Axial Transmission: Techniques, Devices and Clinical Results

Recent progress in quantitative ultrasound have sparked increasing interest towards the measurement of long cortical bones (e.g., radius or tibia), because their ability to sustain loading and resist fractures is known to be related to their mechanical properties at different length scales. In particular, applying guided waves for the assessment of cortical bone is inspired by widely used techniques developed earlier in the field of nondestructive testing and evaluation of different waveguide structures. This approach is based on the experimental evidence that the cortex of long bones can act as a natural waveguide for ultrasound, despite its irregular geometry, attenuation, and heterogeneous material properties. Because guided waves could yield the characterization of several bone properties (e.g., cortical thickness, anisotropic stiffness or porosity) at the mesoscopic level by fitting the dispersion characteristics of a waveguide model to the measured dispersion curves (i.e., solving an inverse problem), this method has a strong clinical potential as a tool for bone status assessment. This chapter revisits the roadmap that allowed the so-called bidirectional axial transmission technique to progress from a pure laboratory concept to a diagnostic tool of clinical interest over the second decade of the twenty-first century and discusses the current clinical challenges associated with cortical bone characterization by ultrasound guided waves.