血管内光声图像的建模与仿真

为了给血管内光声(intravascular photoacoustic,IVPA)成像的图像重建算法和图像后处理算法提供数据源,为培训医师提供图像库,提出一种二维IVPA图像的建模与仿真方法。建立包含斑块的血管横截面模型,并对成像导管发射激光脉冲照射血管壁组织、血管壁组织产生光声信号、利用得到的一组光声信号重建IVPA图像整个过程进行仿真,得到血管横截面模型的二维IVPA灰阶图像,同时根据冠状动脉血管随心脏搏动的规律,实现序列图像的仿真。利用仿真模型得到的IVPA图像与原电磁吸收分布图接近程度较高。仿真模型的物理意义清晰,可得到质量稳定的IVPA图像。      

利用B超视频输出信号测量体内组织声衰减系数

本文介绍了一种利用B超视频输出信号进行体内组织声衰减估计的方法——差比校正法.该法利用已在临床上普及应用的B超,消除了B超的增益,时间增益补偿、声束的扩散和散射等因素的影响,并且不需对现有B超做任何改动,具有较广泛的实用性.本文还从理论上导出了计算体内组织声衰减系数的计算公式,给出了差比校正法对超声仿真模型的声衰减系数的测量结果,并与传输法所得结果进行了比较.实验结果证明了差比校正法的有效性.     

基于换能器驱动信号特征的高强度聚焦超声焦域损伤实时监测

高强度聚焦超声(high intensity focused ultrasound,HIFU)焦域的实时监测是聚焦超声临床治疗面临的关键问题,目前临床常采用B超图像强回声的变化实现焦域组织损伤的监测,而B超图像出现的强回声大多与焦域处的空化及沸腾气泡相关,无法准确、实时地监测治疗状态.HIFU治疗中焦域组织会伴随温度升高、空化、沸腾和组织特性等变化,换能器表面的声学负载也在不断变化,针对该问题,本文构建了换能器电压电流实时检测平台,通过测量换能器电学参数来感知焦域组织的状态变化.以离体牛肝组织作为HIFU辐照对象,并将相位差变化的结果与离体牛肝组织损伤的结果进行了对照,实验结果表明,在HIFU辐照过程中,换能器电压与电流的相位会出现由相对平稳到大幅波动的过程,此时停止辐照可见焦域出现明显损伤,而此时B超图像灰度无明显变化;此外,当焦域出现空化时,其波动幅度与范围将较之更大.此方法可为HIFU焦域组织损伤监测提供一种新的研究方案和手段.     

局域基谐波频谱峰值比的超声图像融合

提出一种局域基谐波频谱峰值比(LFHR)的超声图像融合方法.首先,分离超声射频信号基谐波,计算二者的局域频谱峰值比,确定融合权值;然后,加权平均基谐波成分,得到融合射频信号;最后,经过希尔伯特变换、对数压缩、插值/降采样生成B超融合图像.非均匀囊肿组织模型仿真结果表明,LFHR法获取的融合图像较原始图像的对比度、对比度...     

基于超声散射回波功率谱的热疗无损测温模型

在随机起伏介质超声散射理论下，基于组织超声散射问波功率谱，提出一种无损获取组织温度信息的新方法。组织中声速及密度的随机起伏引起超声散射，散射回波声压谱的均方值与组织中声速ｃ、声速对温度的梯度ｄｃ／ｄｔ和组织的平均温度Ｔ等有关。分析组织散射超声回波信号平均归一化功率谱与组织温度的关系，可以从超声散射回波中有效提取组织温度信息。理论分析和模拟实验为医学上肿瘤热疗提供了一种新的无损测温的方法和思路。     

基于超声RF信号熵分析的声空化时空监测方法

高强度聚焦超声(HIFU)治疗过程中剧烈的空化效应可能损伤靶区周围健康组织,因此,亟需开发可对生物组织内部声空化效应进行高精度时空定量监测的新型技术手段,方能确保临床安全和有效.相对于传统的商用超声灰度值信号,超声射频(RF)信号可以更好地保留声波散射信号更多的细节信息.而信息熵作为非基于数学函数模型的统计参数,可以表征由声空化效应引发的组织内部散射体无序度演变状态.因此,本文提出了一种基于超声RF信号熵分析的声空化实时监测成像系统,在此基础上实时评估HIFU引发的超声空化区域时空演化行为.首先,通过改制后的B超系统获取凝胶生物仿体内部由HIFU引发的空化泡群产生的散射回波原始RF信号,利用二维均值滤波方法抑制HIFU强声束对声空化监测成像回波信号的干扰后,通过数据标准化处理扩展RF信号的动态变化范围,再基于滑动窗信息熵分析重建熵值图像,经过二值化处理后即可实现对HIFU作用下组织内部声空化区域的时空监测.实验结果表明,相比于传统B超灰度成像法,基于RF信号熵分析的声空化监测成像算法可以更灵敏且精确地确定空化发生的起始时间和空间位置,有助于更好地保障HIFU临床治疗的安全性和有效性.本...     

B型超声诊断图像中肿瘤后方的声场分布

本文分析了B型超声诊断图像中肿块后方的增强、阴影和侧后声影带等图像特征。分析中把肿块简化为圆柱,运用射线声学的方法,导出了脉冲入射情况下肿块后方组织的散射回波声压公式。考虑了组织的声衰减和仪器的时间增益补偿的影响,我们用计算机绘出给定参量的回声图像,又用组织仿体进行了定性的实验验证。理论和实验结果定性相符。     

高频聚焦超声声场和温度场的仿真研究*

为探究临床常用的7 MHz高频聚焦超声在多层生物组织中的声传播以及毫秒级时间内的生物传热规律问题,基于Westervelt方程和Pennes传热方程,使用有限元方法建立高频聚焦超声辐照多层组织的非线性热黏性声传播及传热模型。首先分析了线性模型和非线性模型之间的差异,然后在非线性模型下探究换能器的参数对声场和温度场的影响。仿真结果显示：在7 MHz频率下,当换能器输出声功率超过5 W时,声波传播的非线性效应不可忽视(p <0.05);当声功率从5 W增大到15 W时,非线性模型与线性模型预测的温度偏差从20%增加到34.703%;高频聚焦超声波的非线性行为比低频更加显著,基频能量向高次谐波转移的程度增大,声功率为10 W和15 W时4次谐波与基波之比分别达到7.33%和12.12%;高频换能器参数的改变对组织中声场和温度场分布的影响较大,换能器焦距从12 mm减小到11.2 mm,焦点处最高温度增加了77%。结果表明,7 MHz聚焦超声的非线性声传播需要考虑到4次谐波的影响。该文提出的多层组织非线性仿真模型可为高频聚焦超声换能器参数优化及制定安全、有效的术前治疗方案提供理论参考。     

基于一种新型超腔的高亮度激光同步辐射分析

 提出了一种由一对抛物面构成的超腔的技术方案,计算了超腔汇聚点处的总光子密度。利用康普顿散射理论对基于抛物面超腔的激光同步辐射及其光子产额进行了讨论和计算。结果表明:当超腔镜面的反射率等于99.99%时,在超腔碰撞点处的总光子数密度大约是初始激光束在碰撞点处光子数密度的5 000倍,对应康普顿垂直散射的光子产额大约是电子束与初始激光束在碰撞点处发生康普顿垂直散射时的5 000倍。     

超声诊断与非线性声学

超声波在医学诊断和治疗中的研究已有近６０年的历史．近年来发展极为迅速,应用相当广泛．目前超声诊断主要是利用超声良好指向性和与光相似的反射、散射、衰减和多普勒效应等物理规律,不同类型的超声诊断仪采用不同的方法把超声波发射到体内,并在组织中传播．正常组织与病变组织的声阻抗通常是不相同的,当差异超出千分之一时,正常组织与病变组织之间的界面会对声波产生反射、散射,用仪器把反射和散射波接收下来,经过处理便可将其显示为图像（Ｂ超）、波形（Ａ超）,或曲线（Ｍ超）从而作出诊断．超声能显示人体软组织及动态状况,它…     

An ultrasound simulation method for carotid arteries with a wall structure of three membranes

Ultrasound simulation for carotid arteries is helpful to the performance assessments of vessel wall detection and signal processing methods by using ultrasound techniques.An ultrasound simulation method of carotid artery wall with a three-membrane structure is proposed in present study.According to the ultrasound speckle distributions varying with the shapes and densities of scatterer distributions,as well as the statistic results of the clinical images,the parameters of distributions,densities and intensities of scatterers for different kinds of tissues in the carotid artery phantoms are determined.Each region is acoustically characterized using FIELD II software to produce the radio frequency echo signals,from which ultrasound images are derived.The results based on 30 simulations show that the echo distributions of the intimae,mediae,adventitias and blood are consistent with the clinical ones.Moreover,compared with the results from the central frequency of 8 MHz,the mean measurements for thicknesses of the intima,media and adventitia membranes,as well as the lumen diameter from the simulation images based on 12 MHz are the same as the preset ones,and the maximum relative errors are the 4.01%,1.25%,0.04%and 0.15%,respectively.The simulation under this condition is more realistic.     

Asymmetric radial expansion and contraction of rat carotid artery observed using a high-resolution ultrasound imaging system

The geometry of carotid artery bifurcation is of high clinical interest because it determines the characteristics of blood flow that is closely related to the formation and development of atherosclerotic plaque. However, information on the dynamic changes in the vessel wall of carotid artery bifurcation during a pulsatile cycle is limited. This pilot study investigated the cyclic changes in carotid artery geometry caused by blood flow pulsation in rats. A high-resolution ultrasound imaging system with a broadband scanhead centered at 40 MHz was used to obtain longitudinal images of the rat carotid artery. A high frame rate retrospective B-scan imaging technique based on the use of electrocardiogram to trigger signal acquisition was used to examine precisely the fast arterial wall motion. Two-dimensional geometry data obtained from nine rats showed that the rat carotid artery asymmetrically contracts and dilates during each cardiac cycle. Systolic/diastolic vessel diameters near the upstream and downstream regions from the bifurcation were 0.976 ± 0.011/0.825 ± 0.015 mm and 0.766 ± 0.015/0.650 ± 0.016 mm, respectively. Their posterior/anterior wall displacement ratios in the radial direction were 41.0 ± 14.9% and 2.9 ± 1.6%, respectively. These results indicate that in the vicinity of bifurcation, the carotid artery favorably expands to the anterior side during the systolic phase. This phenomenon was observed to be more prominent in the downstream region near the bifurcation. The cyclic variation pattern in wall movement varies depending on the measurement site, which shows different patterns at far upstream and downstream of the bifurcation. The asymmetric radial expansion and contraction of the rat carotid artery observed in this study may be useful in studying the hemodynamic etiology of cardiovascular diseases because the pulsatile changes in vessel geometry may affect the local hemodynamics that determines the spatial distribution of wall shear stress, one of important cardiovascular risk factors. Further systematic study is needed to clarify the effects of wall elasticity, branch angle and vessel diameter ratio on the asymmetric wall motion of carotid artery bifurcation.     

3D reconstruction of a carotid bifurcation from 2D transversal ultrasound images

Visualizing and analyzing the morphological structure of carotid bifurcations are important for understanding the etiology of carotid atherosclerosis, which is a major cause of stroke and transient ischemic attack. For delineation of vasculatures in the carotid artery, ultrasound examinations have been widely employed because of a noninvasive procedure without ionizing radiation. However, conventional 2D ultrasound imaging has technical limitations in observing the complicated 3D shapes and asymmetric vasodilation of bifurcations. This study aims to propose image-processing techniques for better 3D reconstruction of a carotid bifurcation in a rat by using 2D cross-sectional ultrasound images. A high-resolution ultrasound imaging system with a probe centered at 40 MHz was employed to obtain 2D transversal images. The lumen boundaries in each transverse ultrasound image were detected by using three different techniques; an ellipse-fitting, a correlation mapping to visualize the decorrelation of blood flow, and the ellipse-fitting on the correlation map. When the results are compared, the third technique provides relatively good boundary extraction. The incomplete boundaries of arterial lumen caused by acoustic artifacts are somewhat resolved by adopting the correlation mapping and the distortion in the boundary detection near the bifurcation apex was largely reduced by using the ellipse-fitting technique. The 3D lumen geometry of a carotid artery was obtained by volumetric rendering of several 2D slices. For the 3D vasodilatation of the carotid bifurcation, lumen geometries at the contraction and expansion states were simultaneously depicted at various view angles. The present 3D reconstruction methods would be useful for efficient extraction and construction of the 3D lumen geometries of carotid bifurcations from 2D ultrasound images.     

Simulation of blood flow using extended Boltzmann kinetic approach

Lattice Boltzmann (LB) simulations are conducted to obtain the detailed hydrodynamics in a variety of blood vessel setups, including a prototype stented channel and four human coronary artery geometries based on the images obtained from real patients. For a model of stented flow involving an S-shape stent, a pulsatile flow rate is applied as the inlet boundary condition, and the time- and space-dependent flow field is computed. The LB simulation is found to reproduce the analytical solutions for the velocity profiles and wall shear stress distributions for the pulsatile channel flow. For the coronary arteries, the distributions of wall shear stress, which is important for clinical diagnostic purposes, are in good agreement with the conventional CFD predictions.     

Ultrasonically assisted evaluation of the impact of atherosclerotic plaque on the pulse pressure wave propagation: a clinical feasibility study

The purpose of this work was to evaluate ultrasound modality as a non-invasive tool for determination of impact of the degree of the atherosclerotic plaque located in human internal carotid arteries on the values of the parameters of the pulse wave. Specifically, the applicability of the method to such arteries as brachial, common, and internal carotid was examined. The method developed is based on analysis of two characteristic parameters: the value of the mean reflection coefficient modulus |Γ|_a of the blood pressure wave and time delay Δt between the forward (travelling) and backward (reflected) blood pressure waves. The blood pressure wave was determined from ultrasound measurements of the artery’s inner (internal) diameter, using the custom made wall tracking system (WTS) operating at 6.75 MHz. Clinical data were obtained from the carotid arteries measurements of 70 human subjects. These included the control group of 30 healthy individuals along with the patients diagnosed with the stenosis of the internal carotid artery (ICA) ranging from 20% to 99% or with the ICA occlusion. The results indicate that with increasing level of stenosis of the ICA the value of the mean reflection coefficient measured in the common carotid artery, significantly increases from |Γ|_a = 0.45 for healthy individuals to |Γ|_a = 0.61 for patients with stenosis level of 90-99%, or ICA occlusion. Similarly, the time delay Δt decreases from 52 ms to 25 ms for the respective groups. The method described holds promise that it might be clinically useful as a non-invasive tool for localization of distal severe artery narrowing, which can assist in identifying early stages of atherosclerosis especially in regions, which are inaccessible for the ultrasound probe (e.g. carotid sinus or middle cerebral artery).     

In vivo biomicroscopy with ultrasound 2

In the first article of this series it was shown that the use of inverse scattering theory to analyse ultrasound reflections could provide high resolution images of the acoustic impedance profile of the retina. Unlike the retina, most tissue structures of interest, like small tumours and arterial plaque deposits, are shielded from view by intervening layers of tissue of appreciable acoustic impedance and attenuation. By analysing a one-dimensional model for a plaque deposit on the wall of a carotid artery embedded in a 5 cm thick layer of tissue, we demonstrate that a relatively high quality image can be recovered when compensation for the attenuation of the intervening tissue is made. We observe that because of the dearth of low frequency power in the recovered signal of ultrasound transducers, it is important that the field of view imaged is not taken to be too large. We compare the exact iterative distorted wave Born approximation inverse scattering method with the approximate but computationally faster plane wave Born approximation method and find that they give images of comparable quality for this model.     

In vivo attenuation and equivalent scatterer size parameters for atherosclerotic carotid plaque: Preliminary results

We have previously reported on the equivalent scatterer size, attenuation coefficient, and axial strain properties of atherosclerotic plaque ex vivo. Since plaque structure and composition may be damaged during a carotid endarterectomy procedure, characterization of in vivo properties of atherosclerotic plaque is essential. The relatively shallow depth of the carotid artery and plaque enables non-invasive evaluation of carotid plaque utilizing high frequency linear-array transducers. We investigate the ability of the attenuation coefficient and equivalent scatterer size parameters to differentiate between calcified, and lipidic plaque tissue. Softer plaques especially lipid rich and those with a thin fibrous cap are more prone to rupture and can be classified as unstable or vulnerable plaque. Preliminary results were obtained from 10 human patients whose carotid artery was scanned in vivo to evaluate atherosclerotic plaque prior to a carotid endarterectomy procedure. Our results indicate that the equivalent scatterer size obtained using Faran’s scattering theory for calcified regions are in the 120–180 μm range while softer regions have larger equivalent scatterer size distribution in the 280–470 μm range. The attenuation coefficient for calcified regions as expected is significantly higher than that for softer regions. In the frequency bandwidth ranging from 2.5 to 7.5 MHz, the attenuation coefficient for calcified regions lies between 1.4 and 2.5 dB/cm/MHz, while that for softer regions lies between 0.3 and 1.3 dB/cm/MHz.     

Scatterer size estimation in pulse-echo ultrasound using focused sources: theoretical approximations and simulation analysis

The speckle in ultrasound images has long been thought to contain information related to the tissue microstructure. Many different investigators have analyzed the frequency characteristics of the backscattered signals to estimate the scatterer acoustic concentration and size. Previous work has been mostly restricted to unfocused or weakly focused ultrasound sources, thus limiting its implementation with diagnostically relevant fields. Herein, we derive equations capable of estimating the size of a scatterer for any reasonably focused source provided that the velocity potential field in the focal region can be approximated as a three-dimensional Gaussian beam, scatterers are a sufficient distance from the source, and the field is approximately constant across the scatterer. The calculations show that, when estimating the scatterer size, correcting for focusing requires a generalized attenuation-compensation function that includes both attenuation and focusing along the beam axis. The Gaussian approximation is validated by comparing the ideal velocity potential field for three spherically focused sources with f-numbers of 1, 2, and 4 to the Gaussian approximation for frequencies from 2 to 14 MHz. The theoretical derivations are evaluated by simulating the backscatter by using spherically focused sources (f-numbers of 1, 2, and 4) adjacent to attenuating media (0.05 to 1 dB/cm/MHz) that contain scatterers with Gaussian impedance distributions. The generalized attenuation-compensation function yielded results accurate to 7.2% while the traditional attenuation-compensation functions that neglected focusing had errors as high as 103%.     

Gaussian wavelet based dynamic filtering (GWDF) method for medical ultrasound systems

In this paper, a novel dynamic filtering method using Gaussian wavelet filters is proposed to remove noise from ultrasound echo signal. In the proposed method, a mother wavelet is first selected with its central frequency (CF) and frequency bandwidth (FB) equal to those of the transmitted signal. The actual frequency of the received signal at a given depth is estimated through the autocorrelation technique. Then the mother wavelet is dilated using the ratio between the transmitted central frequency and the actual frequency as the scale factor. The generated daughter wavelet is finally used as the dynamic filter at this depth. Frequency-demodulated Gaussian wavelet is chosen in this paper because its power spectrum is well-matched with that of the transmitted ultrasound signal. The proposed method is evaluated by simulations using Field II program. Experiments are also conducted out on a standard ultrasound phantom using a 192-element transducer with the center frequency of 5 MHz. The phantom contains five point targets, five circular high scattering regions with diameters of 2, 3, 4, 5, 6 mm respectively, and five cysts with diameters of 6, 5, 4, 3, 2 mm respectively. Both simulation and experimental results show that optimal signal-to-noise ratio (SNR) can be obtained and useful information can be extracted along the depth direction irrespective of the diagnostic objects.     

3D dynamical ultrasonic model of pulsating vessel walls

The aim of this work is to introduce a novel 3-D model of pulsating vessels, through which the dynamic acoustic response of arterial regions can be predicted. Blood flow is numerically simulated by considering the fluid-dynamic displacements of the scatterers (erythrocytes), while a mechanical model calculates the wall displacement due to fluid pressure. The acoustic characteristics of each region are simulated through the FIELD software. Two numerical phantoms of a carotid artery surrounded by elastic tissue have been developed to illustrate the model. One of them includes a plaque involving a 50% stenosis. B-mode and M-mode images are produced and segmented to obtain the wall displacement profile. A cylindrical holed phantom made of cryogel mimicking material has been constructed for the model validation. In pulsatile flow conditions, fluid and wall displacements have been measured by Doppler ultrasound methods and quantitatively compared to simulated M-mode images, showing a fairly good agreement.