Université François Rabelais de Tours, UMRS Imaging and Brain, Tours, France.
Ultrasound Med Biol. 2011 Jan;37(1):87-101. doi: 10.1016/j.ultrasmedbio.2010.09.006. Epub 2010 Nov 16.
The purpose of this study was to reveal the cause of frequency modulation (FM) present in microembolic Doppler ultrasound signals. This novel explanation should help the development of sensitive microembolus discrimination techniques. We suggest that the frequency modulation detected is caused by the ultrasonic radiation force (URF) acting directly on microemboli. The frequency modulation and the imposed displacement were calculated using a numerical dynamic model. By setting simulation parameters with practical values, it was possible to reproduce most microembolic frequency modulation signatures. The most interesting findings in this study were that: (1) the ultrasound radiation force acting on a gaseous microembolus and its corresponding cumulative displacement were far higher than those obtained for a solid microembolus, and that is encouraging for discrimination purposes; and 2) the calculated frequency modulation indices (FMIs) (≈20 kHz) were in good agreement with literature results. By taking into account the URF, the flow pulsatility, the beam-to-flow angle and both the velocity and the ultrasound beam profiles, it was possible to explain all erratic FM signatures of a microbubble. Finally, by measuring FMI from simulated Doppler signals and by using a constant threshold of 1 KHz, it was possible to discriminate gaseous from solid microemboli with ease.
本研究旨在揭示微栓子多普勒超声信号中出现调频（FM）的原因。这一新的解释应该有助于开发敏感的微栓子鉴别技术。我们认为，检测到的频率调制是由直接作用于微栓子的超声辐射力（URF）引起的。使用数值动力学模型计算了频率调制和施加的位移。通过设置具有实际值的模拟参数，有可能再现大多数微栓子的频率调制特征。本研究中最有趣的发现是：（1）作用于气态微栓子的超声辐射力及其相应的累积位移远高于固态微栓子，这对鉴别目的很有帮助；（2）计算出的频率调制指数（FMIs）（≈20 kHz）与文献结果吻合较好。通过考虑 URF、流动力脉动、束流角度以及速度和超声束剖面，可以解释微泡所有不稳定的 FM 特征。最后，通过测量模拟多普勒信号中的 FMI，并使用 1 kHz 的恒定阈值，很容易区分气态和固态微栓子。
