Cortical bone velocity mapping using leaky surface acoustic waves

Emerging ultrasonic technologies for the assessment of skeletal status have led to acoustic velocity measurements in transmission at peripheral sites (calcaneus, phalanx). Alternatively, cortical bone and non-peripheral sites can be evaluated locally using surface acoustic waves: such devices providing single-point velocity measurements have been developed. However, since bone is a heterogeneous structure, the reliability of single-point measuring techniques is impaired. To overcome this problem, we extended the technique based on leaky longitudinal surface acoustic waves by providing an image of the local propagation velocity over a bone surface with correction for potential errors due to surface shape irregularities. Our measurements were performed on a human femur specimen immersed in a water bath. Transmitted signals of the surface wave were recorded using a pair of broadband unfocused transducers (central frequency 0.5 MHz) facing the specimen surface at an incidence angle close to the critical angle for longitudinal surface wave excitation. Echo signals were also acquired at the same measurement sites for surface correction. Validation of the measuring principle was performed on test materials. The feasibility for measuring human cortical bones was then demonstrated by velocity mapping of a dry femur specimen: the average velocity was 2924±74 m.s^-1 over a 20×30 mm² surface, in agreement with reported values. This method also offers a great potential for characterization of cortical bone strength and anisotropy