Finite Element Simulation of Ultrasound Propagation in Trabecular Bone

Quantitative ultrasound is used to identify healthy versus osteoporotic bone. However the physics of ultrasound propagation in trabecular media is still not sufficiently understood. This lack of understanding is reported to be an obstacle in further development of this bone assessment technique. Numerical models of wave propagation stand as a potentially successful tool to explain the various experimental observations. The main issue in the numerical modeling of wave propagation in trabecular bone is the complex geometry of the trabecular structures surrounded by a fluid (bone marrow). So far, the complex geometrical domain of trabecular structures has been approximated by finite difference grids for wave propagation analyses. In this work, numerical simulation of ultrasound propagation into trabecular bone sample is performed using the finite element method (FEM). A new procedure for numerical modeling of trabecular bone tailored for the FEM is introduced. The entire complex trabecular geometries of two cubic bone samples are reconstructed using computed microtomography data. For the first time a three dimensional finite element mesh using tetrahedral elements is generated for the two-phase medium of a trabecular bone. Separate meshes for the bony part and the filling marrow (considered as non-viscous water) are generated and acoustic-structure interaction condition is imposed on their interface. It is shown that the three-dimensional simulation using the FEM can predict ultrasound propagation phenomena observed in experiments: linear dependency of attenuation on frequency, the effect of bone volume on the attenuation and speed of sound, and the propagation of fast and slow waves. Moreover, the broadband ultrasound attenuation (BUA) for two ultrasonic signals propagating into a healthy and an osteoporotic sample are compared. A distinguishable difference in BUA between the two samples is observed expressing lower BUA for osteoporotic bone. Our developed model is the first three-dimensional finite element analysis model to compare the ultrasound propagation in healthy versus osteoporotic bone. The developed model can be further utilized as a tool to explain various experimental observations of quantitative ultrasound of bone.