A High Frame-rate and Low-cost Elastography System by Generating Shear Waves through Continuous Vibration of the Ultrasound Transducer Daniel C. Mellema 1 , Pengfei Song 1 , Armando Manduca 1 , Matthew W. Urban 1 , Randall R. Kinnick, James F. Greenleaf 1 , Shigao Chen 1 1 Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN, USA Abstract— Ultrasound shear wave elastography (SWE) utilizes the propagation of induced shear waves to characterize the shear modulus of soft tissue. Many methods rely on an acoustic radiation force (ARF) push beam to generate shear waves. However, specialized hardware is required to generate the push beams, and the thermal stress that is placed upon the ultrasound system, transducer, and tissue by the push beams currently limits the frame-rate to about 1 Hz. This paper presents Probe Oscillation Shear Elastography (PROSE) as an alternative method to measure tissue elasticity, by generating shear waves using a continuous mechanical vibration of an ultrasound transducer, while simultaneously detecting motion with the same transducer under pulse-echo mode. Motion of the transducer during detection produces a compression strain artifact that is coupled with the observed shear waves. A novel symmetric sampling scheme is proposed such that pulse-echo detection events are acquired when the ultrasound transducer returns to the same physical position, allowing the shear waves to be decoupled from the compression artifact. Full field-of-view (FOV) two-dimensional (2D) shear wave speed (SWS) images were obtained by applying a local frequency estimation (LFE) method, capable of generating a 2D map from a single frame of shear wave motion, allowing for an imaging frame rate comparable to the vibration frequency. PROSE was able to produce smooth and accurate shear wave images from three homogeneous phantoms with different moduli, with an effective frame rate of 300Hz. An inclusion phantom study showed that increased vibration frequencies improved the accuracy of inclusion imaging, and allowed targets as small as 6.5 mm to be resolved with good contrast (contrast-to-noise ratio ≥19 dB) between the target and background. Index Terms— Mechanical vibration, shear wave, ultrasound elastography. I. INTRODUCTION Elasticity imaging has been developed to provide non- invasive and objective quantification of the mechanical properties of tissue [1]. Many of the techniques developed utilize the propagation of induced shear waves as a surrogate for, or to quantitatively estimate the shear modulus of soft tissue. Assuming soft tissues are incompressible, isotropic, linear, and elastic, the shear modulus (μ) can be related to shear wave propagation speed, c s , by ߩ=ߤ ௦ ଶ , (1) where ρ is density and can be assumed to be 1000 kg/cm 3 for all soft tissues [2]. Acoustic radiation force (ARF) based techniques have been successfully used to generate shear waves within soft tissue [3-9]. However, these ARF-based shear wave elastography (SWE) techniques suffer from two major drawbacks. The first is that the high voltage push beam used to generate shear waves requires upgraded hardware, which limits the implementation of ARF-based SWE to high-tier ultrasound systems [3, 10]. Secondly, the delivered push beams places thermal stress on the hardware, ultrasound transducer, and tissue, ultimately limiting the typical SWE frame-rate to about 1 Hz. Sonoelastography methods are able to generate shear waves through the mechanical coupling of an external vibration source [2, 11], potentially providing higher imaging frame- rates. However, these methods generally require separate devices for the generation and the detection of shear waves, which can cause difficulty when investigating certain tissues due to physiological constraints. Transient elastography methods, such as Fibroscan, have addressed this issue by using a small single element transducer to deliver a mechanical impulse to generate a longitudinally polarized transient shear wave that can be detected by the same ultrasound transducer [7]. This system is widely applied in studying liver fibrosis; however, Fibroscan does not provide two-dimensional (2D) imaging and is not suited for the detection of local changes in tissue elasticity. In this paper we propose Probe Oscillation Shear Wave Elastography (PROSE) as a new method, which utilizes continuous mechanical vibration of the ultrasound transducer to produce harmonic longitudinally polarized shear waves that are similar to those generated using TE. Different from TE, PROSE is capable of 2D quantitative elastography by simultaneously generating and acquiring the resulting shear wave motion using the same transducer under pulse-echo detection mode. Single frame shear wave motion was obtained using a novel symmetric sampling scheme and 2D shear wave speed (SWS) image were reconstructed from each single frame shear wave motion using the Local Frequency Estimation (LFE) method [12]. The efficacy of PROSE was evaluated through homogeneous and inclusion phantom studies. II. METHODS A. Shear Wave Generation and Detection To generate shear wave motion, a custom vibration system was created, by coaxially mounting a linear voice coil actuator (BEI Kimco Magnetics, Vista, CA) to an L11-4v linear array ultrasound transducer (Verasonics Inc., Kirkland, WA) for use with a Verasonics Vantage Ultrasound system (Verasonics Inc., Kirkland, WA). A function generator (Agilent 33250A, 978-1-4799-8182-3/15/$31.00 ©2015 IEEE 2015 IEEE International Ultrasonics Symposium Proceedings 10.1109/ULTSYM.2015.0203