Code Excited Ultrasonic Guided Wave Method In Long Bone Assessment

Osteoporosis will cause the loss of bone mass density and reducing of bone strength, which finally lead to high risk of bone fracture. Ultrasonic guided wave is propagating in cortical bone, it’s sensitive of changes of long bone properties, such as bone mess density, strength and micro-structure. Therefore, using ultrasonic guided wave to assess long bone became one of the research hotspot. However, the propagation of guided waves in long bone suffers from the high attenuation, which results in low amplitude and bad signal-to-noise ratio (SNR).Coded excitation can improve the amplitude and SNR of received signal with normal level inputs. Moreover, the band width of coded excitation is narrower than single pulse. The modulated code excitation can arbitrarily change its center frequency, thus, in ultrasonic guided wave excitation, it’s capable to excite low frequency guided wave modes, which provides great convenience in guided wave mode analysis.This thesis focus on long bone fracture assessment using code excited guided waves. Two kinds of code excitation were used, Barker code and optimal binary code, in platy and tubular long bone models. Both simulation and in vitro experiment have been done, the measured guided waves were decoded by weighted match filter and finite impulse (FIR) response least square inverse filter, respectively. The decoded results were compared with single pulse excited results. The results demonstrate that, the code excitation can obtain larger amplitude and higher SNR than single pulse excitation.The decoded result of weighted match filter is N (N denotes the length of code) times larger than single pulse excited result, while FIR least square inverse filter can get better SNR. Guided waves excited by code excitation can keep high resolution both in time and frequency domain. Moreover, the flexible frequency change can excite low frequency guided waves with less modes, which is more conductive to further analysis.In the last part, time-frequency representation analysis was applied to guided waves, the ridges of energy were picked out to separate and reconstruct the signals of guided wave modes. Energy transformation between predominate modes with change of the depth of long bone fracture, was analyzed. In platy model, the predominate guided wave modes were A1 and S2, while in tubular model, the predominate modes were L(0,1) and L(0,3).With increasing fracture depth, the energy transformation between two modes was confirmed, which provides useful information to long bone fracture assessment.