Guo Yuexin, Wang Yahua, Chang Enoch Jing-Han, Lee Wei-Ning
Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong.
Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong; Medical Engineering Programme, The University of Hong Kong, Hong Kong.
Ultrasound Med Biol. 2018 Apr;44(4):884-896. doi: 10.1016/j.ultrasmedbio.2017.12.009.
We previously found that vascular guided wave imaging (VGWI) could non-invasively quantify transmural wall stiffness in both the longitudinal (r-z plane, 0°) and circumferential (r-θ plane, 90°) directions of soft hollow cylinders. Arterial stiffness estimation in multiple directions warrants further comprehensive characterization of arterial health, especially in the presence of asymmetric plaques, but is currently lacking. This study therefore investigated the multidirectional estimation of the arterial Young's modulus in a finite-element model, in vitro artery-mimicking phantoms and an excised porcine aorta. A longitudinal pre-stretch of 20% and/or lumen pressure (15 or 70 mm Hg) was additionally introduced to pre-condition the phantoms for emulating the intrinsic mechanical anisotropy of the real artery. The guided wave propagation was approximated by a zero-order antisymmetric Lamb wave model. Shape factor, which was defined as the ratio of inner radius to thickness, was calculated over the entire segment of each planar cross section of the hollow cylindrical structure at a full rotation (0°-360° at 10° increments) about the radial axis. The view-dependent geometry of the cross segment was found to affect the guided wave propagation, causing Young's modulus overestimation in four angular intervals along the propagation pathway, all of which corresponded to wall regions with low shape factors (<1.5). As validated by mechanical tensile testing, the results indicate not only that excluding the propagation pathway with low shape factors could correct the overestimation of Young's modulus, but also that VGWI could portray the anisotropy of hollow cylindrical structures and the porcine aorta based on the derived fractional anisotropy values from multidirectional modulus estimates. This study may serve as an important step toward 3-D assessment of the mechanical properties of the artery.
我们之前发现，血管导波成像（VGWI）能够在软质空心圆柱体的纵向（r-z平面，0°）和周向（r-θ平面，90°）方向上非侵入性地量化透壁壁僵硬度。多方向的动脉僵硬度估计对于更全面地表征动脉健康状况很有必要，尤其是在存在不对称斑块的情况下，但目前尚缺乏相关研究。因此，本研究在有限元模型、体外动脉模拟体模和切除的猪主动脉中，对动脉杨氏模量的多方向估计进行了研究。另外引入20%的纵向预拉伸和/或管腔压力（15或70 mmHg）对体模进行预处理，以模拟真实动脉的固有机械各向异性。导波传播通过零阶反对称兰姆波模型进行近似。形状因子定义为内半径与厚度之比，在空心圆柱结构每个平面横截面的整个段上，绕径向轴进行360°全旋转（以10°增量从0°到360°）来计算。发现横截面的视角相关几何形状会影响导波传播，导致在沿传播路径的四个角度区间内杨氏模量被高估，所有这些区间都对应形状因子较低（<1.5）的壁区域。经机械拉伸试验验证，结果表明不仅排除形状因子低的传播路径可以纠正杨氏模量的高估，而且VGWI可以根据多方向模量估计得出的分数各向异性值来描绘空心圆柱结构和猪主动脉的各向异性。本研究可能是迈向动脉力学特性三维评估的重要一步。
