Acoustic radiation force impulse imaging of cardiac tissue

Cardiovascular disease remains the most likely cause of death in developed countries, accounting for approximately 870,000 deaths in the United States alone in 2004. Virtually every form of cardiovascular disease involves modifications in tissue stiffness. Acoustic radiation force impulse (ARFI) imaging shows great promise in the regional characterization of tissue stiffness in a variety of clinical applications. Cardiovascular tissues present unique challenges to ARFI imaging because of their dynamic changes in stiffness, high-amplitude physiological motion, and elevated stiffness levels compared to other tissues. We have implemented ARFI and shear wave elasticity imaging (SWEI) on a commercially available scanner. These imaging methods were employed on: canine and ovine heart models. Myocardial stiffness measurements were obtained throughout the cardiac cycle and with various manipulations of tissue status (ie: radiofrequency ablation, myocardial ischemia, and heart rate) under epicardial and transthoracic imaging conditions. ARFI imaging resolution was similar to corresponding B-mode images. The ARFI-induced displacement and recovery curves exhibited cyclic variations that reflected the expected changes in stiffness through the cardiac cycle. Shear wave velocimetry was successfully implemented. Also, ARFI imaging was capable of visualizing changes in myocardial stiffness due to the presence of a radiofrequency ablation-created lesion, varying heart rates, and acute myocardial ischemia. A parametric pressure-volume and ARFI imaging analysis also suggests that ARFI imaging provides unique and previously unavailable information into myocardial performance and function.