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