Browsing by Author "Dumont, Douglas M"

Several groups have proposed classifying atherosclerotic disease by using acoustic radiation

force (ARF) elasticity methods to estimate the mechanical and material

properties of plaque. However, recent evidence suggests that cardiovascular disease

(CVD), in addition to involving pathological changes in arterial tissue, is also a

hemodynamic remodeling problem. As a result, integrating techniques that can

estimate localized hemodynamics relevant to CVD remodeling with existing ARF based

elastography 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. Techniques

for integrating B-mode, color-flow Doppler, and ARFI imaging were developed

(BACD imaging), validated in tissue-mimicking phantoms, and demonstrated for in

vivo imaging. The resulting system allows for the real-time acquisition (< 20 Hz) of

spatially registered B-mode, flow-velocity, and ARFI displacement images of arterial

tissue throughout the cardiac cycle. ARFI and color-flow Doppler imaging quality,

transducer surface heating, and tissue heating were quantified for different frame-rate

and scan-duration configurations. The results suggest that BACD images can be acquired

at high frame rates with minimal loss of imaging quality for approximately

five seconds, while staying beneath suggested limits for tissue and transducer surface

heating.

Because plaque-burden is potentially a 3D problem, techniques were developed

to allow for the 3D acquisition of color-flow Doppler and ARFI displacement data

using a stage-controlled, freehand scanning approach. The results suggest that a

40mm x 20mm x 25mm BACD volume can be acquired in approximately three seconds.

Jitter, SNR, lesion CNR, soft-plaque detectability, and flow-area assessment were

quantified in tissue mimicking phantoms with a range of elastic moduli relevant

to ARFI imaging applications. Results suggest that both jitter and SNR degrade

with increased sweep velocity, and that degradation is worse when imaging stiffer

materials. The results also suggest that a transition between shearing-dominated

jitter and motion-dominated jitter occurs sooner with faster sweep speeds and in

stiffer materials. These artifacts can be reduced with simple, linear filters. Results

from plaque mimicking phantoms suggest that the estimation of soft-plaque area

and flow area, both important tasks for CVD imaging, are only minimally affected

at 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 developed

for combining spectral Doppler methods with existing ARF-based imaging

techniques to allow for the combined assessment of cross-luminal velocity profiles,

wall-shear rate (WSR), ARFI displacement and ARF-induced wave velocities. These

techniques were validated in controlled phantom experiments, and show good agreement

between previously described ARF-techniques and theory. Initial in vivo feasibility

was then evaluated in five human volunteers. Results show that a cyclic

variability in both ARFI displacement and ARF-generated wave velocity occurs during

the cardiac cycle. Estimates of WSR and peak velocity show good agreement

with previous ultrasonic-based assessments of these metrics. In vivo ARFI and Bmode/

WSR images of the carotid vasculature were successfully formed using ECG gating

techniques.

This thesis demonstrates the potential of these methods for the combined assessment

of vascular hemodynamics and elasticity. However, continued investigation

into optimizing sequences to reduce transducer surface heating, removing the angle

dependency of the SAD-SWEI/SAD-GATED methods, and decreasing processing

time will help improve the clinical viability of the proposed imaging techniques.