Measurements of CMUT neighbour coupling resonances in fluids of different viscosities

In medical imaging it is in most cases necessary to steer the beam to form an ultrasound image. This is done by adding phase shifts between neighbour elements. When using CMUT arrays for this kind of imaging, neighbour coupling between elements through the fluid might give resonances at certain frequencies. We will present measurement results and simulations of input admittance of a linear array of CMUT elements, where neighbour elements are excited 180 degrees out of phase. An excitation phase difference of 180 degrees will not occur in a real imaging situation, the phase difference will in most cases be substantially smaller. Simulations for phase differences that are realistic in an imaging situation are also shown. Measurements have been performed in air, rapeseed oil and kerosene (lamp oil). The measurements show resonances in rapeseed oil between 14 MHz and 16 MHz, and between 15.5 MHz and 16.5 MHz in kerosene depending on the DC bias applied. This corresponds to phase velocities between 700 and 825 m/s. In air the resonances occur between 30.5 MHz and 36 MHz. In air and kerosene the simulations show somewhat higher Q-values than the measurements. By adding case independent losses to the CMUTs, it is possible to obtain good match to all measured Q-values. Comparison of measurements and simulations show that the model is well suited to describe the measurements performed. The results presented indicate that the CMUT model can be used to simulate more realistic neighbour element phase shifts. Such simulations show that resonances will disturb the transmitted ultrasonic wave at frequencies close to the centre frequency at large steering angles.