(a)Dept of Medical Physics, School of Medicine, University of Patras, 26500 Patras, Greece.
Ultrasonics. 2014 Feb;54(2):461-70. doi: 10.1016/j.ultras.2013.07.009. Epub 2013 Jul 19.
A localized modulated radiation force can be produced when two confocal ultrasound beams of nearly equal frequencies interfere in an attenuating medium such as tissue. It is well-established that this force generates both shear and longitudinal waves. By scanning the focal point over a plane and observing the propagation of these waves, the mechanical properties of the medium can be imaged. In this paper, the modulated radiation force is analytically derived in the case of attenuating media, by expanding on the theory of ultrasound-stimulated-vibro-acoustography (USVA) for lossless media. Furthermore, weak nonlinearities are considered in the formulation, since higher source pressures may prove to be necessary to improve the radiation-force profile - only the fundamental component is, however, studied in this paper. An analysis of the generated radiation force is performed and the effects of various parameters are investigated on its amplitude and spatial distribution. It will be shown that by carefully selecting the confocal geometry of the beams, as well as, the source pressure and center frequency, the spatial profile of the radiation force can be optimized. This, subsequently, could improve not only the resolution of the point-spread-function in USVA, but also, the profile of the shear waves in elastography applications.
当两个近频率的共焦超声束在衰减介质（如组织）中干涉时，会产生局部调制辐射力。众所周知，这种力会产生剪切波和纵波。通过在一个平面上扫描焦点并观察这些波的传播，可以对介质的力学性质进行成像。在本文中，通过对无损介质超声刺激振动声学（USVA）理论的扩展，对衰减介质中的调制辐射力进行了分析推导。此外，在公式中考虑了较弱的非线性，因为较高的源压力可能证明对于改善辐射力分布是必要的-然而，本文仅研究了基本分量。对产生的辐射力进行了分析，并研究了各种参数对其幅度和空间分布的影响。结果表明，通过仔细选择光束的共焦几何形状以及源压力和中心频率，可以优化辐射力的空间分布。这随后不仅可以提高 USVA 中点扩散函数的分辨率，还可以提高弹性成像应用中的剪切波的分布。
