Lacefield James C, Waag Robert C
Department of Electrical and Computer Engineering, University of Rochester, Rochester, New York 14627, USA.
J Acoust Soc Am. 2002 Dec;112(6):2558-66. doi: 10.1121/1.1511756.
Complex degree of coherence functions are computed using synthetic and measured ultrasound data to demonstrate noteworthy aspects of coherence analysis in the context of aberration correction. Coherence functions calculated from synthetic data illustrate the importance of proper normalization of the constituent cross-correlation integrals when weak elements and receiver directivity are significant factors. The synthetic data also show that a spike can occur at the zero-lag position of the coherence function when the signal-to-noise ratio is reduced by element directivity near the edges of a large aperture. The latter observation is confirmed by experimental data acquired through tissue-mimicking distributed aberration phantoms using a low f-number two-dimensional array system. The coherence of data acquired at neighboring elements is not changed by time-shift compensation of transmit and receive focusing, but time-shift compensation does improve the coherence of echoes measured over larger separations. The resulting increase in coherence widths evaluated at levels between 0.2 and 0.5 is correlated with narrower -10 dB and -20 dB effective widths in focuses visualized using single-transmit images. Iterative focus compensation methods may benefit from aberration estimation algorithms that take advantage of these longer-range correlations in random-scattering waveforms.
利用合成超声数据和实测超声数据计算复相干度函数，以展示在像差校正背景下相干分析的显著方面。根据合成数据计算出的相干函数表明，当弱元件和接收器指向性是重要因素时，对组成互相关积分进行适当归一化的重要性。合成数据还表明，当大孔径边缘附近的元件指向性降低信噪比时，相干函数的零延迟位置可能会出现一个尖峰。通过使用低f数二维阵列系统，通过组织模拟分布式像差体模获取的实验数据证实了后一种观察结果。相邻元件采集的数据相干性不会因发射和接收聚焦的时移补偿而改变，但时移补偿确实会提高在更大间距上测量的回波相干性。在0.2至0.5之间的水平评估得到的相干宽度增加，与使用单发射图像可视化的焦点中更窄的-10 dB和-20 dB有效宽度相关。迭代聚焦补偿方法可能受益于利用随机散射波形中这些长程相关性的像差估计算法。
