10B-2 Acoustic Radiation Force Impulse Imaging of Mechanical Stiffness Propagation within Myocardial Tissue

Acoustic radiation force impulse (ARFI) imaging has been shown to be capable of imaging the local myocardial stiffness changes of heart throughout the cardiac cycle. Expanding on these results, we have experimentally assessed cardiac ARFI imaging to be a suitable method in visualizing the propagation of mechanical stiffness waves through a heartbeat. ARFI images of the left ventricular free wall of an exposed canine heart were acquired. Images were formed while the heart was external paced by one of two electrodes positioned on the epicardial surface and either side of the imaging plane. Two-line M-mode ARFI images were acquired at a sampling frequency of 120 Hz while the heart was being paced from either stimulating electrode. Passive ARFI images, where the radiation force pulse amplitude was set to zero, were acquired to determine the levels of physiological motion artifact within the ARFI images. Two- dimensional ARFI images that were also triggered off the stimulating electrodes were formed with multi-beat synthesis across seven heartbeats at a frame rates of 65 Hz. Directions and speeds of myocardial stiffness propagation were measured within the ARFI images. In all of these images, the direction myocardial stiffness wave was seen to be away from the stimulating electrode. Within the M-mode ARFI images, the stiffness waves were seen to propagate with similar velocity magnitudes (0.519 m/s and 0.500 m/s). The two dimensional ARFI image estimated a higher stiffness propagation velocity (-0.734 m/s).