Modeling of anomalies due to hydrophones in continuous-wave ultrasound fields

Needle and spot-poled membrane hydrophones using polyvinylidene fluoride (PVDF) sensors are widely used for characterization of biomedical ultrasound fields. It is known that, in measurements of continuous-wave (CW) fields, standing waves may be generated between the transducer and the hydrophone, distorting the field and possibly alternating the signal of the hydrophone. This study uses a three-dimensional, full-wave method to computationally simulate the distortion in the CW field caused by needle and membrane hydrophones. The physical model used in simulations is based on the linear time-harmonic wave equation, which therefore neglects the effects of nonlinear wave propagation. The significance of the distortion is examined by comparing fields emitted by 0.5-5.0 MHz planar circular transducers in the absence and presence of the hydrophones. In addition, the effect of the field distortions on the signal of the hydrophones is studied with simulated measurements. The simulations showed an observable standing wave pattern between the source and the needle hydrophone if the diameter of the needle was larger than a half of the wavelength. However, the standing waves had no clear effect on the signal of the hydrophone. The presence of membrane hydrophone in the CW field generated notable standing waves. Furthermore, the standing waves caused a periodic distortion to the signal of the membrane hydrophone.