Gap Feedback Linearization of CMUTs for Harmonic Imaging and HIFU

描述（由申请人提供）：具有宽带宽和高输出压力能力的线性超声换能器对于改进的组织谐波成像（THI）和高强度超声应用是理想的，这些应用被证明可用于临床环境中许多疾病的诊断和治疗。虽然CMUT显示出具有宽的带宽并且能够产生适合于治疗超声的强度水平，但是其固有的非线性转导机制已经成为这些临床重要应用的显著障碍。在研究CMUT中的非线性来源时，我们最近开发了一种鲁棒且实用的方法，克服了这一重要瓶颈。CMUT的非线性源于CMUT膜上的瞬时力与（V/g）比的平方成比例的事实，其中V是换能器上的电压，g是瞬时膜-基底间隙。通过以所需压力输出的频率的一半用仅AC电信号激励CMUT，我们消除了电压平方非线性。然后，我们迫使CMUT上的电压与瞬时间隙成反比，并且这消除了1/g依赖性，从而导致谐波产生的显著减少。我们通过将明智选择的阻抗元件与CMUT串联放置来实现这一点，或者替代地使用电流驱动电路来驱动CMUT。与用于CMUT非线性降低的早期方法相比，该方法不依赖于复杂的预失真波形。当CMUT通过间隙反馈线性化时，可以避免膜塌陷，并且可以将整个器件间隙用于致动。因此，在没有DC充电问题的情况下，获得了在非塌陷模式下从CMUT可获得的最大压力。与此同时，保留了CMUT用于接收模式操作的固有宽带宽，这对于常规和谐波成像非常重要。我们得到了初步的实验结果，不同的间隙反馈拓扑结构的单元件CMUT工作在1- 10 MHz的范围内，以证明该方法。仿真结果表明，谐波可以减少40分贝以下的基本，适合THI成像。基于这些令人兴奋的结果，在这个项目中，我们将探索这种新的方法CMUT阵列THI，HIFU和双模成像治疗应用。我们将扩展我们的模型，包括相控阵操作和双电极CMUT，以确定不同应用的最佳阵列元件和反馈拓扑结构。我们将制作CMUT阵列，并通过水听器测量评估间隙反馈方法，并与商业压电阵列进行比较。我们将定量评估谐波成像性能的CMUT阵列使用商业研究超声系统的组织模仿商业phantomy和造影剂，并与压电同行进行比较。我们希望这项研究是一个重要的一步，在改善谐波成像和HIFU技术在临床环境中利用CMUT技术的全部潜力。

DESCRIPTION (provided by applicant): A linear ultrasonic transducer with broad bandwidth and high output pressure capability would be ideal for improved tissue harmonic imaging (THI) and high intensity ultrasound applications which are proven to be useful for diagnosis and treatment of many diseases in clinical settings. Although CMUTs are shown to have broad bandwidth and being able to generate intensity levels suitable for therapeutic ultrasound, their inherently nonlinear transduction mechanism has been a significant barrier for these clinically important applications. While investigating the sources of nonlinearity in CMUTs, we recently developed a robust and practical method which overcomes this important bottleneck. The nonlinearity of the CMUT stems from the fact that the instantaneous force on the CMUT membrane is proportional to the square of the (V/g) ratio, where V is voltage on the transducer and g is the instantaneous membrane-substrate gap. By exciting the CMUT with an AC-only electrical signal at half the frequency of the desired pressure output, we cancel the voltage square nonlinearity. We then force the voltage on the CMUT to be inversely proportional to the instantaneous gap, and this cancels the 1/g dependence leading to significant reduction in harmonic generation. We achieve this by placing a judiciously chosen impedance element in series with the CMUT, or alternatively drive the CMUT using a current drive circuit. In contrast with earlier approaches for CMUT nonlinearity reduction, this method does not rely on complex pre-distorted waveforms. When CMUTs are linearized through gap feedback, membrane collapse can be avoided and the full device gap can be used for actuation. Therefore maximum pressure available from CMUT in non-collapse mode is obtained without DC charging problems. In the meantime, the inherent broad bandwidth of the CMUT for receive mode operation is retained, which is important for conventional and harmonic imaging. We obtained initial experimental results with different gap feedback topologies on single element CMUTs operating in the 1-10MHz range to demonstrate the method. The simulations indicate that harmonics can be reduced 40dB below fundamental, suitable for THI imaging. Based on these exciting results, in this project, we will explore this novel approach on CMUT arrays for THI, HIFU and dual-mode imaging-therapy applications. We will extend our model to include phased array operation and dual-electrode CMUTs to determine optimal array element and feedback topology for different applications. We will fabricate the CMUT arrays and evaluate gap feedback method through hydrophone measurements and compare with commercial piezoelectric arrays. We will quantitatively evaluate harmonic imaging performance of CMUT arrays using a commercial research ultrasound system on tissue mimicking commercial phantoms and contrast agents and compare with piezoelectric counterparts. We expect this study to be an important step in improving harmonic imaging and HIFU techniques in clinical settings by exploiting the full potential of the CMUT technology.