Intracardiac Measurements of Elasticity Using Acoustic Radiation Force Impulse (ARFI) Methods: Temporal and Spatial Stability of Shear Wave Velocimetry Peter Hollender ∗ , Richard Bouchard ∗ , Stephen Hsu ∗ , David Bradway ∗ , Patrick Wolf ∗ , Gregg Trahey ∗† ∗ Department of Biomedical Engineering, Duke University, Durham, North Carolina † Department of Radiology, Duke University, Durham, North Carolina Abstract—Acoustic Radiation Force Impulse (ARFI) methods have been validated for measuring tissue elastic properties, with shear wave velocimetry emerging as a quantitative way to measure stiffness. Using ultrasound to interrogate cardiac elasticity holds promise for diagnosis of cardiac dysfunction, but acquiring measurements has previously been an invasive procedure and not clinically viable. This work describes the feasibility of generating and tracking acoustic radiation force generated shear waves in myocardium with an intracardiac echocardiography (ICE) transducer and discusses the spatial and temporal stability of these measurements. In vivo healthy canine data are presented, demonstrating the quantitative contrast of systolic and diastolic shear velocities in the right ventricular free wall (RVFW) as measured by this technique. Although the generated shear wave amplitudes are low, ICE shear-wave velocimetry is shown to provide a much less invasive way to quantify the heart’s stiffening and relaxation through systole and diastole than prior methods. I. I NTRODUCTION Measuring the elastic properties of myocardium using ul- trasound is an area that has been receiving an increasing amount of attention [1]–[3]. Diastolic dysfunction, for exam- ple, is often the result of either passive stiffening or impaired dynamic relaxation of the left ventricle [4]. The propaga- tion velocity of transverse waves in tissue is proportonal to the tissue’s stiffness, so velocimetric methods hold promise for estimating tissue stiffness. Pislaru et al. have used an open-chest vibrometry experiment to measure the systolic to diastolic contrast of dispersive phase velocities in vivo, finding appoximately a 3:1 ratio in velocities [3]. Acoustic Radiation Force Impulse (ARFI)-based methods have also been shown to be able to characterize the stiffness of the heart in vivo both with displacement amplitude [1] and with shear velocimetry [2], though thus far only with an invasive, open chest procedure. Bouchard et al. measured the velocity of shear waves in the left ventricular myocardium with a linear array attached directly to the heart , demonstrating a 2:1 wave velocity ratio between systole and diastole (Figure 1) [5]. Transthoracic cardiac ARFI would be a completely noninvasive way to measure the heart’s elasticity, but faces a number of technical challenges (e.g. viewing angles, distance from transducer, motion). In larger patients, generating enough 0 0.5 1 1.5 2 Shear Velocity (m/s) 0 0.5 1 1.5 T(s) ECG Fig. 1. Bouchard et al’s open-chest in vivo ARF-induced shear wave velocities, measured directly with a linear array on the LVFW radiation force to launch a shear wave from outside of the body is difficult due to attenuation and aberration. This work aims to estimate dynamic myocardial stiffness by the use of ARFI shear wave velocimetry implemented on an intracardiac echocardiography (ICE) transducer. ICE is commonly used in guiding ablation and surgical procedures and is considered a minimally invasive procedure. ICE ARFI has been previously demonstrated to be viable in vivo during diastole [6]. By greatly reducing the distance to the myocardial wall (from 10 cm or more to 1-2 cm), shear wave generation and tracking can be improved. This paper presents some of the challenges of using intrac- ardiac ARFI methods throughout the cardiac cycle, and some techniques used to overcome them. Stability and viability of obtaining shear velocity estimates at different points in the cardiac cycle will be discussed. In vivo data from canine experiments are presented. 682 2010 IEEE International Ultrasonics Symposium Proceedings 10.1109/ULTSYM.2010.0174 978-1-4577-0381-2/10/$25.00 ©2010 IEEE