Duryea Alexander P, Cain Charles A, Roberts William W, Hall Timothy L
IEEE Trans Ultrason Ferroelectr Freq Control. 2015 Dec;62(12):2068-78.
Remanent bubble nuclei generated by primary cavitation collapse can limit the efficiency of histotripsy softtissue fractionation. When these residual bubbles persist from one histotripsy pulse to the next, they can seed the repetitive nucleation of cavitation bubbles at a discrete set of sites within the focal volume. This effect-referred to as cavitation memory- manifests in inefficient lesion formation, because certain sites within the focal volume are overtreated whereas others remain undertreated. Although the cavitation memory effect can be passively mitigated by using a low pulse repetition frequency (PRF) that affords remanent nuclei sufficient time for dissolution between successive pulses, this low PRF also results in slow lesion production. As such, it would be highly desirable to maintain the high per-pulse efficiency associated with low pulse rates when much higher PRFs are utilized. In this vein, we have developed a strategy for the active removal of the remanent bubble nuclei following primary cavitation collapse, using low-amplitude ultrasound sequences (termed bubble-removal sequences) to stimulate the aggregation and subsequent coalescence of these bubbles. In this study, bubbleremoval sequences were incorporated in high-PRF histotripsy treatment (100 Hz) of a red blood cell tissue-mimicking phantom that allows for the visualization of lesion development in real time. A series of reference treatments were also conducted at the low PRF of 1 Hz to provide a point of comparison for which cavitation memory effects are minimal. It was found that bubble-removal sequences as short as 1 ms are capable of maintaining the efficacious lesion development characteristics associated with the low PRF of 1 Hz when the much higher pulse rate of 100 Hz is used. These results were then extended to the treatment of a large volume within the tissue phantom, and optimal bubble-removal sequences identified for the singlefocal- spot case were utilized to homogenize a 10 × 10 mm region at high rate.
由初次空化崩溃产生的残留气泡核会限制组织粉碎术软组织破碎的效率。当这些残留气泡从一个组织粉碎术脉冲持续到下一个脉冲时，它们会在焦区内一组离散的位置引发空化气泡的重复成核。这种效应——称为空化记忆——表现为病变形成效率低下，因为焦区内的某些位置受到过度治疗，而其他位置仍未得到充分治疗。尽管通过使用低脉冲重复频率（PRF）可以被动减轻空化记忆效应，该频率为残留核提供了足够的时间在连续脉冲之间溶解，但这种低PRF也会导致病变产生缓慢。因此，当使用高得多的PRF时，非常希望保持与低脉冲率相关的高单脉冲效率。为此，我们开发了一种策略，在初次空化崩溃后主动去除残留气泡核，使用低振幅超声序列（称为气泡去除序列）来刺激这些气泡的聚集和随后的合并。在本研究中，气泡去除序列被纳入红细胞组织模拟体模的高PRF组织粉碎术治疗（100Hz）中，该体模允许实时观察病变发展。还在1Hz的低PRF下进行了一系列参考治疗，以提供空化记忆效应最小的比较点。结果发现，当使用高达100Hz的脉冲率时，短至1ms的气泡去除序列能够保持与1Hz低PRF相关的有效病变发展特征。然后将这些结果扩展到组织体模内大体积的治疗，并利用针对单焦点情况确定的最佳气泡去除序列以高速率均匀化一个10×10mm的区域。
