Ultrasonic guided wave is sensitive to waveguide microstructure and material property, which has great potential applications in long cortical bone evaluation. Due to the multimodal dispersion effect, low-frequency guided wave is usually used to avoid multimode overlapping and simplify the signal processing. However, the traditional low-frequency ultrasound transducer is usually designed on a large-scale (around several millimeters), leading to relatively low-spatial resolution. In response to such a technique limit, an ultrasound-stimulated vibro-acoustic method is introduced to excite low-frequency ultrasonic guided waves. There are two excitation ways of the ultrasound-stimulated vibro-acoustic method, i.e., a single amplitude-modulated (AM) beam and confocal beam excitation. In the case of the single beam excitation, a high-frequency signal is modulated by using a low-frequency amplitude. In addition, low-frequency vibration can also be produced by a confocal transducer, where two beams are close to the center frequency and focus on a small region. In this way, the frequency difference between two beams can be selected to generate the arbitrary low-frequency excitation in a given bandwidth on the focus point. In this paper, we first introduce the theory of ultrasonic guided wave in the plate and the basic principle of ultrasound-stimulated acoustic emission. Second, the three-dimensional finite element method is used to simulate the phenomena of the low-frequency ultrasonic guided waves excited by the ultrasound-stimulated vibro-acoustic method. Two Gaussian-function enveloped tone-burst signals close to the center frequencies of 5 MHz are used to excite 150 kHz low-frequency guided wave in a 3 mm-thick bone plate. An ex-vivo bovine bone plate is involved in the experiments to test the feasibility of the proposed method. The axial transmission ultrasonic guided waves are recorded at eight different propagation distances. The time-frequency representation method is used to analyze the dispersive guided waves. The results indicate that both the two confocal beams and the single AM beam are capable of stimulating low-frequency ultrasonic guided waves in the bone plate. The first two fundamental guided wave modes, i.e., symmetrical S0 and asymmetrical A0 are observed in the bone plate. Similar spectrum can be obtained in the two different excitation ways. In the simulation and experiment, two wave packets can be separated in the distance-time diagram of the received signals. Good agreement can be found between the results of time-frequency representation and the theoretical group dispersion curves. This study can enhance the spatial resolution of measuring ultrasonic guided wave in long bone, and improve the flexibility of excitation with arbitrary frequency in a given bandwidth. The study can be helpful for developing the new clinical techniques of using low-frequency guided waves for long cortical bone assessment.%为了实现一定频段内任意低频下在长骨中激励导波信号,本文提出一种采用聚焦高频(5 MHz)超声换能器在长骨皮质骨中激发低频(150 kHz)超声导波的振动声方法.首先介绍了板状超声导波理论和双声束共聚焦法与单声束调幅法激发振动声的基本原理;进而采用三维有限元仿真方法分析振动声激发低频超声导波的基本现象,然后结合牛胫骨板离体实验,验证振动声激发低频超声导波的可行性.结果均表明,双声束共焦与单声束振动超声均可在骨板中激发低频超声导波.相关研究方法有助于提高空间域长骨中超声导波测量精度,以及在一定频段内实现任意频率激励等,对发展低频超声导波在体测量长骨皮质骨的新技术具有一定的指导意义.
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