Abstract
Ultrasound imaging is a safe, cost-effective, and real-time modality widely used for clinical diagnosis, providing information that ranges from anatomic structures in B-mode to blood flow dynamics in color Doppler imaging. Ultrasound molecular imaging (USMI) is an advanced technique that combines anatomic visualization with molecular-scale physiological information. In USMI, highly echogenic ultrasound contrast agents, comprising gas-filled microbubbles (MBs), are conjugated with ligands that bind to specific molecular biomarkers expressed on the vascular endothelium. The accumulated adherent MBs generate unique acoustic signatures that allow ultrasound detection and quantification of disease-related specific molecular biomarkers. In this dissertation, several ultrasound sequence designs and image processing methods are presented to enhance the accuracy of adherent MB signal detection and the spatial resolution of USMI.
The dynamics of adherent targeted MBs during the binding process were investigated under both optical and ultrasound conditions using in vitro phantom experiments. The motion of MBs was first observed and measured under a microscope, and subsequently tracked and quantified using ultrasound localization microscopy (ULM) method in Pulse Inversion (PI) contrast imaging. Adherent targeted MBs exhibited significantly lower velocities and distinct motion patterns compared to free-circulating MBs, demonstrating that the slow motion of adherent MBs results in their unique spatiotemporal signal characteristics, that can be differentiated in spatiotemporal filtering-based USMI.
In relation to the in vivo validation of USMI, the modulated Acoustic Radiation Force (mARF) method was developed using a novel dual-probe imaging configuration to provide a rapid and effective measurement of the adherent MB signal intensity. Experiments in a murine abdominal aortic aneurysm (AAA) model with VEGFR-2-targeted MBs indicated that the mARF-based USMI can detect and assess AAA growth at early stages based on the signal intensity of adherent MBs.
To improve the spatial resolution of USMI, a new ultrasound sequence design, Incremental Burst Sequence (IBS), was developed to induce the population of polydisperse adherent targeted MBs to burst progressively, achieving the spatial separation of adherent MBs and allowing localization for high-resolution USMI (HR-USMI, resolved to ~50 micrometers). Furthermore, HR-USMI mappings were superimposed on the super-resolution tumor microvasculature mapping derived from ULM (resolved to <100 micrometers), providing diagnostic information that integrates molecular signatures and anatomic microvascular structures within a single high-resolution image.
