Liu Bei, Qian Shengyou, Hu Weipeng
School of Physics and Electronics, Hunan Normal University, Changsha 410081, China.
Entropy (Basel). 2019 Jul 8;21(7):666. doi: 10.3390/e21070666.
Identification of denatured biological tissue is crucial to high intensity focused ultrasound (HIFU) treatment. It is not easy for intercepting ultrasonic scattered echo signals from HIFU treatment region. Therefore, this paper employed time-frequency entropy based on generalized S-transform (GST) to intercept ultrasonic echo signals. First, the time-frequency spectra of ultrasonic echo signal is obtained by GST, which is concentrated around the real instantaneous frequency of the signal. Then the time-frequency entropy is calculated based on time-frequency spectra. The experimental results indicate that the time-frequency entropy of ultrasonic echo signal will be abnormally high when ultrasonic signal travels across the boundary between normal region and treatment region in tissues. Ultrasonic scattered echo signals from treatment region can be intercepted by time-frequency entropy. In addition, the refined composite multi-scale weighted permutation entropy (RCMWPE) is proposed to evaluate the complexity of nonlinear time series. Comparing with multi-scale permutation entropy (MPE) and multi-scale weighted permutation entropy (MWPE), RCMWPE not only measures complexity of signal including amplitude information, but also improves the stability and reliability of multi-scale entropy. The RCMWPE and MPE are applied to 300 cases of actual ultrasonic scattered echo signals (including 150 cases in normal status and 150 cases in denatured status). It is found that the RCMWPE and MPE values of denatured tissues are higher than those of the normal tissues. Both RCMWPE and MPE can be used to distinguish normal tissues and denatured tissues. However, there are fewer feature points in the overlap region between RCMWPE of denatured tissues and normal tissues compared with MPE. The intra-class distance and the inter-class distance of RCMWPE are less and greater respectively than MPE. The difference between denatured tissues and normal tissues is more obvious when RCMWPE is used as the characteristic parameter. The results of this study will be helpful to guide doctors to obtain more accurate assessment of treatment effect during HIFU treatment.
变性生物组织的识别对高强度聚焦超声（HIFU）治疗至关重要。从HIFU治疗区域截取超声散射回波信号并非易事。因此，本文采用基于广义S变换（GST）的时频熵来截取超声回波信号。首先，通过GST获得超声回波信号的时频谱，其集中在信号的实际瞬时频率附近。然后基于时频谱计算时频熵。实验结果表明，当超声信号穿过组织中的正常区域和治疗区域之间的边界时，超声回波信号的时频熵会异常高。可以通过时频熵截取来自治疗区域的超声散射回波信号。此外，提出了改进的复合多尺度加权排列熵（RCMWPE）来评估非线性时间序列的复杂性。与多尺度排列熵（MPE）和多尺度加权排列熵（MWPE）相比，RCMWPE不仅测量包含幅度信息的信号复杂性，还提高了多尺度熵的稳定性和可靠性。将RCMWPE和MPE应用于300例实际超声散射回波信号（包括150例正常状态和150例变性状态）。发现变性组织的RCMWPE和MPE值高于正常组织。RCMWPE和MPE均可用于区分正常组织和变性组织。然而，与MPE相比变性组织和正常组织的RCMWPE重叠区域中的特征点较少。RCMWPE的类内距离较小而类间距离较大，均大于MPE。当使用RCMWPE作为特征参数时，变性组织和正常组织之间的差异更明显。本研究结果将有助于指导医生在HIFU治疗期间获得更准确的治疗效果评估。
