Nonlinear electrical impedance of the acoustic stack in cardiac ultrasound probe

Understanding the nonlinearity in ultrasound probes is important for tissue harmonic imaging, as 2nd harmonic components transmitted from the probe may interfere with harmonics from the tissue and degrade the image quality. The aim of this paper was to identify the main sources of nonlinearity in a medical ultrasound probe. This was done by investigating the capacitive part of its electrical impedance under high excitation voltages, at frequencies well below and well above resonance. We found that when the excitation voltage amplitude was increased from 10 V to 110 V, the free capacitance increased by 50 %, while the clamped capacitance remained unchanged. We also observed an increase with voltage in the loss tangent under free conditions, but not under clamped conditions. We conclude that the nonlinear electrical impedance of the acoustic stack was associated with the mechanical motion and piezoelectric coupling, while contributions from dielectric nonlinearity was negligible.