Honarvar M, Lobo J, Mohareri O, Salcudean S E, Rohling R
Department of Mechanical Engineering, University of British Columbia, Vancouver, BC, Canada.
Phys Med Biol. 2015 May 7;60(9):3847-68. doi: 10.1088/0031-9155/60/9/3847. Epub 2015 Apr 23.
To produce images of tissue elasticity, the vibro-elastography technique involves applying a steady-state multi-frequency vibration to tissue, estimating displacements from ultrasound echo data, and using the estimated displacements in an inverse elasticity problem with the shear modulus spatial distribution as the unknown. In order to fully solve the inverse problem, all three displacement components are required. However, using ultrasound, the axial component of the displacement is measured much more accurately than the other directions. Therefore, simplifying assumptions must be used in this case. Usually, the equations of motion are transformed into a Helmholtz equation by assuming tissue incompressibility and local homogeneity. The local homogeneity assumption causes significant imaging artifacts in areas of varying elasticity. In this paper, we remove the local homogeneity assumption. In particular we introduce a new finite element based direct inversion technique in which only the coupling terms in the equation of motion are ignored, so it can be used with only one component of the displacement. Both Cartesian and cylindrical coordinate systems are considered. The use of multi-frequency excitation also allows us to obtain multiple measurements and reduce artifacts in areas where the displacement of one frequency is close to zero. The proposed method was tested in simulations and experiments against a conventional approach in which the local homogeneity is used. The results show significant improvements in elasticity imaging with the new method compared to previous methods that assumes local homogeneity. For example in simulations, the contrast to noise ratio (CNR) for the region with spherical inclusion increases from an average value of 1.5-17 after using the proposed method instead of the local inversion with homogeneity assumption, and similarly in the prostate phantom experiment, the CNR improved from an average value of 1.6 to about 20.
为了生成组织弹性图像，振动弹性成像技术包括对组织施加稳态多频振动，从超声回波数据估计位移，并在以剪切模量空间分布为未知量的逆弹性问题中使用估计的位移。为了完全解决逆问题，需要所有三个位移分量。然而，使用超声时，位移的轴向分量比其他方向测量得更准确。因此，在这种情况下必须使用简化假设。通常，通过假设组织不可压缩和局部均匀性，将运动方程转化为亥姆霍兹方程。局部均匀性假设在弹性变化的区域会导致显著的成像伪影。在本文中，我们去除了局部均匀性假设。特别是，我们引入了一种基于有限元的新直接反演技术，其中仅忽略运动方程中的耦合项，因此它可以仅使用位移的一个分量。同时考虑了笛卡尔坐标系和圆柱坐标系。多频激励的使用还使我们能够获得多个测量值，并减少一个频率的位移接近零的区域中的伪影。将所提出的方法在模拟和实验中与使用局部均匀性的传统方法进行了测试。结果表明，与先前假设局部均匀性的方法相比，新方法在弹性成像方面有显著改进。例如在模拟中，使用所提出的方法后，具有球形内含物区域的对比度噪声比（CNR）从平均值1.5提高到17，而不是使用具有均匀性假设的局部反演，同样在前列腺体模实验中，CNR从平均值1.6提高到约20。
