d Original Contribution BIAS OBSERVED IN TIME-OF-FLIGHT SHEARWAVE SPEED MEASUREMENTS USING RADIATION FORCE OFA FOCUSED ULTRASOUND BEAM HENG ZHAO,* PENGFEI SONG,* MATTHEW W. URBAN,* RANDALL R. KINNICK,* MENG YIN, y J AMES F. GREENLEAF ,* and SHIGAO CHEN* * Department of Physiology and Biomedical Engineering; and y Department of Radiology, Mayo Clinic College of Medicine, Rochester, MN, USA (Received 25 February 2011; revised 3 June 2011; in final form 23 July 2011) Abstract—Measurement of shear wave propagation speed has important clinical applications because it is related to tissue stiffness and health state. Shear waves can be generated in tissues by the radiation force of a focused ultra- sound beam (push beam). Shear wave speed can be measured by tracking its propagation laterally from the push beam focus using the time-of-flight principle. This study shows that shear wave speed measurements with such methods can be transducer, depth and lateral tracking range dependent. Three homogeneous phantoms with different stiffness were studied using curvilinear and linear array transducer. Shear wave speed measurements were made at different depths, using different aperture sizes for push and at different lateral distance ranges from the push beam. The curvilinear transducer shows a relatively large measurement bias that is depth depen- dent. The possible causes of the bias and options for correction are discussed. These bias errors must be taken into account to provide accurate and precise time-of-flight shear wave speed measurements for clinical use. (E-mail: chen.shigao@mayo.edu) Ó 2011 World Federation for Ultrasound in Medicine & Biology. Key Words: Shear wave speed, Liver fibrosis, Bias, ARFI. INTRODUCTION Mechanical properties of tissues such as shear modulus (elasticity) are related to the state of tissue health (Sarvazyan et al. 1998). Therefore, noninvasive methods for measuring tissue elasticity have important clinical applications. Assuming isotropy, incompressibility and linearity, the shear modulus m of a soft tissue is related to its shear wave propagation speed c s by eqn (1) m 5 rc 2 s ; (1) where r is density, which can be assumed to be 1000 kg/m 3 for all soft tissues (Yamakoshi et al. 1990). Equa- tion (1) neglects tissue viscosity and frequency effects but generally is considered valid when the frequency of the shear wave is low and narrowband. Therefore, measure- ments of shear wave propagation speed can be used to estimate tissue elasticity for that bandwidth present in the shear wave. Acoustic radiation force from a focused ultrasound beam can be used to generate shear waves within tissues. If a single focused ultrasound beam is used for pushing, it is commonly assumed that the shear waves generated by the ultrasound radiation force at the focal region propa- gate within the transducer imaging plane in a direction perpendicular to the ultrasound beam axis (Parker et al. 2011). Therefore, tissue motion at several lateral positions along the shear wave propagation path at the push beam focal depth can be measured using pulse-echo ultrasound to calculate shear wave propagation speed based on time- of-flight principle. This approach was first proposed in shear wave elasticity imaging (SWEI) and later used by several groups with various modifications to measure tissue elasticity (Chen et al. 2009; Deffieux et al. 2009; Palmeri et al. 2008; Sarvazyan et al. 1998). Among these methods, supersonic shear imaging (SSI) and acoustic radiation force impulse imaging (ARFI) have been implemented on commercial ultrasound scanners and used for human studies (D’Onofrio et al. 2010; Deffieux et al. 2009; Muller et al. 2009; Palmeri et al. 2008; Tanter et al. 2008). The purpose of this study is to evaluate if shear wave speed measured by radiation force of a focused Address correspondence to: Shigao Chen, Department of Physi- ology and Biomedical Engineering, Mayo Clinic College of Medicine, 200 First Street, S.W., Rochester, MN 55905, USA. E-mail: chen. shigao@mayo.edu 1884 Ultrasound in Med. & Biol., Vol. 37, No. 11, pp. 1884–1892, 2011 Copyright Ó 2011 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/$ - see front matter doi:10.1016/j.ultrasmedbio.2011.07.012