Signal Processing And Coded Excitation Methods In Ultrasonic Guided Waves Evaluation Of Long Bones

As the population ages, the incidence of osteoporosis becomes higher and higher. By the end of last century, there had been two hundred million patients who had the disease of osteoporosis. Now the main diagnosis method for osteoporosis is dual X-ray absorption (DXA). However, DXA is not able to provide the full information of bone, such as mechanical properties, micro structures, and so on. Besides, X-ray is radioactive, and it is harmful for human body. On the contrary, the device based on ultrasound is radioactive-free, portable, and cheap. Applying ultrasound to assess bone quality has attracted more and more attention. Since ultrasonic guided waves propagate in the whole layer of cortical bone, they are sensitive to the thickness, modulus of elasticity and other parameters of bone. Thus, many researchers began to study the application of guided waves on bone assessment.Based on the above background, this paper focuses on using signal processing methods and different excitation techniques to analyze the guided waves and obtain the thickness of long bones, which is an important parameter for assessing bone quality. The contents are summarized as follows:1) Applying signal processing methods to obtain the thickness of cortical bone. When osteoporosis occurs, the cortical thickness decreases. Based on dispersion equation of guided waves, the thickness can be calculated by group velocity of single guided mode. In order to verify the effectiveness of time-frequency based blind signal separation (TFBSS) method, this paper first used this method to analyze simulated guided waves. Then the experiment signals were collected from bovine bones, and the single modes were separated by TFBSS method. The group velocities were calculated by Gaussian-Chirplet algorithm and deference method. Finally, the thickness was estimated from group velocity by dispersion equation.TFBSS method is based on time-frequency distribution, which costs so much time. Thus, this paper then introduced joint approximate diagonalization of eigen-matices (JADE) method to do analysis of guided waves. After verifying the effectiveness of JADE method on simulated guided waves, the superimposed guided waves from the in vitro experiments were separated to single modes, and then the group velocity and the thickness were obtained. JADE method provides a fast way to analyze guided waves in long bones. However, TFBSS method can do a better job on extracting single mode in relatively low signal-to-noise ratio (SNR) environment.2) Generating guided waves by different excitations. In the study of separating superimposed guided waves, this paper used traditional single pulse to be the excitation signals. Because of high attenuation of bone, transmission device with large power is needed to improve the SNR of the received signals. Large power is not proper in the use of human bone assessment. Thus, this paper introduced coded excitation to generate guided waves to improve SNR, and for comparison, the sinusoidal pulse and several circles of sine waves were also used to be excitation signals. Both coded excitation and sine waves can improve the SNR of received signals compared with sinusoidal pulse, but only coded excitation still kept the axial resolution as sinusoidal pulse did.Based on dispersion curves of guided waves, fewer modes can be generated by lower frequency. The size of transducer with low century frequency is big, but the measurable region of bone is limited. By coded excitation, this paper enables to obtain fewer modes with relatively high SNR at low generating frequency which is far from the century frequency of transducer in long bones. This provides a solution for applying’low frequency’ array transducers to assess long bones.3) Utilization of guided waves to analyze coated bone phantoms. The cortical bone is covered by a layer of soft tissue, and it will bring attenuation and distortion to guided waves. This paper used two layers of hollow cylinder to mimic coated bone. The base sequence modulated Golay code (BSGC) was used to generate guided waves at low frequencies (50kHz,25kHz), which is far away from the century frequency of the transducer (200kHz), in the coated bone phantoms. Then by using two dimensional Fourier transform (2D-FFT), the fundamental flexural guided wave (FFGW) was identified, and its phase velocity was calculated. The experimental velocity of FFGW increased with the thickness of cortical layer, which indicated the phase velocity was sensitive to the thickness of cortical bone.