Functional ultrafast ultrasound imaging of the cardiovascular system

Abstract

Life-threatening cardiovascular disease (CVD) remains the leading cause of death worldwide and is a major healthcare burden in the elderly. An affordable and reliable diagnostic or prognosis tool of cardiovascular function is imperative to healthy aging. Biomedical ultrasound is the predominant imaging method to map cardiovascular function because it is inexpensive, portable, real-time, and free from ionizing radiation. Ultrafast imaging (> 1000 frame per second (fps)) using plane or diverging waves, instead of focused beams by conventional ultrasound (<100 fps), is a paradigm shift for biomedical ultrasound imaging. Imaging the cardiovascular system at ultrafast frame rates permits detailed quantification of cardiovascular dynamics, including morphological alterations, muscle contraction, and blood flow, at instants of interest in a cardiac cycle. However, current ultrafast ultrasound image acquisition techniques are not optimal for mapping cardiovascular function because they trade additionally induce artifacts and limited signal-to-noise ratio (SNR) for ultrafast frame rates. Ultrafast ultrasound image artifacts consist of both axial and lateral (i.e., side lobe) artifacts. The axial lobe artifact is in the range direction and stems from the ghost echoes produced by unfocused transmissions. The side lobe artifact is caused by the off-axis clutters. Ultrafast ultrasound SNR is limited primarily because a few number of unfocused wave transmissions are used. Image artifacts together with low SNR and thus limited penetration depth hamper the performance of well-established ultrasound signal processing methods for estimation of the heart dynamics, including myocardial motion and blood flow. Besides, analysis of myocardial kinematics has been focused on the spatial distribution and temporal variation. Few studies have reported the time-frequency analysis of the myocardial motion obtained by ultrafast ultrasound. This dissertation therefore aims to develop new functional ultrafast ultrasound imaging (FUUSI) techniques, which incorporate new ultrafast image acquisition methods and post-processing methodologies for mapping myocardial motion and blood flow dynamics of the cardiovascular system. We first developed a new ultrafast image acquisition method using combined transmissions with cross-coherence-based reconstruction (US-CTCC) to suppress both the axial and lateral artifacts. A series of high SNR ultrafast image acquisition frameworks, such as the cascaded dual-polarity wave (CDW) for linear array imaging and cascaded synthetic aperture (CaSA) imaging for phased array configuration, were proposed to improve SNR of ultrafast ultrasound significantly. Results demonstrated the advantage of the proposed CaSA for mapping the myocardial motion in high accuracy, and myocardial motion of in vivo healthy human heart with new post-processing methodologies, such as spectrogram analysis. We further applied the developed ultrafast image acquisition methods (i.e. CaSA) to map blood flow with improved sensitivity in the heart chamber, and observed the spatiotemporal variations of the micro-vessel blood flow in the myocardium and brain. In conclusion, this thesis work demonstrates the feasibility of our developed FUUSI as a reliable imaging tool for monitoring the function of the cardiovascular system.