Browsing by Subject "Elastography"
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Item Open Access Assessment of Cardiac Function by Acoustic Radiation Force (ARF) Based Methods of Ultrasound Elastography(2017) Vejdani Jahromi, MaryamHeart Failure (HF) is a major cause of morbidity and mortality in the world. This disorder is characterized by compromised systolic and/or diastolic function of the myocardium that reduces the pumping and/or filling efficiency resulting in diminished cardiac output. Cardiovascular researchers have been attempting to develop tools for assessment of cardiac function for decades. Evaluating cardiac function helps clinicians to diagnose and to follow the progress of HF patients.
The gold standard technique for cardiac functional assessment, including systolic and diastolic function, is the pressure-volume (PV) loop measurement; however, this measurement is not typically used clinically due to the invasiveness of the technique. PV loop measurement requires the introduction of a pressure or pressure-volume catheter into the left ventricle. Cardiovascular researchers have been attempting to develop non-invasive tools for assessment of cardiac function primarily by measuring surrogates of ventricular contractility and compliance.
These measures are based on imaging and include Ejection Fraction and Doppler and ultrasound strain imaging. These measurements are indirect measures that rely on cardiac motion or volume changes. The measurements are load dependent and could be affected by the heart rhythm and valvular disorders. Despite research toward this goal, there is no clinically accepted noninvasive technique to provide a direct myocardial measurement of cardiac function.
Acoustic radiation force (ARF) based ultrasound elastography techniques were developed in early 2000s and have been used to measure the static stiffness of tissue. These techniques are being used in the clinic for diagnosis of disorders and malignancies in tissues such as liver. When applied to the heart, it was shown that dynamic changes in the stiffness of the myocardium during the cardiac cycle could be recorded using modified versions of these static techniques.
This had the potential to be a direct measure of the time-varying elastance measured during the cardiac cycle using pressure-volume measurements. The question arose as to whether these ARF based measurements of dynamic stiffness could be used for cardiac functional assessments during systole and diastole; and if so, what the relationship is between these measurements and the gold standard method. The goal of this research was to assess the ability of ARF based measurements of cardiac dynamic stiffness to provide meaningful indices of cardiac function.
In this dissertation both acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) ultrasound elastography techniques were studied. These are qualitative and quantitative measures of stiffness, respectively. While the focus of the studies was more on SWEI due to its quantitative nature, ARFI measurements of cardiac function were also investigated and compared to SWEI.
The studies were performed in isolated rabbit hearts in Langendorff or working modes because the preparation has several significant advantages. 1) Parameters including preload, afterload, and coronary perfusion can be accurately controlled. 2) The heart’s left ventricular free wall can be easily imaged from multiple angles. 3) Confounding neurohormonal reflexes of the body can be eliminated.
SWEI measurements of stiffness were used to characterize changes in contractility induced using the Gregg effect. The Gregg effect is the active effect of coronary perfusion on cardiac contractility. It was shown that SWEI measurements of stiffness could detect the changes in contractility induced by this known effect and that the effect was blocked using a Ca channel blocker.
The relationship between ARFI and SWEI measurements was characterized and the possibility of deriving functional indices such as systolic/diastolic ratio and isovolumic relaxation time constant (τ) using either of these techniques was evaluated. It was shown that in the same imaging configuration, the measurements of ARFI and SWEI are linearly related to one another. This could be important, as the ARFI technique will likely be the first cardiac elastography measurement technique to be implemented using transthoracic ultrasound throughout the cardiac cycle.
The Garden Hose effect was used to investigate SWEI’s ability to measure cardiac compliance. SWEI was used to detect the passive effect of coronary perfusion on cardiac compliance and the relationship between perfusion pressure and stiffness was characterized. Finally, SWEI derived measurements of diastolic function were compared to the gold standard PV measurements of cardiac diastolic function including end diastolic stiffness and the relaxation time constant. It was shown that SWEI could detect the changes in cardiac stiffness after induction of global ischemia. These changes were similar to the changes in the PV measure of diastolic stiffness. Furthermore, the results indicated that SWEI could be used to derive the relaxation time constant similar to the relaxation constant derived from intra-ventricular pressure recordings.
In summary, the results of the studies presented in this thesis illustrate the assessments of systolic and diastolic function using ARFI and SWEI ultrasound based elastography. It is concluded that these measurements can be used to derive cardiac functional indices that would have the advantages of an ultrasound based technique; they would be noninvasive, less expensive and could be widely applied outside of the cath lab.
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 Chronic Myocardial Infarct Visualization Using 3D Ultrasound(2011) Byram, BrettThis dissertation aims to demonstrate the feasibility of direct infarct visualization using 3D medical ultrasound. The dissertation proceeds by providing the first ever demonstration of fully-sampled 3D ultrasonic speckle tracking using raw B-Mode data of the heart. The initial demonstration uses a Cramer-Rao lower bound limited displacement estimator. The dissertation then proceeds to develop an implementable method for biased time-delay estimation. Biased time-delay estimation is shown to surpass the traditional limits described by the Cramer-Rao lower bound in a mean square error sense. Additional characterization of this new class of estimator is performed to demonstrate that with easily obtainable levels of prior information it is possible to estimate displacements that do surpass the Cramer-Rao lower bound. Finally, using 2D and 3D realizations of biased displacement estimation (Bayesian speckle tracking) the passive strain induced in the ventricle walls during atrial systole is shown to be sufficient to distinguish healthy and chronically infarcted myocardium.
Item Open Access Comparison of Acoustic Radiation Force Impulse (ARFI) Imaging and Shear Wave Imaging (SWI) in Evaluation of Myocardial Ablation Lesions(2013) Kuo, Lily AnneRadiofrequency ablation (RFA) is commonly used to treat cardiac arrhythmias, by generating a series of discrete RFA lesions in the myocardium to isolate arrhythmogenic conduction pathways. The size of each lesion is controlled by the temperature of the tissue at the surface or the duration of RF power delivery, but feedback on the extent and transmurality of the generated lesion are unavailable with current technology. Intracardiac Echocardiography (ICE) may provide a solution through Acoustic Radiation Force Impulse (ARFI) imaging or Shear Wave Imaging (SWI), which each generate images of local mechanical compliance from very small ultrasonically-induced waves. This work compares ARFI and SWI in an ex-vivo experiment for lesion boundary assessment and lesion gap resolution.
Item Open Access Volumetric Acoustic Radiation Force Impulse Imaging Using Intracardiac Echocardiography(2020) Kim, Young-JoongIntracardiac echocardiography (ICE) based elastography methods have the potential to be useful for a number of clinical purposes including monitoring of ablation lesion formation and myocardial substrate characterization. However, 2-D field-of-view ICE catheters currently in use in the clinic have difficulties imaging face-on regions of myocardial tissue, requiring meticulous and time-consuming translational and rotational scanning of the array. This dissertation investigates the use of helicoid array transducers to perform ICE-based acoustic radiation force impulse (ARFI) imaging on multiple elevation planes at once, improving on current methods in terms of speed and ease-of-use.
The Siemens Acuson SC2000 ultrasound scanner was programmed with sequences to perform SWEI imaging on the Soundstar 8F linear array ICE catheter and to perform volumetric ARFI scans using the AcuNav V helicoid array catheter. These sequences were used respectively to characterize the stiffness contrast in ablated human atrial tissue and to characterize the performance of volumetric ARFI at detecting gaps in atrial tissue phantoms.
The first research chapter is a clinical study showing that shear wave elastography (SWE) using a traditional 2-D field-of-view ICE catheter can be used to distinguish between baseline and ablated left atrial (LA) tissue in patients undergoing radiofrequency ablation (RFA) for atrial fibrillation (AF). Shear wave velocities of baseline LA and right atrium (RA), low electrogram voltage areas of the LA, and ablated LA are reported. The second chapter investigates through simulation and experiments the volumetric B-mode imaging performance of helicoid array transducers. Experimental verification of pressure field simulations is done by the use of the Siemens Acuson AcuNav V, a 128-element helicoid array transducer. Guided by these results, a discussion of the design of helicoid array transducer imaging sequences is presented. The final chapter is about the use of the helicoid array transducer for volumetric ARFI imaging. Experiments in tissue phantoms of varying elasticities and inclusions demonstrate that it is possible to identify gaps as narrow as 1 mm when the contrast is similar to that of baseline and ablated human LA myocardium.
This work demonstrates the feasibility of using helicoid array transducers for volumetric elastography imaging of the heart and establishes a foundation for future clinical investigations using this technology.