Coded excitation of the fundamental flexural guided wave in coated bone phantoms

Abstract

There is an increasing interest of using ultrasonic guided waves to assess long cortical bones. In particular, a method of ultrasonic estimation of cortical thickness based on the fundamental flexural guided wave (FFGW), generally consistent with the A0 Lamb mode, has proven to be promising in vitro and modeling studies. Soft-tissue coating on top of the bone makes, however, the related in vivo application challenge. Visibility of FFGW on top of the soft tissue is not good due to the characteristic displacement profile of this mode, and due to its relatively high attenuation in the bone and surrounding tissue. Moreover, the soft tissue provides a direct propagation path for ultrasonic modes that give rise to interferences. As a result, the effective signal-to-noise ratio (SNR) of FFGW becomes low, and it is thus difficult to identify and extract this mode on top of the coating. So as to improve the FFGW detection, a tailored excitation of the ultrasound signal is introduced. To this end, the suitability of a base-sequence modulated Golay code (BSGC) is investigated on coated (2.5 mm) tubular bone phantoms with a wall thickness of 1.0–5.0 mm. The proposed approach improved the SNR, so that FFGW excitation by a custom-made 200-kHz piezo transducer was enabled at very low ultrasonic frequencies (67 kHz) and with much higher amplitude. This method facilitated the detection of thickness sensitivity of the FFGW phase velocity through the soft coating. These results indicate that BSGC excitation may also facilitate FFGW-based in vivo estimation of cortical thickness.