6B-6 Transthoracic Cardiac Acoustic Radiation Force Impulse Imaging: A Feasibility Study

We describe new non-invasive techniques for visualizing local variations of stiffness in cardiac tissue through the use of transthoracic acoustic radiation force impulse (ARFI) imaging. Custom M-mode and two-dimensional ARFI sequences were implemented on the Siemens SONOLINE Antares scanner, and a porcine model was used to demonstrate the feasibility of transthoracic cardiac ARFI. The hearts of two live 20 kg Sinclair pigs were imaged through intercostal and subcostal acoustic windows, using M-mode and two-dimensional B-mode imaging. An abdominal low-frequency (2.2 MHz) probe was used with an extended radiation force pulse length (320 mus) to produce tissue displacements at depths up to 7 cm. Quadratic motion filters were used to separate ARFI displacements from cardiac and respiratory physiological motion artifacts. During M-mode imaging, matched ECG signals were acquired to enable registration to the cardiac cycle. M-mode ARFI displacement images reflect the expected myocardial stiffness changes through the cardiac cycle. During two-dimensional ARFI imaging, ECG gating was used during image acquisition, and while the images show local variations in displacement, some noise and artifacts remain. These preliminary results indicate the feasibility of real-time imaging of cardiac stiffness in vivo.