Fundamental and third harmonic operation of a medical phased array transducer

An experimental PZT-based phased array probe (f_c = 3 MHz, 64 elements, elevation focus at 60 mm) was used to investigate the performance at the third harmonic resonance mode. Impedance analyzer measurements of electrically untuned elements indicated that the third harmonic thickness resonance was located at 10 MHz. Serial tuning inductors were then replaced to tune around 10 MHz. Measurements of the transmit and two-way transfer function indicated the transmit and receive potential at both the fundamental and third harmonic for a re-tuned element. As compared to a conventionally tuned element, the transmit transfer function at the third harmonic increased more than 23 dB, while the sensitivity at the fundamental was only 6 dB lower. Simulated transfer functions, from both a 1D KLM and 2D finite element model of an element of the experimental phased array transducer, confirmed the expected second sensitivity peak at the third harmonic thickness resonance.