Across the varieties of waves that have been studied in physics, it is well established that group velocities can be significantly greater than or less than phase velocities measured within comparable frequency bands, depending on the particular mechanisms involved. The distinction between group and phase velocities is important in elastography, because diagnoses are made based on shear wave speed estimations from a variety of techniques. We review the general definitions of group and phase velocity and examine their specific relations within an important general class of rheological models. For the class of tissues and materials exhibiting power law dispersion, group velocity is significantly greater than phase velocity, and simple expressions are shown to interrelate the commonly measured parameters. Examples are given from phantoms and tissues.

中文翻译：

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软组织中剪切波的群速度与相速度

在物理学中研究过的各种波中，众所周知，群速度可以显着大于或小于在可比频带内测量的相速度，这取决于所涉及的特定机制。群速度和相速度之间的区别在弹性成像中很重要，因为诊断是基于来自各种技术的剪切波速度估计进行的。我们回顾了群速度和相速度的一般定义，并检查了它们在一类重要的流变模型中的具体关系。对于表现出幂律色散的组织和材料类，群速度明显大于相速度，并且显示了简单的表达式来关联常用的测量参数。示例来自体模和组织。