This paper deals with modeling of the ultrasound axial transmission technique for in vivo cortical long bone which is known as being a anisotropic solid medium with functionally graded porosity. The bone is modeled as an anisotropic poroelastic medium using the Biot’s theory. We develop a hybrid spectral/finite element formulation to obtain the time-domain solution of ultrasonic waves propagating in a poroelastic plate immersed two fluid halfspaces. The numerical method is based on a combined Laplace-Fourier transform which solves the problem in the frequency-wavenumber domain. In the spectral domain, as radiation conditions may be exactly introduced in the infinite fluid halfspaces, only the heterogeneous solid layer needs to be analyzed using finite element method. Several numerical tests are presented showing very good performances of the proposed approach. A preliminary study on the FAS (First Arrived Signal) velocities computed by using equivalent elastic and poroelastic models will be presented.