Erkamp Ramon Q, Emelianov Stanislav Y, Skovoroda Andrei R, O'Donnell Matthew
Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
IEEE Trans Ultrason Ferroelectr Freq Control. 2004 May;51(5):532-9.
In tissue the Young's modulus cannot be assumed constant over a wide deformation range. For example, direct mechanical measurements on human prostate show up to a threefold increase in Young's modulus over a 10% deformation. In conventional elasticity imaging, these effects produce strain-dependent elastic contrast. Ignoring these effects generally leads to suboptimal contrast (stiffer tissues at lower strain are contrasted against softer tissues at higher strain), but measuring the nonlinear behavior results in enhanced tissue differentiation. To demonstrate the methods extracting nonlinear elastic properties, both simulations and measurements were performed on an agar-gelatin phantom. Multiple frames of phase-sensitive ultrasound data are acquired as the phantom is deformed by 12%. All interframe displacement data are brought back to the geometry of the first frame to form a three-dimensional (3-D) data set (depth, lateral, and preload dimensions). Data are fit to a 3-D second order polynomial model for each pixel that adjusts for deformation irregularities. For the phantom geometry and elastic properties considered in this paper, reconstructed frame-to-frame strain images using this model result in improved contrast to noise ratios (CNR) at all preload levels, without any sacrifice in spatial resolution. From the same model, strain hardening at all preload levels can be extracted. This is an independent contrast mechanism. Its maximum CNR occurs at 5.13% preload, and it is a 54% improvement over the best case (preload 10.6%) CNR for frame-to-frame strain reconstruction. Actual phantom measurements confirm the essential features of the simulation. Results show that modeling of the nonlinear elastic behavior has the potential to both increase detectability in elasticity imaging and provide a new independent mechanism for tissue differentiation.
在组织中，杨氏模量在较大的变形范围内不能被假定为常数。例如，对人体前列腺进行的直接力学测量表明，在10%的变形范围内，杨氏模量会增加高达三倍。在传统的弹性成像中，这些效应会产生应变依赖的弹性对比度。忽略这些效应通常会导致对比度欠佳（低应变下较硬的组织与高应变下较软的组织形成对比），但测量非线性行为会增强组织分化。为了演示提取非线性弹性特性的方法，在琼脂 - 明胶模型上进行了模拟和测量。当模型变形12%时，采集多帧相敏超声数据。所有帧间位移数据都被恢复到第一帧的几何形状，以形成三维（3 - D）数据集（深度、横向和预加载维度）。针对每个像素，数据拟合到一个三维二阶多项式模型，以校正变形不规则性。对于本文所考虑的模型几何形状和弹性特性，使用该模型重建的逐帧应变图像在所有预加载水平下都能提高对比度噪声比（CNR），而不会牺牲任何空间分辨率。从同一模型中，可以提取所有预加载水平下的应变硬化情况。这是一种独立的对比度机制。其最大CNR出现在5.13%的预加载水平，相较于逐帧应变重建的最佳情况（预加载10.6%）的CNR提高了54%。实际模型测量结果证实了模拟的基本特征。结果表明，对非线性弹性行为进行建模有可能提高弹性成像中的可检测性，并为组织分化提供一种新的独立机制。
