Lipman Samantha L, Rouze Ned C, Palmeri Mark L, Nightingale Kathryn R
Department of Biomedical Engineering, Duke University, Durham, North Carolina, USA.
Department of Biomedical Engineering, Duke University, Durham, North Carolina, USA.
Ultrasound Med Biol. 2018 Apr;44(4):897-908. doi: 10.1016/j.ultrasmedbio.2017.12.019. Epub 2018 Feb 5.
Shear wave elasticity imaging (SWEI) characterizes the mechanical properties of human tissues to differentiate healthy from diseased tissue. Commercial scanners tend to reconstruct shear wave speeds for a region of interest using time-of-flight methods reporting a single shear wave speed (or elastic modulus) to the end user under the assumptions that tissue is elastic and shear wave speeds are not dependent on the frequency content of the shear waves. Human tissues, however, are known to be viscoelastic, resulting in dispersion and attenuation. Shear wave spectroscopy and spectral methods have been previously reported in the literature to quantify shear wave dispersion and attenuation, commonly making an assumption that the acoustic radiation force excitation acts as a cylindrical source with a known geometric shear wave amplitude decay. This work quantifies the bias in shear dispersion and attenuation estimates associated with making this cylindrical wave assumption when applied to shear wave sources with finite depth extents, as commonly occurs with realistic focal geometries, in elastic and viscoelastic media. Bias is quantified using analytically derived shear wave data and shear wave data generated using finite-element method models. Shear wave dispersion and attenuation bias (up to 15% for dispersion and 41% for attenuation) is greater for more tightly focused acoustic radiation force sources with smaller depths of field relative to their lateral extent (height-to-width ratios <16). Dispersion and attenuation errors associated with assuming a cylindrical geometric shear wave decay in SWEI can be appreciable and should be considered when analyzing the viscoelastic properties of tissues with acoustic radiation force source distributions with limited depths of field.
剪切波弹性成像（SWEI）通过表征人体组织的力学特性来区分健康组织和病变组织。商用扫描仪倾向于使用飞行时间方法重建感兴趣区域的剪切波速度，并在组织为弹性且剪切波速度不依赖于剪切波频率成分的假设下，向最终用户报告单一的剪切波速度（或弹性模量）。然而，已知人体组织具有粘弹性，会导致频散和衰减。此前文献中已报道了剪切波光谱法和频谱方法来量化剪切波频散和衰减，通常假设声辐射力激发作为具有已知几何剪切波振幅衰减的柱形源。这项工作量化了在弹性和粘弹性介质中，当将这种柱形波假设应用于具有有限深度范围的剪切波源（实际聚焦几何形状中常见）时，与剪切频散和衰减估计相关的偏差。使用解析推导的剪切波数据和有限元方法模型生成的剪切波数据来量化偏差。对于横向范围（高宽比<16）相对较小且景深较小的更紧密聚焦的声辐射力源，剪切波频散和衰减偏差（频散高达15%，衰减高达41%）更大。在分析具有有限景深的声辐射力源分布的组织粘弹性特性时，SWEI中假设柱形几何剪切波衰减所产生的频散和衰减误差可能相当可观，应予以考虑。
