Chen Hao, Varghese Tomy
Department of Medical Physics, The University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI-53706, USA.
Ultrasonics. 2009 May;49(4-5):472-83. doi: 10.1016/j.ultras.2008.12.003. Epub 2009 Jan 1.
Shear stresses are always present during quasi-static strain imaging, since tissue slippage occurs along the lateral and elevational directions during an axial deformation. Shear stress components along the axial deformation axes add to the axial deformation while perpendicular components introduce both lateral and elevational rigid motion and deformation artifacts into the estimated axial and lateral strain tensor images. A clear understanding of these artifacts introduced into the normal and shear strain tensor images with shear deformations is essential. In addition, signal processing techniques for improved depiction of the strain distribution is required. In this paper, we evaluate the impact of artifacts introduced due to lateral shear deformations on the normal strain tensors estimated by varying the lateral shear angle during an axial deformation. Shear strains are quantified using the lateral shear angle during the applied deformation. Simulation and experimental validation using uniformly elastic and single inclusion phantoms were performed. Variations in the elastographic signal-to-noise and contrast-to-noise ratios for axial deformations ranging from 0% to 5%, and for lateral deformations ranging from 0 to 5 degrees were evaluated. Our results demonstrate that the first and second principal component strain images provide higher signal-to-noise ratios of 20 dB with simulations and 10 dB under experimental conditions and contrast-to-noise ratio levels that are at least 20 dB higher when compared to the axial and lateral strain tensor images, when only lateral shear deformations are applied. For small axial deformations, the lateral shear deformations significantly reduces strain image quality, however the first principal component provides about a 1-2dB improvement over the axial strain tensor image. Lateral shear deformations also significantly increase the noise level in the axial and lateral strain tensor images with larger axial deformations. Improved elastographic signal and contrast-to-noise ratios in the first principal component strain image are always obtained for both simulation and experimental data when compared to the corresponding axial strain tensor images in the presence of both axial and lateral shear deformations.
在准静态应变成像过程中，剪切应力始终存在，因为在轴向变形期间，组织会沿横向和纵向方向发生滑动。沿轴向变形轴的剪切应力分量会叠加到轴向变形上，而垂直分量则会在估计的轴向和横向应变张量图像中引入横向和纵向的刚性运动及变形伪像。清楚了解这些因剪切变形而引入到法向应变张量图像和剪切应变张量图像中的伪像至关重要。此外，还需要用于更好地描绘应变分布的信号处理技术。在本文中，我们通过在轴向变形期间改变横向剪切角度，评估了横向剪切变形引入的伪像对估计的法向应变张量的影响。在施加变形期间，使用横向剪切角度对剪切应变进行量化。使用均匀弹性和单包含体模型进行了模拟和实验验证。评估了轴向变形范围为0%至5%以及横向变形范围为0至5度时弹性成像的信噪比和对比噪声比的变化。我们的结果表明，当仅施加横向剪切变形时，与轴向和横向应变张量图像相比，第一和第二主成分应变图像在模拟中提供了20 dB的更高信噪比，在实验条件下提供了10 dB的更高信噪比，并且对比噪声比水平至少高出20 dB。对于小轴向变形，横向剪切变形会显著降低应变图像质量，然而第一主成分比轴向应变张量图像提供了约1 - 2 dB的改善。横向剪切变形还会在较大轴向变形时显著增加轴向和横向应变张量图像中的噪声水平。与存在轴向和横向剪切变形时的相应轴向应变张量图像相比，对于模拟和实验数据，在第一主成分应变图像中总是能获得更高的弹性成像信号和对比噪声比。
