Optimization of capacitive micromachined ultrasound transducers (CMUTs) for a high-frequency medical ultrasonic imaging system
Pakdaman Zangabad, Reza (2014) Optimization of capacitive micromachined ultrasound transducers (CMUTs) for a high-frequency medical ultrasonic imaging system. [Thesis]
Conventional ultrasonic imaging systems use piezoelectric transducers for the generation and reception of the acoustic signal. Since its invention in 1994, the Capacitive Micromachined Ultrasound Transducer (CMUT) has been subjected to research as an alternative technology. Major advantages of the CMUT over traditional piezoelectric ultrasound transducers include higher bandwidth, higher sensitivity, CMOS compatibility, and ease of manufacturing (by the use of standard lithography techniques.) Increasing the dynamic range, decreasing the parasitic capacitance and cross coupling are the major goals in CMUT designing specially for medical imaging applications. The work in this thesis aims the optimization of a high-frequency (20 MHz) CMUT array to be used for high-resolution medical imaging. The figure of merit has been chosen as the signal-to-noise ratio of the electrical return signal, which required the construction of a model for the entire pulse-echo operation. Such a model consists of: (1) a circuit model for the device itself, (2) a model for the radiation impedance, and (3) a model for the propagation medium. The CMUT model has been extensively studied in the literature. An already existing circuit model has been used in the simulations. The radiation impedance of the CMUT array was computed using Finite Element Analysis (FEA) software packages COMSOL Multiphysics® and ANSYS®, and converted to an equivalent circuit to represent the load in the circuit simulator. The pulse-echo model, which is entirely implemented in LTspice circuit simulator, was then used to optimize CMUT parameters that include radius, membrane thickness, and gap height to maximize signal-to-noise ratio.
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