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将是改善组织谐波成像（THI）和高强度超声应用的理想选择，这些应用已被证明对临床环境中许多疾病的诊断和治疗有用。尽管CMUTs被证明具有宽带宽并且能够产生适合治疗性超声的强度水平，但其固有的非线性转导机制已经成为这些临床重要应用的重大障碍。在研究cmut中非线性的来源时，我们最近开发了一种鲁棒且实用的方法来克服这一重要瓶颈。CMUT的非线性源于CMUT膜上的瞬时力与（V/g）比值的平方成正比，其中V为换能器上的电压，g为瞬时膜-衬底间隙。通过用期望压力输出频率的一半的纯交流电信号激励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，这消除了1/g依赖性，导致谐波产生显着减少。我们通过将一个明智选择的阻抗元件与CMUT串联，或者使用电流驱动电路驱动CMUT来实现这一点。与早期的CMUT非线性还原方法相比，该方法不依赖于复杂的预畸变波形。当cmut通过间隙反馈线性化时，可以避免膜崩溃，并且可以利用整个器件间隙进行驱动。因此，在没有直流充电问题的情况下，可以获得CMUT在非折叠模式下的最大压力。同时，CMUT保持了接收模式工作的固有宽带带宽，这对常规成像和谐波成像非常重要。我们在工作在1-10MHz范围内的单元件cmut上获得了不同间隙反馈拓扑的初步实验结果来验证该方法。仿真结果表明，谐波可以在基频下降低40dB，适合于THI成像。基于这些令人兴奋的结果，在本项目中，我们将探索这种用于THI， HIFU和双模式成像治疗的CMUT阵列的新方法。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素和反馈拓扑。我们将制作CMUT阵列，并通过水听器测量评估间隙反馈方法，并与商用压电阵列进行比较。我们将定量评估CMUT阵列的谐波成像性能，使用商业研究超声系统模拟商业模型和造影剂，并与压电对应物进行比较。我们期望这项研究通过充分利用CMUT技术的潜力，在改善谐波成像和临床HIFU技术方面迈出重要的一步。

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.