flow velocity. However, errors exist in the estimation when used to measure blood flow velocity. In this paper, the relationship between speckle size and blood flow velocity is investigated further with both simulated flow data and measurements from a blood flow phantom. It can be seen that: 1) when the blood flow velocity is much greater than the scan velocity (spatial rate of A-line acquisition), the velocity will be significantly underestimated because of speckle decorrelation caused by quick blood movement out of the ultrasound beam; 2) modeled flow gradients increase the average estimation error from a range between 1.4% and 4.4%, to a range between 4.4% and 6.8%; and 3) estimation performance in a blood flow phantom with both flow gradients and random motion of scatterers increases the average estimation error to between 6.1% and 7.8%. Initial attempts at a multiple-scan strategy for estimating flow by a least-squares model suggest the possibility of increased accuracy using multiple scan velocities.     

Comparison of 3-D multi-lag cross- correlation and speckle brightness aberration correction algorithms on static and moving targets

Phase correction has the potential to increase the image quality of 3-D ultrasound, especially transcranial ultrasound. We implemented and compared 2 algorithms for aberration correction, multi-lag cross-correlation and speckle brightness, using static and moving targets. We corrected three 75-ns rms electronic aberrators with full-width at half-maximum (FWHM) auto-correlation lengths of 1.35, 2.7, and 5.4 mm. Cross-correlation proved the better algorithm at 2.7 and 5.4 mm correlation lengths (P < 0.05). Static cross-correlation performed better than moving-target cross-correlation at the 2.7 mm correlation length (P < 0.05). Finally, we compared the static and moving-target cross-correlation on a flow phantom with a skull casting aberrator. Using signal from static targets, the correction resulted in an average contrast increase of 22.2%, compared with 13.2% using signal from moving targets. The contrast-to-noise ratio (CNR) increased by 20.5% and 12.8% using static and moving targets, respectively. Doppler signal strength increased by 5.6% and 4.9% for the static and moving-targets methods, respectively.     

Analysis of correlation coefficient filtering in elasticity imaging

Correlation-based speckle tracking methods are commonly used in elasticity imaging to estimate displacements. In the presence of local strain, a larger window size results in larger displacement error. To reduce tracking error, we proposed a short correlation window followed by a correlation coefficient filter. Although simulation and experimental results demonstrated the efficacy of the method, it was not clear why correlation coefficient filtering reduces tracking error since tracking error increases if normalization before filtering is not applied. In this paper, we analyzed tracking errors by estimating phase variances of the cross-correlation function and the correlation coefficient at the true time lag based on statistical properties of these functions' real and imaginary parts. The role of normalization is clarified by identifying the effect of the cross-correlation function's amplitude fluctuation on the function's imaginary part. Furthermore, we present analytic forms for predicting axial displacement error as a function of strain, system parameters (signal-to-noise ratio, center frequency, and signal and noise bandwidths), and tracking parameters (window and filter sizes) for cases with and without normalization before filtering. Simulation results correspond to theory well for both noise-free cases and general cases with an empirical correction term included for strains up to 4%.     

Estimation and reduction of decorrelation effect due to tissue lateral displacement in elastography

In cross-correlation based elastography, the quality of the strain image is degraded by the distortion of echo waveforms due to tissue axial and lateral displacement. To study the effects of tissue lateral displacement on echo decorrelation, a tissue axial stretching model is developed and a concept called correlation signal-to-noise ratio (CSNR) is introduced to quantify the decorrelation effect due to tissue lateral displacement. A computer simulation based on the tissue stretching model is carried out to study the influence of several important elastographic parameters on echo decorrelation due to tissue lateral displacement. Finally, guided by the CSNR concept, a 2-D spatial comprehensive cross-correlation method is proposed to reduce the decorrelation noise. Results indicate that CSNR can be used as a quality indicator of elastography and the 2-D spatial comprehensive cross-correlation method can effectively reduce the decorrelation noise while slightly decreasing the lateral resolution of the strain image     

High frame-rate blood vector velocity imaging using plane waves: simulations and preliminary experiments

Conventional ultrasound methods for acquiring color images of blood velocity are limited by a relatively low frame-rate and are restricted to give velocity estimates along the ultrasound beam direction only. To circumvent these limitations, the method presented in this paper uses 3 techniques: 1) The ultrasound is not focused during the transmissions of the ultrasound signals; 2) A 13-bit Barker code is transmitted simultaneously from each transducer element; and 3) The 2-D vector velocity of the blood is estimated using 2-D cross-correlation. A parameter study was performed using the Field II program, and performance of the method was investigated when a virtual blood vessel was scanned by a linear array transducer. An improved parameter set for the method was identified from the parameter study, and a flow rig measurement was performed using the same improved setup as in the simulations. Finally, the common carotid artery of a healthy male was scanned with a scan sequence that satisfies the limits set by the Food and Drug Administration. Vector velocity images were obtained with a frame-rate of 100 Hz where 40 speckle images are used for each vector velocity image. It was found that the blood flow approximately followed the vessel wall, and that maximum velocity was approximately 1 m/s, which is a normal value for a healthy person. To further evaluate the method, the test person was scanned with magnetic resonance (MR) angiography. The volume flow derived from the MR scanning was compared with that from the ultrasound scanning. A deviation of 9% between the 2 volume flow estimates was found.     

Three-dimensional motion measurements using feature tracking

Feature tracking was developed to efficiently compute motion measurements from volumetric ultrasound images. Prior studies have demonstrated the motion magnitude accuracy and computation speed of feature tracking. However, the previous feature tracking implementations were limited by performance of their calculations in rectilinear coordinates. Also, the previous feature tracking approaches did not fully explore the three dimensional (3- D) nature of volumetric image analysis or utilize the 3-D directional information from the tracking calculations. This study presents an improved feature tracking method which achieves further computation speed gains by performing all calculations in the native spherical coordinates of the 3-D ultrasound image. The novel method utilizes a statistical analysis of tracked directions of motion to achieve better rejection of false tracking matches. Results from in vitro tracking of a speckle target show that the new feature tracking method is significantly faster than correlation search and can accurately determine target motion magnitude and 3-D direction.     

Development and evaluation of a high-frequency ultrasound-based system for in vivo strain imaging of the skin

The elastic properties of skin are of great interest in dermatology because they are affected by many pathological conditions. In this paper, a technique for in vivo mechanical strain imaging of the skin based on high-frequency ultrasound (HFUS) is presented. Elastic skin properties are assessed applying suction to the skin surface with a stepwise increased vacuum and estimating the resulting displacements in a spatially resolved manner. Acquired radio frequency (RF) echo signals and their envelope are analyzed for this purpose. A computer-controlled vacuum system with a digital pressure control loop was developed for precise and reproducible deformation. In a first processing step, the skin surface is segmented. Local axial strains inside the skin are estimated from axial displacements, which are estimated from consecutive echo signal frames analyzing the phase of the complex cross correlation function of analytical echo signals. Furthermore, speckle tracking is applied to estimate axial and lateral displacements and to quantify axial and lateral strains. The correlation coefficient of windowed echo signals compensated for displacements are used as a measure to validate the estimated strains, which is essential to accomplish reliable in vivo measurements. Phantom experiments were performed to validate the proposed technique. Results of in vivo measurements are presented, showing the potential for mechanical strain imaging in the skin in vivo.     

激光散斑测量电流变液中颗粒的运动速度

利用激光散斑测量装置,观察悬浮在电流变液中的淀粉颗粒在外加直流电场下的运动情况,表明当激光穿过电流变液时可以形成散斑。利用激光散斑测速法(LSV)对不同外加电场强度和不同浓度的淀粉电流变液中颗粒的运动速度进行了测量,通过自编的互相关计算软件对散斑图像进行了分析,获得了淀粉颗粒沿电场方向和垂直于电场方向的运动速度。实验结果表明在同一电场作用下,淀粉颗粒运动速度随时间的变化呈现震荡递减的趋势;并且存在一个临界颗粒浓度和临界电场强度,当低于此临界值时,颗粒运动速度增加,反之则减小。     

Blood flow imaging--A new real-time, 2-D flow imaging technique

This paper presents a new method for the visualization of two-dimensional (2-D) blood flow in ultrasound imaging systems called blood flow imaging (BFI). Conventional methods of color flow imaging (CFI) and power Doppler (PD) techniques are limited as the velocity component transversal to the ultrasound beam cannot be estimated from the received Doppler signal. The BFI relies on the preservation and display of the speckle pattern originating from the blood flow scatterer signal, and it provides qualitative information of the blood flow distribution and movement in any direction of the image. By displaying speckle pattern images acquired with a high frame rate in slow motion, the blood flow movement can be visually tracked from frame to frame. The BFI is easily combined with conventional CFI and PD methods, and the resulting display modes have been shown to have several advantages compared to CFI or PD methods alone. Two different display modes have been implemented: one combining BFI with conventional CFI, and one combining BFI with PD. Initial clinical trials have been performed to assess the clinical usefulness of BFI. The method especially has potential in vascular imaging, but it also shows potential in other clinical applications.     

2-D high-frame-rate dynamic elastography using delay compensated and angularly compounded motion vectors: preliminary results

This paper describes a new ultrasound-based system for high-frame-rate measurement of periodic motion in 2-D for tissue elasticity imaging. Similarly to conventional 2-D flow vector imaging, the system acquires the RF signals from the region of interest at multiple steering angles. A custom sector subdivision technique is used to increase the temporal resolution while keeping the total acquisition time within the range suitable for real-time applications. Within each sector, 1-D motion is estimated along the beam direction. The intra- and inter-sector delays are compensated using our recently introduced delay compensation algorithm. In-plane 2-D motion vectors are then reconstructed from these delay-compensated 1-D motions. We show that Young's modulus images can be reconstructed from these 2-D motion vectors using local inversion algorithms. The performance of the system is validated quantitatively using a commercial flow phantom and a commercial elasticity phantom. At the frame rate of 1667 Hz, the estimated flow velocities with the system are in agreement with the velocity measured with a pulsed-wave Doppler imaging mode of a commercial ultrasound machine with manual angle correction. At the frame rate of 1250 Hz, phantom Young's moduli of 29, 6, and 54 kPa for the background, the soft inclusion, and the hard inclusion, are estimated to be 30, 11, and 53 kPa, respectively.     

Angular correlation function of speckle patterns scattered from a one-dimensional rough dielectric film on a glass substrate

We present the experimental results of the angular correlation function of far-field speckle patterns scattered by a one-dimensionally random rough surface of a thin dielectric film on a glass substrate when a polarized beam of light is incident upon the rough surface from vacuum. This surface, which separates the vacuum and the dielectric, is rough enough that only diffused speckles are observed. The experiment for the correlation measurement was set up to make use of a CCD camera to obtain the image of the speckle pattern in the specular direction for each given angle of incidence; the cross-correlation function is then calculated from the digitized images. It is found that the intensity correlation functions exhibit two distinct maxima: one arises from the autocorrelation and the other from the reciprocity condition. It is also found that different scattering processes give rise to quite different correlation functions: multiple-scattering processes produce narrow peaks with secondary maxima and single-scattering processes produce relatively broad peaks.