Assessment of Mechanical and Hemodynamic Vascular Properties using Radiation-Force Driven Methods

Abstract

Several groups have proposed classifying atherosclerotic disease by using acoustic radiationforce (ARF) elasticity methods to estimate the mechanical and materialproperties of plaque. However, recent evidence suggests that cardiovascular disease(CVD), in addition to involving pathological changes in arterial tissue, is also ahemodynamic remodeling problem. As a result, integrating techniques that canestimate localized hemodynamics relevant to CVD remodeling with existing ARF basedelastography methods may provide a more complete assessment of CVD.This thesis describes novel imaging approaches for combining clinically-accepted,ultrasound-based flow velocity estimation techniques (color-flow Doppler and spectral-Doppler imaging) with ARF-based elasticity characterization of vascular tissue. Techniquesfor integrating B-mode, color-flow Doppler, and ARFI imaging were developed(BACD imaging), validated in tissue-mimicking phantoms, and demonstrated for invivo imaging. The resulting system allows for the real-time acquisition (< 20 Hz) ofspatially registered B-mode, flow-velocity, and ARFI displacement images of arterialtissue throughout the cardiac cycle. ARFI and color-flow Doppler imaging quality,transducer surface heating, and tissue heating were quantified for different frame-rateand scan-duration configurations. The results suggest that BACD images can be acquiredat high frame rates with minimal loss of imaging quality for approximatelyfive seconds, while staying beneath suggested limits for tissue and transducer surfaceheating.Because plaque-burden is potentially a 3D problem, techniques were developedto allow for the 3D acquisition of color-flow Doppler and ARFI displacement datausing a stage-controlled, freehand scanning approach. The results suggest that a40mm x 20mm x 25mm BACD volume can be acquired in approximately three seconds.Jitter, SNR, lesion CNR, soft-plaque detectability, and flow-area assessment werequantified in tissue mimicking phantoms with a range of elastic moduli relevantto ARFI imaging applications. Results suggest that both jitter and SNR degradewith increased sweep velocity, and that degradation is worse when imaging stiffermaterials. The results also suggest that a transition between shearing-dominatedjitter and motion-dominated jitter occurs sooner with faster sweep speeds and instiffer materials. These artifacts can be reduced with simple, linear filters. Resultsfrom plaque mimicking phantoms suggest that the estimation of soft-plaque areaand flow area, both important tasks for CVD imaging, are only minimally affectedat faster sweep velocities.Current clinical assessment of CVD is guided by spectral Doppler velocity methods.As a result, novel imaging approaches (SAD-SWEI, SAD-GATED) were developedfor combining spectral Doppler methods with existing ARF-based imagingtechniques to allow for the combined assessment of cross-luminal velocity profiles,wall-shear rate (WSR), ARFI displacement and ARF-induced wave velocities. Thesetechniques were validated in controlled phantom experiments, and show good agreementbetween previously described ARF-techniques and theory. Initial in vivo feasibilitywas then evaluated in five human volunteers. Results show that a cyclicvariability in both ARFI displacement and ARF-generated wave velocity occurs duringthe cardiac cycle. Estimates of WSR and peak velocity show good agreementwith previous ultrasonic-based assessments of these metrics. In vivo ARFI and Bmode/WSR images of the carotid vasculature were successfully formed using ECG gatingtechniques.This thesis demonstrates the potential of these methods for the combined assessmentof vascular hemodynamics and elasticity. However, continued investigationinto optimizing sequences to reduce transducer surface heating, removing the angledependency of the SAD-SWEI/SAD-GATED methods, and decreasing processingtime will help improve the clinical viability of the proposed imaging techniques.