Li He, Lee Wei-Ning
Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong Kong.
Phys Med Biol. 2017 Feb 21;62(4):1456-1479. doi: 10.1088/1361-6560/aa530b. Epub 2017 Jan 23.
The anisotropic mechanical properties (mechanical anisotropy) and view-dependent ultrasonic backscattering (acoustic anisotropy) of striated muscle due to the underlying myofiber arrangement have been well documented, but whether they impact on ultrasound strain imaging (USI) techniques remains unclear. The aim of this study was therefore to investigate the performance of a cross-correlation-based two-dimensional (2D) USI method in anisotropic media under controlled quasi-static compression in silico and in vitro. First, synthetic pre- and post-deformed 2D radiofrequency images of anisotropic phantoms were simulated in two scenarios to examine the individual effect of the mechanical and acoustic anisotropies on strain estimation. In the first scenario, the phantom was defined to be transversely isotropic with the scatterer amplitudes following a zero-mean Gaussian distribution, while in the second scenario, the phantom was defined to be mechanically isotropic with Gaussian distributed scatterer amplitudes correlated along the principal directions of pre-defined fibers. These two anisotropies were then jointly incorporated into the ultrasound image simulation model with additional depth-dependent attenuation. Three imaging planes-the fiber plane with the fiber direction perpendicular to the ultrasound beam (TIS), the fiber plane with the fiber direction parallel to the beam (TIS), and the transverse fiber plane (TIS)-were studied. The absolute relative error (ARE) of the lateral strain estimates in TIS (20.99 ± 15.65%) was much higher than that in TIS (4.14 ± 3.17%). The ARE in TIS was unavailable owing to the large spatial extent of false peaks. The effect of tissue anisotropy on the performance of the 2D USI was further confirmed in an in vitro porcine skeletal muscle phantom. The best in-plane strain quality was again shown in TIS (elastographic signal-to-noise ratio, or SNR: >25 dB), whereas the most unreliable strain estimates were found as expected in TIS (SNR: <10 dB). The strain filter explained the effect of the mechanical anisotropy and required the underlying strain to be within an optimal range for estimation. Sonographic SNR (SNR) was found to be altered by the acoustic anisotropy and was much lower in TIS (10 dB) than in TIS (50 dB) in vitro, which affected the accuracy of the strain estimation. Speckle size showed no evident impact on strain estimation but requires further examination.
由于潜在的肌纤维排列，横纹肌的各向异性力学特性（力学各向异性）和与视角相关的超声背向散射（声学各向异性）已有充分记录，但它们是否会影响超声应变成像（USI）技术仍不清楚。因此，本研究的目的是在计算机模拟和体外控制的准静态压缩条件下，研究基于互相关的二维（2D）USI方法在各向异性介质中的性能。首先，在两种情况下模拟了各向异性体模的合成变形前和变形后的二维射频图像，以检验力学和声学各向异性对应变估计的单独影响。在第一种情况下，将体模定义为横向各向同性，散射体振幅服从零均值高斯分布，而在第二种情况下，将体模定义为力学各向同性，高斯分布的散射体振幅沿预定义纤维的主方向相关。然后，将这两种各向异性与额外的深度相关衰减一起纳入超声图像模拟模型。研究了三个成像平面——纤维方向垂直于超声束的纤维平面（TIS）、纤维方向平行于束的纤维平面（TIS）和横向纤维平面（TIS）。TIS中横向应变估计的绝对相对误差（ARE）（20.99±15.65%）远高于TIS中的（4.14±3.17%）。由于假峰的空间范围较大，TIS中的ARE无法获得。在体外猪骨骼肌模型中进一步证实了组织各向异性对二维USI性能的影响。再次在TIS中显示出最佳的平面内应变质量（弹性成像信噪比，或SNR：>25 dB），而正如预期的那样，在TIS中发现最不可靠的应变估计（SNR：<10 dB）。应变滤波器解释了力学各向异性的影响，并要求潜在应变在估计的最佳范围内。发现超声信噪比（SNR）受声学各向异性的影响，在体外TIS中（10 dB）远低于TIS中（50 dB），这影响了应变估计的准确性。散斑大小对应变估计没有明显影响，但需要进一步研究。
