Obstructing propagation of interfering modes improves detection of guided waves in coated bone models

Interference due to wave propagation in soft tissue that covers the bone is a major challenge to in vivo assessment of the fundamental flexural guided wave (FFGW) in bone. To improve signal-to-interference ratio (SIR) we propose to obstruct the propagation of interfering modes by locally deforming the coating by external mechanical compression. This approach was modeled by 2D finite-element transient domain (FEMTD) simulations in a fluid-coated (7 mm) solid plate (3 mm). The fluid layer mimics the soft tissue that covers the bone. A single emitter or a 6-element phased array excited ultrasound pulses at 50 kHz on the surface of the coating, and a receiver array was placed on the surface, 20-50 mm apart from the last emitter. Localized deformation of the coating was modeled by a sine-shaped notch (7 mm half-width and 0-7 mm depth), centered between the emitting and receiving array. The deformation of the coating increased the SIR of FFGW by 20.3 ± 2.3 dB. A 50% deformation of the coating thickness was effective and only a small improvement in SIR was achieved by further compression. Combining a 50% deformation of the soft coating with phased excitation resulted in a 31.4 ± 2.6 dB SIR gain, and revealed a distinct FFGW wave packet. It was impossible to distinguish FFGW from the interference by a conventional measurement. These results thus suggest that obstructing the propagation of interfering modes may improve the detection of FFGW in in vivo measurements.