The gold standard to assess bone loss with age is the Bone Mineral Density (BMD), because this measurement highly correlates to bone mass. However, the BMD cannot fully explain the decrease in bone strength and the associated risk of fracture, indicating that other factors beside bone mass play an important role. Ultrasound is also used to evaluate bone properties by measuring the velocity and the attenuation of longitudinal waves. Unfortunately, most of current clinical ultrasound devices aim to determine bone mass density, as DXA does, without taking advantage of the fact that ultrasound is sensitive to geometry, trabecular orientation and tissue composition. Recent in vitro studies have provided evidence of the propagation of two different longitudinal wave modes in cancellous bone, as predicted by the poroelastic approach of wave propagation originally developed by Maurice A. Biot. However, most current ultrasound approaches consider only one wave to propagate in cancellous bone, and the wave velocity/attenuation is usually analyzed as a function of the bone mass density only. It is important to note that reported velocities in highly osteoporotic bone (for instance 90% fluid and 10% solid) never shows values below 1450m/s, which is the speed of sound in marrow. This observation suggests that in highly porous samples, the fraction of the media being characterized by the measured wave is the fluid one, as opposed to the solid trabecular structure. The relevance of the ultrasound black box approach to distinguish the changes in the trabecular microstructure is therefore challenged. In this proposal, we intent to combine Biot's poroelastic approach with a tomography based (multidirectional) assessment of acoustic properties in bone. The major advantage of the poroelastic approach is that it predicts the existence of two waves and characterizes the relative contribution of the solid and fluid bone fractions on ultrasound wave velocity/attenuation. The role of the anisotropic solid structure and fluid in the behavior of the fast and slow wave velocities is examined. From the acoustic properties, the elastic properties of bone are derived and then compared to mechanical testing measurements. This approach has the potential to better characterize changes due to bone loss with age and to provide a first step for defining a criterion for bone loss beyond bone mineral density. Project Narrative and Relevance: The proposed study has the potential to produce a highly medically significant result. Achievement of the proposed aims would enable further research possibly leading to the development of new diagnostic devices capable of greater sensitivity and specificity for the detection and grading of osteoporosis risk.
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