Gap Feedback Linearization of CMUTs for Harmonic Imaging and HIFU

描述(申请人提供)：具有宽带和高输出压力能力的线性超声换能器将是改进的组织谐波成像(THI)和高强度超声应用的理想选择，这些应用被证明在临床环境中对许多疾病的诊断和治疗是有用的。尽管CMUT具有很宽的带宽，能够产生适合治疗性超声的强度水平，但其固有的非线性传导机制一直是这些临床重要应用的重大障碍。在研究CMUT中的非线性来源的同时，我们最近开发了一种稳健而实用的方法来克服这一重要的瓶颈。CMUT的非线性源于CMUT膜上的瞬时作用力与(V/g)比的平方成正比，其中V是换能器上的电压，g是膜-衬底的瞬时间隙。通过以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，这取消了1/g的依赖关系，从而显著减少了谐波产生。我们通过将精心选择的阻抗元件与CMUT串联来实现这一点，或者使用电流驱动电路来驱动CMUT。与以往的CMUT非线性抑制方法不同，该方法不依赖于复杂的预失真波形。当通过间隙反馈线性化CMUT时，可以避免薄膜崩塌，并且可以利用整个器件间隙来驱动。因此，CMUT在非坍塌模式下可以获得最大可用压力，而不会出现直流充电问题。同时，CMUT用于接收模式操作的固有的宽带宽被保留，这对于常规成像和谐波成像是重要的。为了验证该方法，我们对工作在1-10 MHz范围内的单元件CMUT获得了不同间隙反馈拓扑的初步实验结果。仿真结果表明，该方法可以将谐波降低到基波以下40d B，适合于THI成像。基于这些令人兴奋的结果，在这个项目中，我们将探索这种用于THI、HIFU和双模成像治疗应用的CMUT阵列的新方法。我们将扩展我们的模型，以包括相控阵运算和双电极CMUT，以确定不同应用的最佳阵元和反馈拓扑。我们将制作CMUT阵列，并通过水听器测量对间隙反馈方法进行评估，并与商用压电阵列进行比较。我们将使用商业研究超声系统在模拟商业体模和造影剂的组织上定量评估CMUT阵列的谐波成像性能，并与压电式阵列进行比较。我们期望这项研究通过充分挖掘CMUT技术的潜力，在临床环境中改进谐波成像和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.