Numerical analysis of fast and slow waves backscattered from various depths in cancellous bone

Ultrasound backscattering in cancellous bone was numerically investigated using finite-difference time-domain (FDTD) simulations. The cancellous bone models with various thicknesses were prepared, and artificial absorbing layers were set at the back surfaces. An ultrasound pulse wave was radiated toward the front surface and was transmitted in the direction parallel to the main trabecular orientation. The backscattered signal inside cancellous bone was isolated from the difference between two simulated waveforms obtained using the bone models with different thicknesses. In the simulated results of the incident and backscattered waveforms, the fast and slow waves, which could propagate mainly in the respective parts of the trabecular elements and the pore spaces, could be clearly observed. For each wave, the backscatter coefficient, which was defined as the amplitude ratio of the backscattered wave to the incident wave, was derived at various bone depths. At all bone depths, the backscatter coefficients of the fast wave were not correlated with porosity, but those of the slow wave were moderately correlated.