Krongold B S, Sayeed A M, Moehring M A, Ritcey J A, Spencer M P, Jones D L
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign 61801, USA.
IEEE Trans Biomed Eng. 1999 Sep;46(9):1081-9. doi: 10.1109/10.784139.
Small formed elements and gas bubbles in flowing blood, called microemboli, can be detected using Doppler ultrasound. In this application, a pulsed constant-frequency ultrasound signal insonates a volume of blood in the middle cerebral artery, and microemboli moving through its sample volume produce a Doppler-shifted transient reflection. Current detection methods include searching for these transients in a short-time Fourier transform (STFT) of the reflected signal. However, since the embolus transit time through the Doppler sample volume is inversely proportional to the embolus velocity (Doppler-shift frequency), a matched-filter detector should in principle use a wavelet transform, rather than a short-time Fourier transform, for optimal results. Closer examination of the Doppler shift signals usually shows a chirping behavior apparently due to acceleration or deceleration of the emboli during their transit through the Doppler sample volume. These variations imply that a linear wavelet detector is not optimal. We apply linear and quadratic time-frequency and time-scale detectors to a set of noise-corrupted embolus data. Our results show improvements of about 1 dB using the time-scale detectors versus an STFT-based detector signifying that embolus detection is best approached as a time-scale problem. A time-scale-chirp detector is also applied and is found to have the overall best performance by about 0.5-0.7 dB while coming fairly close (about 0.75 dB) to a theoretical upper bound.
流动血液中的小的成形成分和气泡，即微栓子，可以使用多普勒超声进行检测。在该应用中，一个脉冲恒频超声信号对大脑中动脉中的一定体积的血液进行声照射，穿过其采样体积的微栓子会产生一个多普勒频移的瞬态反射。当前的检测方法包括在反射信号的短时傅里叶变换（STFT）中搜索这些瞬态信号。然而，由于栓子穿过多普勒采样体积的通过时间与栓子速度（多普勒频移频率）成反比，原则上匹配滤波器检测器应该使用小波变换而不是短时傅里叶变换来获得最佳结果。对多普勒频移信号的仔细检查通常会发现一种啁啾行为，这显然是由于栓子在穿过多普勒采样体积时加速或减速所致。这些变化意味着线性小波检测器并非最优。我们将线性和二次时频及时间尺度检测器应用于一组含有噪声的栓子数据。我们的结果表明，与基于STFT的检测器相比，使用时间尺度检测器可提高约1dB，这表明将栓子检测视为一个时间尺度问题是最佳方法。还应用了一种时间尺度啁啾检测器，发现其总体性能最佳，比理论上限低约0.75dB，比其他方法高出约0.5 - 0.7dB。
