Hajim School of Engineering and Applied Sciences, University of Rochester, Rochester, NY 14627, USA.
Ultrasound Med Biol. 2011 Nov;37(11):1893-908. doi: 10.1016/j.ultrasmedbio.2011.07.009. Epub 2011 Oct 1.
Model-based elastography is an emerging technique with clinical applications in imaging vascular tissues, guiding minimally invasive therapies and diagnosing breast and prostate cancers. Its usage is limited because ultrasound can measure only the axial component of displacement with high precision. The goal of this study was to assess the effect of lateral sampling frequency, lateral beam-width and the number of active transmission elements on the quality of axial and lateral strain elastograms. Elastographic imaging was performed on gelatin-based phantoms with a modified commercial ultrasound scanner. Three groups of radio-frequency (RF) echo frames were reconstructed from fully synthetic aperture data. In the first group, all 128 transmission elements (corresponding to a lateral beamwidth of 0.22 mm at the center of the field of view) were used to reconstruct RF echo frames with A-line densities that varied from 6.4 lines/mm to 51.2 lines/mm. In the second group, the size of the aperture was varied to produce RF echo frames with lateral beamwidths ranging from 0.22 mm to 0.43 mm and a fixed A-line density of 25.6 lines/mm. In the third group, sparse arrays with varying number of active transmission elements (from 2 to 128) were used to reconstruct RF echo frames, whose A-line density and lateral beamwidth were fixed to 25.6 lines/mm and 0.22 mm, respectively. Applying a two-dimensional (2-D) displacement estimator to the pre- and post-deformed RF echo frames produced displacement elastograms. Axial and lateral strain elastograms were computed from displacement elastograms with a least squares strain estimator. The quality of axial and lateral strain elastograms improved with increasing applied strain and A-line density but decreased with increasing lateral beamwidth and deteriorated as the number of active transmission elements in the sparse arrays were reduced. This work demonstrated that the variance incurred when estimating the lateral component of displacement was reduced considerably when elastography was performed with a synthetic aperture ultrasound imaging system. Satisfactory axial and lateral strain elastograms were produced using a sparse array with as few as 16 active transmission elements.
基于模型的弹性成像是一种新兴技术，具有在血管组织成像、引导微创治疗以及诊断乳腺癌和前列腺癌方面的临床应用。它的使用受到限制，因为超声只能高精度地测量位移的轴向分量。本研究的目的是评估横向采样频率、横向波束宽度和有效发射元件数量对轴向和横向应变弹性成像质量的影响。弹性成像在基于明胶的体模上进行，使用改良的商业超声扫描仪。从完全合成孔径数据重建三组射频（RF）回波帧。在第一组中，使用所有 128 个发射元件（对应于视场中心的横向波束宽度为 0.22mm），以 A 线密度从 6.4 线/mm 到 51.2 线/mm 重建 RF 回波帧。在第二组中，改变孔径的大小以产生具有从 0.22mm 到 0.43mm 的横向波束宽度和固定 A 线密度为 25.6 线/mm 的 RF 回波帧。在第三组中，使用具有不同数量的有效发射元件（从 2 到 128）的稀疏阵列重建 RF 回波帧，其 A 线密度和横向波束宽度分别固定为 25.6 线/mm 和 0.22mm。将二维（2-D）位移估计器应用于预变形和后变形的 RF 回波帧，生成位移弹性图像。通过最小二乘应变估计器从位移弹性图像中计算轴向和横向应变弹性图像。轴向和横向应变弹性图像的质量随着施加应变和 A 线密度的增加而提高，但随着横向波束宽度的增加而降低，并且当稀疏阵列中的有效发射元件数量减少时会变差。这项工作表明，在使用合成孔径超声成像系统进行弹性成像时，横向位移分量的估计方差大大降低。使用只有 16 个有效发射元件的稀疏阵列可以生成令人满意的轴向和横向应变弹性图像。
