Walker W F, Trahey G E
Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708, USA.
J Acoust Soc Am. 1997 Apr;101(4):1847-58. doi: 10.1121/1.418235.
Tissue speed of sound inhomogeneities cause significant degradation of medical ultrasound images. In certain cases these inhomogeneities can be modeled as a thin, spatially varying time delay screen located at the face of the transducer. Correction of such aberrators requires the addition of compensating time delays to the normal system focusing delays. These compensating delays are estimated from the arrival time differences between echoes received on different array elements. The accuracy with which these arrival time differences can be estimated is limited by the level of correlation between received speckle signals. This paper derives analytical expressions predicting the correlation between speckle signals acquired by a pulse echo system with either point or larger receive elements in the presence of near-field phase aberrations. Simulations are presented which are in good agreement with theoretical predictions. Similarities between the derived expressions and the Van Cittert-Zernike Theorem are discussed. These results indicate that near-field phase aberration correction may be far more difficult than previous analyses suggest because of the low correlation between echoes received by adjacent elements in elevation in 1.5-D arrays. Transmit aperture amplitude apodization and a new translating aperture technique are presented as methods for improving speckle correlation.
组织中声速的不均匀性会导致医学超声图像显著退化。在某些情况下，这些不均匀性可被建模为位于换能器表面的一个薄的、空间变化的时间延迟屏。校正此类像差需要在正常系统聚焦延迟上添加补偿时间延迟。这些补偿延迟是根据在不同阵列元件上接收到的回波之间的到达时间差来估计的。这些到达时间差的估计精度受到接收散斑信号之间的相关程度的限制。本文推导了在存在近场相位像差的情况下，预测由具有点或较大接收元件的脉冲回波系统采集的散斑信号之间相关性的解析表达式。给出的仿真结果与理论预测吻合良好。讨论了推导表达式与范西特 - 泽尼克定理之间的相似性。这些结果表明，由于1.5维阵列中相邻元件在仰角方向上接收的回波之间相关性较低，近场相位像差校正可能比以前的分析所表明的要困难得多。提出了发射孔径幅度切趾和一种新的平移孔径技术作为改善散斑相关性的方法。
