Investigation of Non-Linearities in Medical Ultrasound Imaging Probes by Characterizing Free and Clamped Capacitances

Tissue harmonic imaging requires good control of the nonlinearity in the ultrasound probe, as transmitted second harmonics from the probe may interfere with tissue harmonics and degrade image quality. We have studied the nonlinearity in four different medical ultrasound probes by measuring the capacitive part of their electrical impedances under varying electric fields, at frequencies well below and above the resonances. The probes were made with two different piezoelectric materials, piezoceramic PZT and single-crystal PMN-PT, with either soft backing operating at half-wavelength resonance or hard backing operating at quarter-wavelength resonance. When the applied electric field amplitude E was increased from \mathrm {0.05~V/\mu m} to \mathrm {0.5~V/\mu m} , we observed an increase in both the capacitance at high frequency, interpreted as clamped conditions, and at low frequency, interpreted as free conditions. This is a nonlinear phenomenon as these capacitances will not change in the linear regime. The increase in free capacitance was from 4 to 10 times larger than the increase in clamped capacitance for all the investigated probes. This indicates a stronger nonlinearity for the free capacitance. At the low-frequency excitation, we observed distortion in the current passing through the acoustic stack corresponding to a relative second harmonic level of −20 dB. We conclude that the nonlinear impedance of the acoustic stack in the investigated probes was primarily caused by nonlinearities in the mechanical coefficients, while contributions from dielectric nonlinearity were negligible.