| dc.description.abstract |
<p>Acoustic radiation force impulses (ARFI) have been used to generated transverse-traveling
mechanical waves in various biological tissues. The velocity of these waves is related
to a medium's stiffness and thus can offer useful diagnostic information. Consequently,
shear wave velocimetry has the potential to investigate cardiac disease states that
manifest themselves as changes in tissue stiffness (e.g., ischemia).</p><p> The
work contained herein focuses on employing ARFI-based shear wave velocimetry techniques,
similar to those previously utilized on other organs (e.g., breast, liver), for the
investigation of cardiac tissue. To this end, ARFI excitations were used to generate
slow-moving (under 3 m/s) mechanical waves in exposed myocardium (with access granted
through a thoracotomy); these waves were then tracked with ultrasonic methods. Imaging
techniques to increase frame-rate, decrease transducer/tissue heating, and reduce
the effects of physiological motion were developed. These techniques, along with
two shear wave velocimetry methods (i.e., the Lateral Time-to-Peak and Radon sum transformation
algorithms), were utilized to successfully track shear wave propagation through the
mid-myocardial layer <italic>in vitro</italic> and <italic>in vivo</italic>. <italic>In
vitro</italic> experiments focused on the investigation of a shear wave anisotropy
through the myocardium. This experimentation suggests a moderate shear wave velocity
anisotropy through regions of the mid-myocardial layer. <italic>In vivo</italic>
experiments focused on shear wave anisotropy (which tend to corroborate the aforementioned
<italic>in vitro</italic> results), temporal/spatial stability of shear wave velocity
estimates, and estimation of wave velocity through the cardiac cycle. Shear wave velocity
was found to cyclically vary through the cardiac cycle, with the largest estimates
occurring during systole and the smallest occurring during diastole. This result
suggests a cyclic stiffness variation of the myocardium through the cardiac cycle.
A novel, on-axis technique, the displacement ratio rate (DRR) method, was developed
and compared to conventional shear wave velocitmetry and ARFI imaging results; all
three techniques suggest a similar cyclic stiffness variation.</p><p> Shear wave
velocimetry shows promise in future investigations of myocardial elasticity. The
DRR method may offer a means for transthoracic characterization of myocardial stiffness.
Additionally, the future use of transesophageal and catheter-based transducers presents
a way of generating and tracking shear waves in a clinical setting (i.e., when epicardial
imaging is not feasible). Lastly, it is hoped that continued investigations into
the physical basis of these ARFI-generated mechanical waves may further clarify the
relationship between their velocity in myocardium and material stiffness.</p>
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