Frijlink Martijn E, Løvstakken Lasse, Torp Hans
Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, ISB/MTFS/NTNU, 7489 Trondheim, Norway.
Ultrasonics. 2009 Dec;49(8):601-4. doi: 10.1016/j.ultras.2009.03.007. Epub 2009 Apr 7.
In this study, the phenomenon of higher harmonic thickness resonance of a piezoelectric transducer was used to investigate potentially additional sensitivity at the third harmonic frequency for conventional medical transducers. The motivation for this research is that some applications in medical ultrasound (e.g. third harmonic transmit phasing and contrast imaging) need probes which are sensitive around both the fundamental and third harmonic frequencies, and that these higher harmonic thickness modes, although often considered as undesired, might be used beneficially. The novelty aspect in this study is the presented transmit and receive potential at both the fundamental and third harmonic of a conventional cardiac probe with modified electrical tuning. Elements of an experimental PZT-based phased-array probe (f(c)=3 MHz, 64 elements, element width=0.3mm, elevation aperture=13 mm) were electrically retuned with series inductors around the third harmonic resonance frequency at 10 MHz. Hydrophone measurements with 10-MHz-tuned elements showed that, 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 frequency was only 6 dB lower. Pulse-echo measurements showed that the two-way transfer function of a 10-MHz-tuned element resulted in 20 dB increased sensitivity around the third harmonic as compared to an untuned element. Simulated transfer functions, from both a 1D KLM and 2D finite element model of an element of the experimental array transducer, confirmed the measured sensitivity peaks at the fundamental and third harmonic. In conclusion, this study demonstrated the effect of changing the electrical tuning on a conventional array transducer which increased the sensitivity around the third harmonic resonance frequency, while maintaining good sensitivity at the fundamental frequency.
在本研究中，利用压电换能器的高次谐波厚度共振现象，来研究传统医用换能器在三次谐波频率下可能具有的额外灵敏度。开展这项研究的动机在于，医学超声中的一些应用（如三次谐波发射相位控制和造影成像）需要在基波和三次谐波频率附近都具有灵敏度的探头，而且这些高次谐波厚度模式虽然通常被认为是不理想的，但可能会有有益的用途。本研究的新颖之处在于，展示了经过改进电调谐的传统心脏探头在基波和三次谐波处的发射和接收潜力。一个基于实验性PZT的相控阵探头（f(c)=3MHz，64个阵元，阵元宽度=0.3mm，仰角孔径=13mm）的阵元，在10MHz的三次谐波共振频率附近用电感进行了重新调谐。对调谐到10MHz的阵元进行水听器测量结果表明，与传统调谐的阵元相比，三次谐波处的发射传递函数增加了超过23dB，而基波频率处的灵敏度仅低6dB。脉冲回波测量表明，与未调谐的阵元相比，调谐到10MHz的阵元的双向传递函数在三次谐波附近使灵敏度提高了20dB。从实验阵列换能器一个阵元的一维KLM和二维有限元模型得到的模拟传递函数，证实了在基波和三次谐波处测得的灵敏度峰值。总之，本研究证明了改变传统阵列换能器的电调谐的效果，即在保持基波频率良好灵敏度的同时，提高了三次谐波共振频率附近的灵敏度。
