Lin Kuang-Wei, Hall Timothy L, McGough Robert J, Xu Zhen, Cain Charles A
IEEE Trans Ultrason Ferroelectr Freq Control. 2014 Jul;61(7):1123-36. doi: 10.1109/TUFFC.2014.3012.
In diagnostic ultrasound, broadband transducers capable of short acoustic pulse emission and reception can improve axial resolution and provide sufficient bandwidth for harmonic imaging and multi-frequency excitation techniques. In histotripsy, a cavitation-based ultrasound therapy, short acoustic pulses (<2 cycles) can produce precise tissue ablation wherein lesion formation only occurs when the applied peak negative pressure exceeds an intrinsic threshold of the medium. This paper investigates a frequency compounding technique to synthesize nearly monopolar (half-cycle) ultrasound pulses. More specifically, these pulses were generated using a custom transducer composed of 23 individual relatively-broadband piezoceramic elements with various resonant frequencies (0.5, 1, 1.5, 2, and 3 MHz). Each frequency component of the transducer was capable of generating 1.5-cycle pulses with only one high-amplitude negative half-cycle using a custom 23-channel high-voltage pulser. By varying time delays of individual frequency components to allow their principal peak negative peaks to arrive at the focus of the transducer constructively, destructive interference occurs elsewhere in time and space, resulting in a monopolar pulse approximation with a dominant negative phase (with measured peak negative pressure [P-]: peak positive pressure [P+] = 4.68: 1). By inverting the excitation pulses to individual elements, monopolar pulses with a dominant positive phase can also be generated (with measured P+: P- = 4.74: 1). Experiments in RBC phantoms indicated that monopolar pulses with a dominant negative phase were able to produce very precise histotripsy-type lesions using the intrinsic threshold mechanism. Monopolar pulses with a dominant negative phase can inhibit shock scattering during histotripsy, leading to more predictable lesion formation using the intrinsic threshold mechanism, while greatly reducing any constructive interference, and potential hot-spots elsewhere. Moreover, these monopolar pulses could have many potential benefits in ultrasound imaging, including axial resolution improvement, speckle reduction, and contrast enhancement in pulse inversion imaging.
在诊断超声中，能够发射和接收短声脉冲的宽带换能器可提高轴向分辨率，并为谐波成像和多频率激发技术提供足够的带宽。在组织超声破碎术（一种基于空化的超声治疗方法）中，短声脉冲（<2个周期）可产生精确的组织消融，其中只有当施加的峰值负压超过介质的固有阈值时才会形成损伤。本文研究了一种频率复合技术，以合成近乎单极（半周期）的超声脉冲。更具体地说，这些脉冲是使用一个定制的换能器产生的，该换能器由23个具有不同共振频率（0.5、1、1.5、2和3兆赫）的相对宽带压电陶瓷元件组成。换能器的每个频率分量能够使用定制的23通道高压脉冲发生器产生仅具有一个高振幅负半周期的1.5周期脉冲。通过改变各个频率分量的时间延迟，使它们的主峰值负峰在换能器的焦点处相长干涉，在时间和空间的其他地方会发生相消干涉，从而产生具有主导负相位的单极脉冲近似（测量的峰值负压[P-]：峰值正压[P+]=4.68:1）。通过反转施加到各个元件的激励脉冲，也可以产生具有主导正相位的单极脉冲（测量的P+：P-=4.74:1）。在红细胞模型中的实验表明，具有主导负相位的单极脉冲能够利用固有阈值机制产生非常精确的组织超声破碎术类型的损伤。具有主导负相位的单极脉冲可以在组织超声破碎术中抑制冲击波散射，从而利用固有阈值机制导致更可预测的损伤形成，同时大大减少任何相长干涉以及其他地方潜在的热点。此外，这些单极脉冲在超声成像中可能有许多潜在益处，包括提高轴向分辨率、减少斑点以及增强脉冲反转成像中的对比度。
