Varghese T, Ophir J
Department of Radiology, University of Texas Medical School, Houston 77030, USA.
Ultrasound Med Biol. 1998 May;24(4):543-55. doi: 10.1016/s0301-5629(98)00008-8.
A theoretical formulation characterizing the noise performance of strain estimation using envelope signals is presented for the cross-correlation based strain estimator in elastography, using a modified strain filter approach. The strain filter describes the relationship among the elastographic signal-to-noise ratio (SNRe), sensitivity, contrast-to-noise ratio and dynamic range for a given resolution in the elastogram, as determined by the cross-correlation window length and window overlap. Theoretical results indicate that the envelope strain filter noise performance (SNRe level) is about half that obtained in the ratio frequency (RF) case (fo = 7.5 MHz). Simulation results corroborate the trend predicted using the strain filter. Experimental SNRe vs. strain plots presented in this article illustrate the same trend as the theoretical results. These plots allow a quantitative comparison of the elastograms obtained with RF and envelope signal processing. For small strains, the performance obtained using RF signals is superior to that obtained for envelope signals (since jitter errors are smaller due to the utilization of phase information in RF signals). However, for large tissue strains, envelope analysis provides an accurate estimate of the tissue strain (since envelope signal decorrelation is smaller than RF signal decorrelation at large strains). An algorithm that combines the low-noise characteristics of the cross-correlation analysis using RF signals at small strains and envelope signals for estimation of large tissue strains is proposed to improve the dynamic range in the elastogram.
针对弹性成像中基于互相关的应变估计器，采用改进的应变滤波器方法，提出了一种描述使用包络信号进行应变估计的噪声性能的理论公式。应变滤波器描述了弹性图中给定分辨率下弹性成像信噪比（SNRe）、灵敏度、对比度噪声比和动态范围之间的关系，该关系由互相关窗口长度和窗口重叠确定。理论结果表明，包络应变滤波器的噪声性能（SNRe水平）约为比率频率（RF）情况（fo = 7.5 MHz）下的一半。仿真结果证实了使用应变滤波器预测的趋势。本文给出的实验SNRe与应变关系图展示了与理论结果相同的趋势。这些图允许对通过RF和包络信号处理获得的弹性图进行定量比较。对于小应变，使用RF信号获得的性能优于使用包络信号获得的性能（因为由于RF信号中相位信息的利用，抖动误差较小）。然而，对于大组织应变，包络分析提供了组织应变的准确估计（因为在大应变下包络信号去相关小于RF信号去相关）。提出了一种算法，该算法结合了小应变时使用RF信号进行互相关分析的低噪声特性和用于估计大组织应变的包络信号，以提高弹性图中的动态范围。
