Doinikov Alexander A, Haac Jillian F, Dayton Paul A
Institute of Nuclear Problems, Belarus State University, Minsk, Belarus.
Ultrasonics. 2009 Feb;49(2):263-8. doi: 10.1016/j.ultras.2008.09.006. Epub 2008 Sep 30.
Knowledge of resonant frequencies of contrast microbubbles is important for the optimization of ultrasound contrast imaging and therapeutic techniques. To date, however, there are estimates of resonance frequencies of contrast microbubbles only for the regime of linear oscillation. The present paper proposes an approach for evaluating resonance frequencies of contrast agent microbubbles in the regime of nonlinear oscillation. The approach is based on the calculation of the time-averaged oscillation power of the radial bubble oscillation. The proposed procedure was verified for free bubbles in the frequency range 1-4 MHz and then applied to lipid-shelled microbubbles insonified with a single 20-cycle acoustic pulse at two values of the acoustic pressure amplitude, 100 kPa and 200 kPa, and at four frequencies: 1.5, 2.0, 2.5, and 3.0 MHz. It is shown that, as the acoustic pressure amplitude is increased, the resonance frequency of a lipid-shelled microbubble tends to decrease in comparison with its linear resonance frequency. Analysis of existing shell models reveals that models that treat the lipid shell as a linear viscoelastic solid appear may be challenged to provide the observed tendency in the behavior of the resonance frequency at increasing acoustic pressure. The conclusion is drawn that the further development of shell models could be improved by the consideration of nonlinear rheological laws.
了解造影剂微泡的共振频率对于优化超声造影成像和治疗技术非常重要。然而，迄今为止，仅在线性振荡范围内对造影剂微泡的共振频率进行了估计。本文提出了一种评估非线性振荡范围内造影剂微泡共振频率的方法。该方法基于径向气泡振荡的时间平均振荡功率的计算。所提出的程序在1-4MHz频率范围内对自由气泡进行了验证，然后应用于在100kPa和200kPa两个声压幅值以及1.5、2.0、2.5和3.0MHz四个频率下用单个20周期声脉冲照射的脂质壳微泡。结果表明，随着声压幅值的增加，脂质壳微泡的共振频率与其线性共振频率相比趋于降低。对现有壳模型的分析表明，将脂质壳视为线性粘弹性固体的模型可能难以解释在声压增加时共振频率行为中观察到的趋势。得出的结论是，通过考虑非线性流变学定律可以改进壳模型的进一步发展。
