Evaluating Bone by Ultrasound
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Bone fractures associated with osteoporosis, a major bone disease characterized by low density and high fracture risk, are common causes of disability and large medical care expenses around the world. Considering its low cost, high portability, and non-ionizing nature, non-invasive ultrasound techniques have been investigated as tools for evaluating bone quality and biomechanical competence. Quantitative ultrasound has been used clinically as a surrogate for the current gold standard measure in osteoporosis diagnosis - Bone Mineral Densitometry (BMD), which unfortunately utilizes ionizing radiation. This study proposes the application of a reflection ultrasound method to evaluate non-BMD properties of cancellous bone, including porosity and the microstructure of the trabecular network, all of which are directly related to bone morphological changes caused by osteoporosis and could result in better predictions of fracture risk. Computer simulations and phantom studies were adopted to guide the measurement of bone properties. In the computer simulations, the cellular model and the wire model of cancellous bone predict the backscattering dependence on porosity from two different perspectives, but reach the same result. This leads to the first conclusion that reflection ultrasound is not sensitive to the shape of a scatterer of wavelength size but to the spacing between scatterers. The in vitro cancellous bone study demonstrated that the average porosity is correlated with the density, while the local porosity depends upon the heterogeneity of the cancellous bone. The average porosity of cancellous bone can be directly determined from ultrasound signals reflected from the bone. Results of the ex vivo and in vivo short bone studies in patella are in agreement with that of Ultrasound Critical-angle Reflectometry (UCR). Thus, the second conclusion of this dissertation is that reflection ultrasound can be an effective tool for assessing bone properties in vivo. During the short bone-mimicking phantom study, the first critical angle detected by UCR was shown to correspond to the solid ultrasound velocity and is independent of porosity, but its amplitude is strongly related to porosity; the second critical angle, corresponding to bulk ultrasound velocity, is strongly related to porosity, but the correlation between its amplitude and the porosity is weak.