Vascular ultrasound elastography for comprehensive characterization of the artery : from linear to nonlinear wall behaviors

Abstract

Abnormality of arteries is a major contributor to cardiovascular diseases (CVDs), which remain the leading cause of global death. Arterial stiffness, one index of arterial mechanical properties denoting the ability to resist deformation, is believed to be a predictor for CVDs, and has attracted substantial efforts. Vascular ultrasound elastography serves as one representative category of noninvasive techniques for direct or indirect stiffness assessment. Examples include ultrasound strain imaging (USI) for deformation, pulse wave imaging (PWI) for circumferential Young’s modulus, and vascular guided wave imaging (VGWI) for multidirectional shear moduli. Arterial stiffness alteration is common in various cardiovascular complications, so itself alone is not unique to an etiology. Primarily because of spontaneous cyclic loading from blood pressure (P), arterial mechanical properties are intrinsically dynamic and exhibit nonlinearity, anisotropy and viscoelasticity. Noninvasively assessing the dynamic behaviors of arterial mechanical properties under physiological pressure in multi-perspectives helps understand artery mechanics and physiology in disease progression, but relevant studies are few. Separately analyzing single index from any of the three interdependent attributes—kinematics (e.g., displacement, strain), mechanical properties and hemodynamics (e.g., blood pressure, flow)—remains the mainstream in related research fields, thus leading to segmented knowledge and insights about the vascular system. This dissertation thus aims at developing ultrasound imaging methods to study arterial dynamic mechanical properties, establish relationships among aforementioned attributes, and propose new predictors for comprehensive characterization of the artery. An ultrasound elastographic imaging framework (UEIF) is first developed and encompasses bidirectional VGWI and USI to obtain strain (ε) and shear modulus (μ) simultaneously. The ε-μ relationship (or, loop) is further proposed and validated in vitro as a new graphical diagnostic index of the artery, which infers nonlinearity, anisotropy and hysteresis of the arterial wall. Since dynamic behaviors of the arterial wall are mainly caused by P, which is difficult to measure deep inside the body, building a relationship among P and wall properties is necessary. A mathematical model is hereby derived for localized P change as a function of ε and μ in validation against finite element simulations and in vitro experiments on vessels with normal or abnormal geometries. In parallel, a novel weighted line-focused (wLF) beam is developed to effectively induce elastic waves for μ estimation of moving tissues, e.g., arterial wall, by producing an adjustable focused zone while strictly abiding by safety guidelines. This wLF beam is combined with cascaded dual-polarity waves (CDW) imaging, which is separately developed by our group, to assess ε-μ relationship of in vivo human carotid arteries with high image quality. Results show good repeatability within the same subject and consistent trends with clear distinction among different subjects, thereby demonstrating the potential of the ε-μ relationship as a new predictor for arterial conditions. In summary, this dissertation presents a novel system of vascular ultrasound imaging methods to 1) noninvasively quantify multiple dynamic mechanical properties of the artery; 2) lump kinematics, mechanical properties, and hemodynamics together by one indicator; and 3) lay a foundation for comprehensive characterization and personalized care of the vascular system.