Measurement of Wall Shear Stress Exerted by Flowing Blood in the Human Carotid Artery: Ultrasound Doppler Velocimetry and Echo Particle Image Velocimetry

Vascular endothelial cells lining the arteries are sensitive to wall shear stress (WSS) exerted by flowing blood. An important component of the pathophysiology of vascular diseases, WSS is commonly estimated by centerline ultrasound Doppler velocimetry (UDV). However, the accuracy of this method is uncertain. We have previously validated the use of a novel, ultrasound-based, particle image velocimetry technique (echo PIV) to compute 2-D velocity vector fields, which can easily be converted into WSS data. We compared WSS data derived from UDV and echo PIV in the common carotid artery of 27 healthy participants. Compared with echo PIV, time-averaged WSS was lower using UDV (28 ± 35%). Echo PIV revealed that this was due to considerable spatiotemporal variation in the flow velocity profile, contrary to the assumption that flow is steady and the velocity profile is parabolic throughout the cardiac cycle. The largest WSS underestimation by UDV was found during peak systole (118 ± 16%) and the smallest during mid-diastole (4.3± 46%). The UDV method underestimated WSS for the accelerating and decelerating systolic measurements (68 ± 30% and 24 ± 51%), whereas WSS was overestimated for end-diastolic measurements (?44 ± 55%). Our data indicate that UDV estimates of WSS provided limited and largely inaccurate information about WSS and that the complex spatiotemporal flow patterns do not fit well with traditional assumptions about blood flow in arteries. Echo PIV-derived WSS provides detailed information about this important but poorly understood stimulus that influences vascular endothelial pathophysiology.     

Flow Velocity Mapping Using Contrast Enhanced High-Frame-Rate Plane Wave Ultrasound and Image Tracking: Methods and Initial in Vitro and in Vivo Evaluation

Ultrasound imaging is the most widely used method for visualising and quantifying blood flow in medical practice, but existing techniques have various limitations in terms of imaging sensitivity, field of view, flow angle dependence, and imaging depth. In this study, we developed an ultrasound imaging velocimetry approach capable of visualising and quantifying dynamic flow, by combining high-frame-rate plane wave ultrasound imaging, microbubble contrast agents, pulse inversion contrast imaging and speckle image tracking algorithms. The system was initially evaluated in vitro on both straight and carotid-mimicking vessels with steady and pulsatile flows and in vivo in the rabbit aorta. Colour and spectral Doppler measurements were also made. Initial flow mapping results were compared with theoretical prediction and reference Doppler measurements and indicate the potential of the new system as a highly sensitive, accurate, angle-independent and full field-of-view velocity mapping tool capable of tracking and quantifying fast and dynamic flows.     

2-D Left Ventricular Flow Estimation by Combining Speckle Tracking With Navier–Stokes-Based Regularization: An In Silico,In Vitro and In Vivo Study

Despite the availability of multiple ultrasound approaches to left ventricular (LV) flow characterization in two dimensions, this technique remains in its childhood and further developments seem warranted. This article describes a new methodology for tracking the 2-D LV flow field based on ultrasound data. Hereto, a standard speckle tracking algorithm was modified by using a dynamic kernel embedding Navier–Stokes-based regularization in an iterative manner. The performance of the proposed approach was first quantified in synthetic ultrasound data based on a computational fluid dynamics model of LV flow. Next, an experimental flow phantom setup mimicking the normal human heart was used for experimental validation by employing simultaneous optical particle image velocimetry as a standard reference technique. Finally, the applicability of the approach was tested in a clinical setting. On the basis of the simulated data, pointwise evaluation of the estimated velocity vectors correlated well (mean r = 0.84) with the computational fluid dynamics measurement. During the filling period of the left ventricle, the properties of the main vortex obtained from the proposed method were also measured, and their correlations with the reference measurement were also calculated (radius, r = 0.96; circulation, r = 0.85; weighted center, r = 0.81). In vitro results at 60 bpm during one cardiac cycle confirmed that the algorithm properly measures typical characteristics of the vortex (radius, r = 0.60; circulation, r = 0.81; weighted center, r = 0.92). Preliminary qualitative results on clinical data revealed physiologic flow fields.     

Blood Flow Quantification with High-Frame-Rate,Contrast-Enhanced Ultrasound Velocimetry in Stented Aortoiliac Arteries: In Vivo Feasibility

Local flow patterns influence stent patency, while blood flow quantification in stents is challenging. The aim of this study was to investigate the feasibility of 2-D blood flow quantification using high-frame-rate, contrast-enhanced ultrasound (HFR-CEUS) and particle image velocimetry (PIV), or echoPIV, in patients with aortoiliac stents. HFR-CEUS measurements were performed at 129 locations in 62 patients. Two-dimensional blood flow velocity fields were obtained using echoPIV. Visual inspection was performed by five observers to evaluate feasibility. The contrast-to-background ratio and average vector correlation were calculated and compared between stented and native vessel segments. Flow quantification with echoPIV was feasible in 128 of 129 locations (99%), with optimal quantification in 40 of 129 locations (31%). Partial quantification was achieved in 88 of 129 locations (68%), where one or multiple limiting issues occurred (not related to the stent) including loss of correlation during systole (57/129), short vessel segments (20/129), loss of contrast during diastole (20/129) and shadow regions (20/129). The contrast-to-background ratio and vector correlation were lower downstream in the imaged blood vessel, independent of the location of the stent. In conclusion, echoPIV was feasible in stents placed in the aortoiliac region, and the stents did not adversely affect flow tracking.     

Ultrasound Imaging Velocimetry: Effect of Beam Sweeping on Velocity Estimation

As an emerging flow-mapping tool that can penetrate deep into optically opaque media such as human tissue, ultrasound imaging velocimetry has promise in various clinical applications. Previous studies have shown that errors occur in velocity estimation, but the causes have not been well characterised. In this study, the error in velocity estimation resulting from ultrasound beam sweeping in image acquisition is quantitatively investigated. The effects on velocity estimation of the speed and direction of beam sweeping relative to those of the flow are studied through simulation and experiment. The results indicate that a relative error in velocity estimation of up to 20% can be expected. Correction methods to reduce the errors under steady flow conditions are proposed and evaluated. Errors in flow estimation under unsteady flow are discussed.     

Aortic Valve Stenosis Increases Helical Flow and Flow Complexity: A Study of Intra-Operative Cardiac Vector Flow Imaging

Aortic valve stenosis alters blood flow in the ascending aorta. Using intra-operative vector flow imaging on the ascending aorta, secondary helical flow during peak systole and diastole, as well as flow complexity of primary flow during systole, were investigated in patients with normal, stenotic and replaced aortic valves. Peak systolic helical flow, diastolic helical flow and flow complexity during systole differed between the groups (p < 0.0001), and correlated to peak systolic velocity (R = 0.94, 0.87 and 0.88, respectively). The study indicates that aortic valve stenosis increases helical flow and flow complexity, which are measurable with vector flow imaging. For assessment of aortic stenosis and optimization of valve surgery, vector flow imaging may be useful.     

Use of Optical Flow to Estimate Continuous Changes in Muscle Thickness from Ultrasound Image Sequences

Muscle thickness is one of the most widely used parameters for quantifying muscle function. Ultrasonography is frequently used to estimate changes in muscle thickness in both static and dynamic contractions. Conventionally, most such measurements are conducted by manual analysis of ultrasound images. This manual approach is time consuming, subjective, susceptible to error and not suitable for measuring dynamic change. In this study, we developed an automated tracking method based on an optical flow algorithm using an affine motion model. The goal of the study was to evaluate the performance of the proposed method by comparing it with the manual approach and by determining its repeatability. Real-time B-mode ultrasound was used to examine the rectus femoris during voluntary contraction. The coefficient of multiple correlation (CMC) was used to quantify the level of agreement between the two methods and the repeatability of the proposed method. The two methods were also compared by linear regression and Bland-Altman analysis. The findings indicated that the results obtained using the proposed method were in good agreement with those obtained using the manual approach (CMC = 0.97 ± 0.02, difference = −0.06 ± 0.22 mm) and were highly repeatable (CMC = 0.91 ± 0.07). In conclusion, the automated method proposed here provides an accurate, highly repeatable and efficient approach to the estimation of muscle thickness during muscle contraction.     

彩色多普勒显像检测下肢深静脉瓣功能不全的应用价值

目的 评价彩超检测下肢深静脉瓣膜功能不全的应用价值。方法 以速度5－25cm/s,时间大于0．5s为诊断标准。对45例深静脉瓣功能不全患者原发性35例共40条患肢,继发性10例,共12条患肢进行检测及对比分析。结果 CDFI不但能清晰显示静脉瓣返流频谱,而且可显示双向彩色血流信号。结论 对本病CDFI可为临床手术和治疗提供可靠依据。     

Echo Particle Image Velocimetry for Estimation of Carotid Artery Wall Shear Stress: Repeatability,Reproducibility and Comparison with Phase-Contrast Magnetic Resonance Imaging

Measurement of hemodynamic wall shear stress (WSS) is important in investigating the role of WSS in the initiation and progression of atherosclerosis. Echo particle image velocimetry (echo PIV) is a novel ultrasound-based technique for measuring WSS in vivo that has previously been validated in vitro using the standard optical PIV technique. We evaluated the repeatability and reproducibility of echo PIV for measuring WSS in the human common carotid artery. We measured WSS in 28 healthy participants (18 males and 10 females, mean age: 56 ± 12 y). Echo PIV was highly repeatable, with an intra-observer variability of 1.0 ± 0.1 dyn/cm² for peak systolic (maximum), 0.9 dyn/cm² for mean and 0.5 dyn/cm² for end-diastolic (minimum) WSS measurements. Likewise, echo PIV was reproducible, with a low inter-observer variability (max: 2.0 ± 0.2 dyn/cm², mean: 1.3 ± 0.1 dyn/cm², end-diastolic: 0.7 dyn/cm²) and more variable inter-scan (test–retest) variability (max: 7.1 ± 2.3 dyn/cm², mean: 2.9 ± 0.4 dyn/cm², min: 1.5 ± 0.1 dyn/cm²). We compared echo PIV with the reference method, phase-contrast magnetic resonance imaging (PC-MRI); echo PIV-based WSS measurements agreed qualitatively with PC-MRI measurements (r = 0.89, p < 0.05). Significant differences were observed in some WSS measurements (echo PIV vs. PC-MRI): WSS at peak systole: 21 ± 7.0 dyn/cm² vs. 15 ± 5.0 dyn/cm²; time-averaged WSS: 8.9 ± 3.0 dyn/cm² vs. 7.1 ± 3.0 dyn/cm² (p < 0.05); WSS at end diastole: 3.8 ± 2.8 dyn/cm² vs. 3.9 ± 2 dyn/cm² (p > 0.05). For the first time, we report that echo PIV can measure WSS with good repeatability and reproducibility in adult humans with a broad age range. Echo PIV is feasible in humans and offers an easy-to-use, ultrasound-based, quantitative technique for measuring WSS in vivo in humans with good repeatability and reproducibility.     

Two-Dimensional Flow Imaging in the Carotid Bifurcation Using a Combined Speckle Tracking and Phase-Shift Estimator: A Study Based on Ultrasound Simulations and in vivo Analysis

A two-dimensional (2-D) blood velocity estimator is presented combining speckle tracking (ST) and phase-shift estimation (PE) to measure lateral (vx) and axial (vz) velocities respectively. Estimator properties were assessed in a carotid bifurcation using ultrasound simulations based on computational fluid dynamics, allowing validation toward a ground truth. Simulation results were supported with in vivo data of a healthy carotid. ST and PE estimates were combined as: (1) vx from 2D-ST and vz from PE, (2) vx from 2D-ST and vz from PE with aliasing correction based on ST and (3) vz from PE and only lateral ST for vx. Regression analysis showed a 35% to 77% decrease in standard deviation for vz for PE compared with ST. Aliasing correction based on ST improved results but also introduced spurious artifacts. A marginal decrease in performance was observed when only tracking laterally. Further work will focus on in vivo trials in patients with carotid plaques. (E-mail: abigail.swillens@ugent.be)     

门静脉海绵样变性的二维超声彩色多普勒血流显像的诊断

本文应用二维超声、彩色多普勒血流显像诊断的１７例门静脉海绵样变性，其中７例为先天性，肝脏损害轻微，１０例继发性，肝脏损害严重。手术证实５例，ＣＴ及ＤＳＡ各证实５例。门静脉海绵样变性二维超声显像肝门区结构紊乱，呈蜂窝或盘曲管状液性暗区；ＣＤＦＩ呈红蓝相间彩色血流图像；脉冲多普勒在蜂窝状或盘曲的管状暗区内显示平坦的连续带状门静脉频谱；ＰＤＩ在病变区以星点状、短线状彩色血流信号充填，血管网络连续性好。本文认为，二维超声、ＣＤＦＩ、脉冲多普勒及ＰＤＩ可以准确地为门静脉海绵样变性做出诊断。     

下肢动脉血栓形成所致血管狭窄的彩超诊断及定量分析

本文应用彩超检查了下肢动脉血栓形成患者50例。二维图像显示血栓形成；彩色多普勒（CD-FI）彩色血流束包绕血栓，血栓附着于血管壁导致管腔狭窄。狭窄度＜25％时，远端脉冲多普勒频谱无明显变化；狭窄度达25％～50％时，动脉远端血流速度下降，但脉冲多普勒频谱无原则变化；狭窄度＞50％时，动脉远端脉冲多普勒频谱为收缩期峰值速度降低的单相波形；闭塞时，血流信号消失。讨论了血栓形成致血管狭窄程度的分级及其与临床体征的相关性。     

超声测量前列腺体积——三种方法的精确度比较及误差原因分析

目的：本文通过比较经腹部超声、经直肠端射式及经直肠双平面超声测量前列腺体积的精确度,找出其测量误差原因和改进的方法。方法：对50例病人分别用经腹部凸阵探头、经直肠端射式及经直肠双平面探头进行前列腺体积的超声测量,同时以面积容积法的测值作为金标准对照。结果：三种不同方法测量前列腺体积的测值与金标准测值比较,其差值绝对值均数在三组之间有高度显著意义（P＜0．01）;当把前列腺边界显示不清者剔除以后,其差值绝对值均数在三组之间无显著意义（P＞0．05）。结论：三种不同超声方法测量前列腺体积,其测值存在一定误差,其误差原因与前列腺边界清晰与否关系甚大。     

Electromagnetic Interference of Digital and Analog Cellular Telephones with Implantable Cardioverter Defibrillators: In Vitro and In Vivo Studies

The present study examines the potential electromagnetic interference effects induced by cellular telephones on ICDs. We developed ad hoc protocols to conduct both in vitro and in vivo trials on most of the implantable cardioverter defibrillators available on the international market. Trials were conducted with three cellular telephones: two GSM (Global System for Mobile Communication) and one TACS (Total Access Communication System). A human trunk simulator was used to carry out in vitro observations on six ICDs from five manufacturers. In vivo tests were conducted on 13 informed patients with eight different ICD models. During the trials in air, GSM telephones induced interference effects on 4 out of the 6 cardioverter defibrillators tested. Specifically, pulse inhibition, reprogramming, false ventricular fibrillation, and ventricular tachycardia detections occurred, which would have entailed inappropriate therapy delivery had this been activated. Effects were circumscribed to the area closely surrounding the connectors. When the ICD was immersed in saline solution, no effects were observed. Three cases of just ventricular triggering with the interfering signal were observed in vivo.     

Non-Invasive Identification of Vulnerable Atherosclerotic Plaques Using Texture Analysis in Ultrasound Carotid Elastography: An In Vivo Feasibility Study Validated by Magnetic Resonance Imaging

The aims of this study were to quantify the textural information of strain rate images in ultrasound carotid elastography and evaluate the feasibility of using the textural features in discriminating stable and vulnerable plaques with magnetic resonance imaging as an in vivo reference. Ultrasound radiofrequency data were acquired in 80 carotid plaques from 52 patients, mainly in the longitudinal imaging view, and axial strain rate images were estimated with an ultrasound carotid elastography technique based on an optical flow algorithm. Four textural features of strain rate images—contrast, homogeneity, correlation and angular second moment—were derived based on the gray-level co-occurrence matrix in plaque regions to quantify the deformation distribution pattern. Conventional elastographic indices based on the magnitude of the absolute strain rate, such as the maximum, mean, median, standard deviation and 99th percentile of the axial strain rate, were also obtained for comparison. Composition measurement with magnetic resonance imaging identified 30 plaques as vulnerable and the other 50 as stable. The four textural features, as well as the magnitude of strain rate images, significantly differed between the two groups of plaques. The best performing features for plaque classification were found to be the contrast and 99th percentile of the absolute strain rate, with a comparative area under the receiver operating characteristic curve of 0.81; a slightly higher maximum accuracy of plaque classification can be achieved by the textural feature of contrast (83.8% vs. 81.3%). The results indicate that the use of texture analysis in plaque classification is feasible and that larger local deformations and higher level of complexity in deformation patterns (associated with the elastic or stiffness heterogeneity of plaque tissues) are more likely to occur in vulnerable plaques.     

Intra-Operative Vector Flow Imaging Using Ultrasound of the Ascending Aorta among 40 Patients with Normal,Stenotic and Replaced Aortic Valves

Stenosis of the aortic valve gives rise to more complex blood flows with increased velocities. The angle-independent vector flow ultrasound technique transverse oscillation was employed intra-operatively on the ascending aorta of (I) 20 patients with a healthy aortic valve and 20 patients with aortic stenosis before (IIa) and after (IIb) valve replacement. The results indicate that aortic stenosis increased flow complexity (p < 0.0001), induced systolic backflow (p < 0.003) and reduced systolic jet width (p < 0.0001). After valve replacement, the systolic backflow and jet width were normalized (p < 0.52 and p < 0.22), but flow complexity was not (p < 0.0001). Flow complexity (p < 0.0001), systolic jet width (p < 0.0001) and systolic backflow (p < 0.001) were associated with peak systolic velocity. The study found that aortic stenosis changes blood flow in the ascending aorta and valve replacement corrects some of these changes. Transverse oscillation may be useful for assessment of aortic stenosis and optimization of valve surgery.     

脉冲多普勒外周血管血流定量可信性的实验研究

为探讨脉冲多普勒技术在外周血管血流定量中的准确性、可行性及影响因素，本文以电磁流量计和血泵流率（实际值）为金标准，采用频谱外和中线包络线的ＶＴＩ或ＭＶ计算每分流量（测量值），进行动物和体外模拟脉动及稳流两种血流状态的实验。结果表明：频谱外包络线的测量值明显高于实际值（ｐ＜０．０１），但两者之间有良好的相关性（ｐ＜０．０１），如除以校正系数则接近实际值。在相同条件下，变化取样容积和探头频率均对血流定量有影响。我们认为，如注意避免影响血流定量的各因素，脉冲多普勒技术仍能提供较准确血流定量信息。