Shear wave elasticity imaging (SWEI) was employed to track acoustic radiation force (ARF)-induced shear waves
in the myocardium of a beating heart. Shear waves were generated in and tracked through the myocardium of
the left ventricular free wall (LVFW) in an in vivo heart that was exposed through a thoracotomy; matched
studies were also preformed on an ex vivo myocardial specimen. Average shear wave velocities ranged from 2.22
to 2.53 m/s for the ex vivo specimen and 1.5 to 2.9 m/s (1.5-2.09 m/s during diastole; 2.9 m/s during systole)
for in vivo specimens. Despite the known rotation of myocardial fiber orientation with tissue depth, there was
no statistically significant shear wave velocity depth dependence observed in any of the experimental trials.
Acoustic radiation force impulse (ARFI) imaging has been demonstrated to provide insight into the mechanical properties of tissue. The quality of ARFI images is dependent on the amount of acoustic energy from the radiation force pulse reaching the focus. Intra-cardiac probes provide an advantage for ARFI imaging of cardiac tissue, as the probe can be positioned close to the region of interest. The resulting ARFI images display local variations in tissue stiffnesses and show promise for monitoring and assessing the progress of cardiac ablations. The Siemens AcuNav intra-cardiac probe was used to image a tissue-mimicking phantom having 3 mm diameter spherical inclusions with an elastic modulus eight times greater than the surrounding tissue. The ARFI sequences formed high contrast, high resolution images of these inclusions up to depths of approximately 1.5 cm. The ARFI pulse sequences resulted in 0.8°C temperature increase on the transducer face, and the time constant associated with the return to equilibrium temperature was approximately 300 ms. The probe was used to examine an excised segment of an ovine right ventricle with a surface lesion created from radiofrequency ablations (RFA). In areas of healthy tissue, the ARFI images did not show any stiffer regions that would indicate the presence of a lesion. Although the lesion was not visible in conventional B-mode images, the ARFI images were able to show the boundaries between the lesion and the surrounding tissue.