Time - Frequency Analysis Of The Measurement Of Ultrasonic Guided Waves In Long Bones And Its Thickness

Osteoporotic bones have thinner cortical bone layers than healthy ones.The velocities of guided waves are very sensitive to bone properties and cortical thickness when propagating in long bone.Therefore,ultrasonic evaluation of long bone based on measurement of guided waves become an attractive topic in recent years.Many attentions have been absorbed on using the velocity of ultrasoinc guided waves to measure cortical thickness,but some challenges still exit including the complexity of the propagation of ultrasonic guided waves in long bone and the superposition of multiple ultrasonic guided modes contained the received signal.The main work of this thesis is to propose time-frequency methods to analyze the properties of ultrasonic guided waves in long bones.The cortical thickness of long bones can be determined after the group velocity of the guided modes is measured in the analysis with the proposed methods.(1)The disperse properties of guided waves in long bone was analyzed with the assumption that the long bone is regarded as a pipe filled with a viscous liquid.(2)Hilbert-Huang transform(HHT) and adaptive Gaussian-Chirplet decomposition(AGCD) were proposed to analyze and identify multi-mode guided modes and decompose to independent modes.(3)The ultrasonic guided waves were measured and analyzed in long bone phantoms(steel plate and plastic pipe),and bovine tibias without skin and with skin.HHT and AGCD methods were used to process every data of ultrasonic guided waves.(4)The group velocity of the independent mode was obtained at corresponding frequency and compared with the results of short time Fourier transform(STFT).(5) The cortical thickness was calculated by comparing the experimental group velocity data from time-frequency methods with the theoretical ones.Finally,the experimental thickness was compared with the actual thickness.Results suggest that the ultrasonic determined thickness of long bone phantoms (steel plate and plastic pipe) and long cortical bones were in agreement with the actual thickness.The average errors of cortical thickness obtained from the HHT and AGCD were respectively 2.9%and 2.3%.Thus,the HHT and AGCD can be provided for the measurement of the group velocity of the guided wave modes.Ultrasonic guided waves can assess the cortical thickness of long bone.Also,it is indicated that the HHT and AGCD are effective methods to identify multimode waves.