Research On Key Technique Of Bone Quantitative Ultrasound Measurementand Its Acoustic Behavior

Ultrasound technique is always used in bone density measurement due to its nondestructive and nonradiative characteristics. However, the bone strength depends on not only the mass density, but also the trabeculae microarchitectures. The ultrasonic backscattering method can moderately detect the structures and can be a potential technique in telemedicine, but in cancellous bone this method has very limited sensitivity and accuracy, and the solid matrix of trabeculae deeply affects the acoustical scattering and attenuation. This makes it a necessary work to establish an accurate model of the real bone structures. Meanwhile, improving the ultrasound transmission depth without increasing the incident ultrasound power is also of many advantages.In this work, with the equivalent scatterer model and the model from μCT, we investigated the accuracy enhancement in ultrasonic backscattering measurement. In addition, the influences of trabeculae microstructures on ultrasound transmission and the ultrasound biological effects in bone were explored.We studied the improvement of the mean scatterer spacing(MSS) measurement with coded excitation(CE) based approaches. The trabeculae pattern is quasi-periodical and can be regarded as distributing scatterers. Based on this hypothesis, we encoded the incident wave to improve the incident ultrasound energy without increasing the power. This helps to solve the problem of low resolution and attenuation in MSS measurement, and improves the accuracy and SNR. In addition, in this work the influences of noise on the CE based method were studied. Simulations were performed with the particle vibration theory and no frequency superposition in linear elastic media was included. The results indicate that with CE enhancement, the echo power increases and the robustness of MSS measurement improves.The ultrasonic backscattering signals from trabeculae is composed of many different components including the noise and the backscattered data from trabecular structures. In this work, we investigated the intrinsic mode function(IMF) based MSS measurement. By decomposing the backscattering signals, we extracted the backscattering components in different time scales and then made MSS measurements separately. Intrinsic wave modes were obtained with various frequencies by using empirical mode decomposition(EMD) technique, of which the original backscattering data and the energy in various time scales were divided in the form of IMFs. MSS was then measured with the first three mode functions. To solve the problem of mode mixing, a new decomposition method – ensemble empirical mode decomposition(EEMD) performed by adding noise and changing signal poles was introduced. EEMD generated new IMFs and then the measurement of MSS was re-performed. The IMF based MSS measurement worked like a high-pass filter in which the residual components were removed from the trabeculae backscattering signals.Since quantitative bone ultrasound is of poor reliability and repeatability, we believe that in addition to the noise and attenuation, the acoustic behavior of trabeculae has influences on MSS measurement. In this work, we proposed a μCT based method to investigate the influences of trabeculae anisotropy and inhomogeneity on ultrasound transmission. Instead of apparent modulus, we were interested in how the tissue modulus affected the acoustic parameters. Since the apparent modulus also depends on bone structures, the analysis of trabeculae microstructures was included. In this work, highresolution DICOM slices were used to reconstruct the trabeculae architectures, and then within certain range of tissue modulus, the bone anisotropy and inhomogeneity were studied with μFE technique. The dependence of acoustic behavior on trabeculae structures, the relationship between ultrasound propagation and the tissue modulus were also studied.The propagation of ultrasound in tissue causes mechanical and non-mechanical biological effects. In this work, we investigated the ultrasound induced stress distribution in bone. The stress concentration fields in bone may cause tissue damage and should be well handled, so in bone we presented the FEM analysis method of ultrasound-induced stress fields based on an optimized equivalent model. By coupling the interface between different tissues, finite element technique was used to solve the elastic wave equation by introducing into the energy functional, and then based on the von-Mises stresses, the stress fields in bone were studied. The Lamb wave modes in finite bone structures were investigated.