Influences of frequency-dispersive guided waves on contrast-enhanced ultrasound imaging

When emission waves (EWs) of imaging scanner hit bone, especially at an oblique incidence, guided waves (GWs) are generated from the bone cortex. GWs are multimodal and frequency-dispersive due to their non-linear propagation. Meanwhile, GWs leak to surrounding tissues and disturb SNR of acoustic signals. When microbubbles (MBs) flow through microvessels near a bone cortex, GWs can interact with MBs. The vibration and nonlinear scattering of MBs are unavoidably affected by GWs, which may disturb the image quality of ultrasound contrast imaging (UCI) near the bone surface. However, such disturbances on UCI with sub-harmonic (SH), fundamental (F), and ultra-harmonic (UH) have not been adequately addressed. This study aimed to elucidate the influences of frequency-dispersive GWs on UCI, especially with pulse inversion (PI) mode. The effects of GWs on UCI with B and PI modes were first elucidated through simulated comparison based on the modified Herring equation derived by GWs coupled EWs together and EWs alone, respectively. The GWs were reconstructed by the single-mode GWs, which were detected by using a transmitted-air gap-received (TAR) method and identified by using a smoothed pseudo-Wigner-Ville (SPWV) energy distribution superimposed with theoretical dispersion curves. Such effects were further verified by UCI in a vessel-plate flow phantom at incidence angles 0° and 22°. The influences of GWs on contrast and resolution of UCI were finally quantified by the differences in peak value (DPV) and half peak width (DHPW) of MB echoes, respectively. Based on TAR combined with SPWV methods, the coupling disturbances of EWs were removed; and the GWs with four single-modes (A0, A1, S0, and S2) and double-frequencies (3.35 and 1.62 MHz) were detected and identified. Due to the changes in MB echoes caused by GWs, the echo aliasing and gray changing occurred in all UCI methods. And DPV of MB echoes of PI SH was 4.0 times greater than that of SH, and DHPW of SH was 3.9 and 11.4 times higher than these of F and UH. Contrast of UCI was enhanced by GWs, but corresponding resolution was decreased, especially for SH and PI mode. So as a double-edged sword, the effects of GWs should be further explored to benefit UCI near the cortex.