Nonlinear ultrasound imaging experiments using a CMUT probe

Electrostatic nonlinearity limits the use of Capacitive Micromachined Ultrasonic Transducers (CMUTs) in nonlinear ultrasound modalities, such as contrast and tissue harmonic imaging. In this paper, we experimentally investigate the performance of a linearization method based on the application of pre-distorted analog excitation signals whose spectral content compensates the harmonic distortion introduced by the CMUT. The pre-distortion is based on the accurate knowledge of the CMUT non-linear behavior, achieved by estimating the electrical impedance at several bias voltages. We tested the method using a 256-element, 7.5MHz CMUT linear array probe connected to the ULA-OP 256 imaging system. We measured the acoustic pressure generated in water by a CMUT array element excited with both 4 MHz undistorted and pre-distorted signals. When using the pre-distorted signal, the second harmonic pressure level was reduced by 13 dB, resulting in a final harmonic-to-fundamental level of -30 dB. Fundamental and second harmonic B-mode images of a commercial tissue-mimicking phantom were finally generated using both undistorted and pre-distorted signals. By computing the second harmonic residual power in a region of interest, a reduction of 13 dB was confirmed using the proposed linearization method in a nonlinear ultrasound imaging context.