Gap Feedback Linearization of CMUTs for Harmonic Imaging and HIFU

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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.