Towards 3-D plus time ultrafast ultrasound investigation of human arterial wall : wave physiology, computational mechanics, and image formation

Abstract

Physiological and pathological conditions of the human arterial system are closely related to the development of cardiovascular diseases, which threaten the elderly’s life expectancy and remain the leading cause of death globally. An in-depth investigation of physiological features with long-term monitoring of morphological and functional progress of the arteries promotes reliable diagnosis and prognosis. Biomedical ultrasound imaging enjoys non-invasive, real-time, cost-effective, portable, and free of ionizing radiation characteristics, making it an indispensable modality for artery examination in clinical practices. Currently well-established ultrasound-based methods to investigate the mechanical properties (e.g., elasticity) of the arterial wall include regional pulse wave imaging (PWI) and vascular guided wave imaging (VGWI). However, these methods work under their respective assumptions and partially characterize the arterial wall, which is known to be structurally and mechanically anisotropic. For example, PWI, which uses spontaneous pressure pulse wave, evaluates the arterial circumferential stiffness only; although externally induced guided waves propagating in multi-directions enabled VGWI to obtain directional stiffness estimates, precise excitation of acoustic radiation force to generate the desired guided waves in vivo is technically tricky. This dissertation first explored physics of waves that spontaneously occur inside human arteries, its link to vascular physiology, and associated wall mechanics. Specifically, a two-layer, two/four-fiber family reinforced constitutive arterial model was visited to mimic the mechanical response of arteries. Finite-element simulations with the aforementioned constitutive model were thereafter realized to simulate spontaneous waves arising from the blood-wall-interaction. An imaging framework that integrates spatial angular compounding, PWI, and VGWI was then realized to conduct in vivo human carotid artery experiments. In addition to pulse wave, another type of natural wave, which was characterized by µm-level longitudinal wall motion and longitudinal propagation, was unprecedentedly observed. It is coined as extension wave and was demonstrated to be capable of grading arterial wall anisotropy together with the concurrent pulse wave. Our prior natural wave observation and analysis were performed in a single 2-D image plane. How natural waves propagate in three dimensions remained unclear. 4-D (3-D plus time) ultrafast ultrasound image formation combining a cascaded synthetic aperture (CaSA) strategy and a low-rank tensor completion (LRTC) algorithm was then developed for high volume rate imaging of the human artery. CaSA was encoded in the 3-D spatiotemporal domain to increase the signal-to-noise ratios (SNR) of volumetric images; an explicit random sparse reception strategy was designed to partially collect pre-beamformed raw radiofrequency signals; LRTC was thereafter applied to recover full channel-domain echoes. Few acquisition events were required to form one volume. Results from simulations and a wire phantom proved that LRTC could recover the original signals accurately. A calibration phantom study demonstrated that CaSA with LRTC outperformed well-established benchmark methods in both SNR and volume rates. In vivo results showed that our proposed method depicted reasonably the arterial wall dynamics in the 4-D domain. The thesis presents novel ultrasound imaging methods that facilitate investigation of wave physiology, computational analysis, and experimental measurements of arterial mechanics and dynamics, paving the way towards clinical translational applications.