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

Abstract

<p>Several groups have proposed classifying atherosclerotic disease by using acoustic radiation</p><p>force (ARF) elasticity methods to estimate the mechanical and material</p><p>properties of plaque. However, recent evidence suggests that cardiovascular disease</p><p>(CVD), in addition to involving pathological changes in arterial tissue, is also a</p><p>hemodynamic remodeling problem. As a result, integrating techniques that can</p><p>estimate localized hemodynamics relevant to CVD remodeling with existing ARF based</p><p>elastography methods may provide a more complete assessment of CVD.</p><p>This thesis describes novel imaging approaches for combining clinically-accepted,</p><p>ultrasound-based flow velocity estimation techniques (color-flow Doppler and spectral-</p><p>Doppler imaging) with ARF-based elasticity characterization of vascular tissue. Techniques</p><p>for integrating B-mode, color-flow Doppler, and ARFI imaging were developed</p><p>(BACD imaging), validated in tissue-mimicking phantoms, and demonstrated for in</p><p>vivo imaging. The resulting system allows for the real-time acquisition (< 20 Hz) of</p><p>spatially registered B-mode, flow-velocity, and ARFI displacement images of arterial</p><p>tissue throughout the cardiac cycle. ARFI and color-flow Doppler imaging quality,</p><p>transducer surface heating, and tissue heating were quantified for different frame-rate</p><p>and scan-duration configurations. The results suggest that BACD images can be acquired</p><p>at high frame rates with minimal loss of imaging quality for approximately</p><p>five seconds, while staying beneath suggested limits for tissue and transducer surface</p><p>heating.</p><p>Because plaque-burden is potentially a 3D problem, techniques were developed</p><p>to allow for the 3D acquisition of color-flow Doppler and ARFI displacement data</p><p>using a stage-controlled, freehand scanning approach. The results suggest that a</p><p>40mm x 20mm x 25mm BACD volume can be acquired in approximately three seconds.</p><p>Jitter, SNR, lesion CNR, soft-plaque detectability, and flow-area assessment were</p><p>quantified in tissue mimicking phantoms with a range of elastic moduli relevant</p><p>to ARFI imaging applications. Results suggest that both jitter and SNR degrade</p><p>with increased sweep velocity, and that degradation is worse when imaging stiffer</p><p>materials. The results also suggest that a transition between shearing-dominated</p><p>jitter and motion-dominated jitter occurs sooner with faster sweep speeds and in</p><p>stiffer materials. These artifacts can be reduced with simple, linear filters. Results</p><p>from plaque mimicking phantoms suggest that the estimation of soft-plaque area</p><p>and flow area, both important tasks for CVD imaging, are only minimally affected</p><p>at faster sweep velocities.</p><p>Current clinical assessment of CVD is guided by spectral Doppler velocity methods.</p><p>As a result, novel imaging approaches (SAD-SWEI, SAD-GATED) were developed</p><p>for combining spectral Doppler methods with existing ARF-based imaging</p><p>techniques to allow for the combined assessment of cross-luminal velocity profiles,</p><p>wall-shear rate (WSR), ARFI displacement and ARF-induced wave velocities. These</p><p>techniques were validated in controlled phantom experiments, and show good agreement</p><p>between previously described ARF-techniques and theory. Initial in vivo feasibility</p><p>was then evaluated in five human volunteers. Results show that a cyclic</p><p>variability in both ARFI displacement and ARF-generated wave velocity occurs during</p><p>the cardiac cycle. Estimates of WSR and peak velocity show good agreement</p><p>with previous ultrasonic-based assessments of these metrics. In vivo ARFI and Bmode/</p><p>WSR images of the carotid vasculature were successfully formed using ECG gating</p><p>techniques.</p><p>This thesis demonstrates the potential of these methods for the combined assessment</p><p>of vascular hemodynamics and elasticity. However, continued investigation</p><p>into optimizing sequences to reduce transducer surface heating, removing the angle</p><p>dependency of the SAD-SWEI/SAD-GATED methods, and decreasing processing</p><p>time will help improve the clinical viability of the proposed imaging techniques.</p>