Browsing by Subject "Blood flow"
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Item Open Access Assessment of Mechanical and Hemodynamic Vascular Properties using Radiation-Force Driven Methods(2011) Dumont, Douglas MSeveral 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.
Item Open Access Enhanced Vasculature Imaging of the Retina Using Optical Coherence Tomography(2013) Hendargo, HansfordOptical coherence tomography (OCT) is a non-invasive imaging modality that uses low coherence interferometry to generate three-dimensional datasets of a sample's structure. OCT has found tremendous clinical applications in imaging the retina and has demonstrated great utility in the diagnosis of various retinal diseases. However, such diagnoses rely upon the ability to observe abnormalities in the structure of the retina caused by pathology. By the time an ocular disease has progressed to the point of affecting the morphology of the retina, irreversible vision loss in the eye may already occur. Changes in the functionality of the tissue often precede changes to the structure. Thus, if imaging methods are developed to provide additional functional information about the behavior and response of the retinal tissue and vasculature, earlier treatment for disease may be prescribed, thus preserving vision for the patient.
Within the last decade, significant technological advances in OCT systems have enabled high-speed and high sensitivity image acquisition using either spectral domain OCT (SDOCT) or swept-source OCT (SSOCT) configurations. Such systems use Fourier processing to extract structural information of a sample from interferometric principles. But such systems also have access to the optical phase information, which allows for functional analysis of sample dynamics. This dissertation details the development and application of methods using both intensity and phase information as a tool for studying interesting biological phenomena. The goal of this work is an extension of techniques to image the vasculature in the retina and enhance the clinical utility of OCT.
I first outline basic theory necessary for understanding the principles of OCT. I then describe OCT phase imaging in cellular applications as a demonstration of the ability of OCT to provide functional information on biological dynamics. Phase imaging methods suffer from an artifact known as phase wrapping, and I have developed a software technique to overcome this problem in OCT, thus extending its usefulness in providing quantitative information. I characterize the limitations in measuring moving scatterers with Doppler OCT in both SDOCT and SSOCT system. I also show the ability to image the vasculature in the retina using variance imaging with a high-speed retinal imaging system and software based methods to correct for patient motion and create a widefield mosaic in an automated manner. Finally, future directions for this work are discussed.
Item Open Access High Resolution X-ray Microscopy Using Digital Subtraction Angiography for Small Animal Functional Imaging(2008-08-04) Lin, Ming DeResearch using mice and rats has gained interest because they are robust test beds for clinical drug development and are used to elucidate disease etiologies. Blood vessel visualization and blood flow measurements are important anatomic and physiologic indicators to drug/disease stimuli or genetic modification. Cardio-pulmonary blood flow is an important indicator of heart and lung performance. Small animal functional imaging provides a way to measure physiologic changes minimally-invasively while the animal is alive, thereby allowing for multiple measurements in the same animal with little physiologic perturbation. Current methods of measuring cardio-pulmonary blood flow suffer from some or all of these limitations-they produce relative measurements, are limited to global or whole animal or organ regions, do not provide vasculature visualization, limited to a few or singular samples per animal, are not able to measure acute changes, or are very invasive or requires animal sacrifice. The focus of this work was the development of a small animal x-ray imaging system capable of minimally invasive real-time, high resolution vascular visualization, and cardio-pulmonary blood flow measurements in the live animal. The x-ray technique used was digital subtraction angiography (DSA). This technique is a particularly appealing approach because it is easy to use, can capture rapid physiological changes on a heart beat-to-beat basis, and provides anatomical and functional vasculature information. This DSA system is special because it was designed and implemented from the ground up to be optimized for small animal imaging and functional measurements. This system can perform: 1) minimally invasive in vivo blood flow measurements, 2) multiple measurements in the same animal in a rapid succession (every 30 seconds-a substantial improvement over singular measurements that require minutes to acquire by the Fick method), 3) very high resolution (up to 46 micron) vascular visualization, 4) quantitative blood flow measurements in absolute metrics (mL/min instead of arbitrary units or velocity) and relative blood volume dynamics from discrete ROIs, and 5) relative mean transit time dynamics on a pixel-by-pixel basis (100 µm x 100 µm). The end results are 1) anatomical vessel time course images showing the contrast agent flowing through the vasculature, 2) blood flow information of the live rat cardio-pulmonary system in absolute units and relative blood volume information at discrete ROIs of enhanced blood vessels, and 3) colormaps of relative transit time dynamics. This small animal optimized imaging system can be a useful tool in future studies to measure drug or disease modulated blood flow dynamics in the small animal.