IEEE Trans Ultrason Ferroelectr Freq Control. 2020 Feb;67(2):308-317. doi: 10.1109/TUFFC.2019.2944305. Epub 2019 Sep 27.
The objective of this article is to demonstrate the feasibility of estimating the backscatter coefficient (BSC) using an in situ calibration source. Traditional methods of estimating the BSC in vivo using a reference phantom technique do not account for transmission losses due to intervening layers between the ultrasonic source and the tissue region to be interrogated, leading to increases in bias and variance of BSC-based estimates. To account for transmission losses, an in situ calibration approach is proposed. The in situ calibration technique employs a titanium sphere that is well-characterized ultrasonically, biocompatible, and embedded inside the sample. A set of experiments was conducted to evaluate the embedded titanium spheres as in situ calibration targets for BSC estimation. The first experiment quantified the backscattered signal strength from titanium spheres of three sizes: 0.5, 1, and 2 mm in diameter. The second set of experiments assessed the repeatability of BSC estimates from the titanium spheres and compared these BSCs to theory. The third set of experiments quantified the ability of the titanium bead to provide an in situ reference spectrum in the presence of a lossy layer on top of the sample. The final set of experiments quantified the ability of the bead to provide a calibration spectrum over multiple depths in the sample. All experiments were conducted using an L9-4/38 linear array connected to a SonixOne system. The strongest signal was observed from the 2-mm titanium bead with the signal-to-noise ratio (SNR) of 11.6 dB with respect to the background speckle. Using an analysis bandwidth of 2.5-5.5 MHz, the mean differences between the experimentally derived BSCs and BSCs derived from the Faran theory were 0.54 and 0.76 dB using the array and a single-element transducer, respectively. The BSCs estimated using the in situ calibration approach without the layer and with the layer and using the reference phantom approach with the layer were compared to the reference phantom approach without the layer present. The mean differences in BSCs were 0.15, 0.73, and -9.69 dB, respectively. The mean differences of the BSCs calculated from data blocks located at depths that were either 30 pulse lengths above or below the actual bead depth compared to the BSC calculated at bead depth were -1.55 and -1.48 dB, respectively. The results indicate that an in situ calibration target can account for overlaying tissue losses, thereby improving the robustness of BSC-based estimates.
本文旨在演示使用原位校准源估计反向散射系数 (BSC) 的可行性。传统的使用参考体模技术在体内估计 BSC 的方法不能考虑到超声波源和要探测的组织区域之间的层间传输损耗，导致基于 BSC 的估计的偏差和方差增加。为了考虑传输损耗，提出了一种原位校准方法。原位校准技术采用钛球作为校准目标，钛球具有良好的超声特性、生物相容性，并且嵌入在样品内部。进行了一组实验来评估嵌入钛球作为 BSC 估计的原位校准目标的效果。第一项实验量化了三种直径（0.5、1 和 2 毫米）的钛球的反向散射信号强度。第二项实验评估了从钛球获得的 BSC 估计的重复性，并将这些 BSCs 与理论进行了比较。第三组实验量化了在样品顶部存在损耗层的情况下，钛珠提供原位参考光谱的能力。最后一组实验量化了珠子在样品的多个深度提供校准光谱的能力。所有实验均使用连接到 SonixOne 系统的 L9-4/38 线性阵列进行。在 2 毫米钛球上观察到最强的信号，其相对于背景散斑的信噪比 (SNR) 为 11.6dB。使用分析带宽为 2.5-5.5MHz，实验得出的 BSCs 与 Faran 理论得出的 BSCs 之间的平均差值分别为 0.54 和 0.76dB，使用的是阵列和单个换能器。没有层的情况下使用原位校准方法、有层的情况下使用参考体模方法和没有层的情况下使用参考体模方法的 BSCs 进行了比较。BSCs 的平均差值分别为 0.15、0.73 和-9.69dB。与位于实际珠子深度上方或下方 30 个脉冲长度处的数据块计算的 BSCs 相比，位于珠子深度处计算的 BSCs 的平均差值分别为-1.55 和-1.48dB。结果表明，原位校准目标可以考虑重叠组织的损耗，从而提高基于 BSC 的估计的稳健性。
