Shear wave based quantitative ultrasound imaging methods

背景技术背景：临床超声（US）扫描仪通常提供亮度模式（B模式）图像，其中组织结构之间的声阻抗对比用于识别诊断边界。现代US系统还包括用于流动和组织运动分析的多普勒模式（有或没有造影剂），以及用于评估组织机械特性的弹性成像模式（基于应变和剪切波（SW））。定量US（QUS）成像是旨在量化声压缩波与生物组织的相互作用的另一个领域。QUS技术提取组织的基本物理特性，以提供关于在B模式或其他成像模式上不可见的亚分辨率特性的信息。最先进的定位：由于生物软组织是水合的，粘弹性适合于使用弹性来表示它们的固体状行为，并且使用粘度来表示它们的流体状行为。然而，通过US成像评估弹性比粘度更经常被利用。在超声弹性成像中，通常通过聚焦推射束生成SW，然后立即监测SW运动。在某些假设下，弹性依赖于SW速度的测量，粘性依赖于其频率分散或SW衰减。我将继续开发用于SW弹性成像的稳健粘度方法。此外，有趣的初步数据的基础上，在SW传播过程中QUS后向散射特性的变化，我还提出了一个新的领域，我的标签的发展：“动态QUS后向散射成像”。目的：1）使用SW传播特性提高组织粘度评估的鲁棒性; 2）基于SW传播开发新的QUS概念; 3）使用模拟、体外体模数据和可用的体内人体扫描验证基于SW的粘度和QUS成像方法;以及4）开发结合SW组织激发和压缩波反向散射分析的开放访问模拟包，用于动态QUS反向散射成像。假设：1）SW衰减成像可以通过允许SW的光谱内容在空间上变化、通过不再假设衰减的线性幂律依赖性、以及通过使用随机化样本一致性算法来在优化过程的框架中寻求衰减与距离的关系来改进。2)动态QUS后向散射方法应反映散射体的相关长度，这决定了由SW传播调制的建设性和破坏性压缩波干扰。新的动态QUS反向散射方法可以对局部组织振荡散射体尺寸和位置变化进行成像，并且提供反映病理状态的新的图像对比度（不同于标准反向散射QUS、SW速度或SW衰减）。结论：基于SW的技术（粘度和反向散射）的发展应提供新的成像模式，以更好地生物组织表征。

BACKGROUND: Clinical ultrasound (US) scanners typically provide brightness mode (B-mode) images where the acoustic impedance contrast between tissue structures is used to identify boundaries for diagnosis. Modern US systems also include Doppler modes (with or without contrast agents) for flow and tissue motion analysis, and elastography modes (strain and shear wave (SW) based) for assessing mechanical properties of tissues. Quantitative US (QUS) imaging is another field that aims at quantifying acoustic compression wave interactions with biological tissues. QUS techniques extract fundamental physical properties of tissues to provide information on sub-resolution properties that are not visible on B-mode or other imaging modes. STATE-OF-THE-ART POSITIONING: Since biological soft tissues are hydrated, viscoelasticity is suitable to represent their solid-like behavior using elasticity, and fluid-like behavior using viscosity. However, the assessment of elasticity through US imaging has been more often exploited than viscosity. In US elastography, SWs are usually generated through focused push beams followed immediately by the monitoring of the SW motion. Under certain assumptions, elasticity relies on the measurement of the SW speed, and viscosity on its frequency dispersion or SW attenuation. I will pursue the development of robust viscosity methods for SW elastography imaging. Also, based on intriguing preliminary data on changes in QUS backscatter properties during SW propagation, I am also proposing the development of a new field that I label: "Dynamic QUS backscatter imaging". OBJECTIVES: 1) Improve robustness of tissue viscosity assessment using SW propagation properties; 2) develop a new QUS concept based on SW propagation; 3) validate SW-based viscosity and QUS imaging methods using simulations, in vitro phantom data, and available in vivo human scans; and 4) develop an open-access simulation package combining SW tissue excitation and compression wave backscatter analysis for dynamic QUS backscatter imaging. HYPOTHESES: 1) SW attenuation imaging can be improved by allowing the spectral content of SWs to vary spatially, by no longer assuming a linear power law dependency for the attenuation, and by using a randomized sample consensus algorithm to seek attenuation versus distance in the framework of an optimization process. 2) The dynamic QUS backscatter method should reflect the correlation length of scatterers, which determines constructive and destructive compression wave interferences modulated by the SW propagation. The new dynamic QUS backscatter method may image local tissue oscillating scatterer size and position changes, and provide a new image contrast reflecting pathological states (different from standard backscatter QUS, SW speed, or SW attenuation). CONCLUSION: The development of SW-based technologies (viscosity and backscatter) should provide new imaging modalities for better biological tissue characterization.