Le L H
Department of Radiology and Diagnostic Imaging, Walter C Mackenzie Health Sciences Centre, The University of Alberta, Edmonton, Canada.
Phys Med Biol. 1998 Aug;43(8):2295-308. doi: 10.1088/0031-9155/43/8/021.
Berlage wavelets are used to simulate ultrasonic pulses in an unbounded, homogeneous, isotropic and absorptive medium. Intrinsic absorption of the medium is properly described by Kolsky's attenuation, which considers velocity dispersion to meet the causality condition. Several current time-domain velocity measurement techniques have been investigated using numerically simulated pulses for three normalized BUA values: 20, 40 and 60 dB MHz(-1) cm(-1), which mimic experimentally determined values for cancellous bone. The velocities, calculated using first motion transit times, are used as references supported by the Fermat principle of least time. The simulated results for fixed sample thickness indicate that pulse-broadening increases with the transit time of the reference point and the intrinsic absorption of the medium. Comparison shows that the first zero-crossing method yields 3-6% errors in velocity results, better than the cross-correlation method. However, the zero-crossing method gives inconsistent velocity measurement for a medium of 40 dB MHz(-1) cm(-1)1 absorption and three different thicknesses: 0.2, 0.4 and 0.6 cm. A novel technique for velocity measurement is presented using the peak of the envelope of a signal as a reference point to measure transit time difference. The envelope of a signal represents the instantaneous amplitude of the associated analytic signal. The velocities derived using this method differ from the true velocities by only 1.2-2.4%, more accurate than those obtained by the first zero-crossing method. The envelope peak has the additional merits of easy detection and robustness. Most importantly, the envelope technique may be used to yield accurate velocity measurement in cases where an accurate determination of the first motion transit time is sometimes prohibited due to the presence of noise.
贝拉尔热小波用于模拟在无界、均匀、各向同性和吸收性介质中的超声脉冲。介质的固有吸收通过科尔兹基衰减来恰当描述，该衰减考虑了速度色散以满足因果条件。使用数值模拟脉冲针对三个归一化宽带超声衰减（BUA）值：20、40和60 dB MHz⁻¹ cm⁻¹，研究了几种当前的时域速度测量技术，这三个值模拟了松质骨的实验测定值。使用初动传播时间计算的速度用作由费马最短时间原理支持的参考值。固定样本厚度的模拟结果表明，脉冲展宽随参考点的传播时间和介质的固有吸收而增加。比较表明，首次过零法在速度结果中产生3 - 6%的误差，优于互相关法。然而，对于吸收为40 dB MHz⁻¹ cm⁻¹且厚度分别为0.2、0.4和0.6 cm的三种不同介质，过零法给出的速度测量结果不一致。提出了一种新的速度测量技术，使用信号包络的峰值作为参考点来测量传播时间差。信号的包络代表相关解析信号的瞬时幅度。使用该方法得出的速度与真实速度的差异仅为1.2 - 2.4%，比首次过零法获得的结果更准确。包络峰值具有易于检测和稳健性的额外优点。最重要的是，在由于噪声存在有时无法准确确定初动传播时间的情况下，包络技术可用于进行准确的速度测量。
