Mapping mechanical properties of the artery using multi-directional ultrasound guided wave imaging

Abstract

Arterial stiffness, which plays an important role in arterial function, has been widely recognized as an independent predictor for various cardiovascular diseases (CVD), constituting the top one killer nowadays. Therefore, it has attracted efforts to develop techniques for the evaluation of arterial stiffness. The developed techniques can be categorized into invasive ones, e.g., intravascular ultrasound elastography, or non-invasive ones, e.g., strain imaging, pulse wave velocity (PWV), acoustic radiation force imaging (ARFI), and shear wave imaging (SWI). Among them, PWV, which depends on the intrinsic pulse wave velocity, is currently regarded as the gold standard. However, due to the low temporal sampling rate, i.e., only one pulse wave event per heartbeat, PWV cannot monitor dynamic variations of arterial stiffness under different lumen pressures within one entire cardiac cycle. Moreover, the aforementioned techniques all assume material isotropy for arteries and provide arterial stiffness only in one particular direction, which is deemed insufficient for fully understanding the relationship between arterial stiffness and vascular function. Therefore, this thesis presents a new technique termed as vascular guided wave imaging (VGWI), which utilizes SWI and is based on the Lamb wave approximation, to non-invasively quantify the dynamic multi-directional arterial stiffness under pulsatile flow for the first time. We first introduce the idea of guided circumferential wave, which is well-known in non-destructive testing (NDT), to describe the guided wave propagation in the circumferential direction of a tube. We further demonstrate the capability of VGWI in quantifying anisotropic multi-directional arterial stiffness under pulsatile flows based on our proposed geometry correction method and validate VGWI against mechanical tensile testing. In addition, VGWI estimates achieve comparable reproducibility to PWV. Prior to clinical applications, we also investigate the impact of the coupling medium. The performance of VGWI degrades when coupled by solid medium instead of water. In summary, this thesis presents the development of a new ultrasound elastographic technique, VGWI, to non-destructively quantify arterial stiffness. We have demonstrated it as a potential diagnostic tool that benefits not only the early diagnosis of CVD but also a further understanding of vascular mechanics and physiology.