Three-dimensional visualization of shear wave propagation generated by dual acoustic radiation pressure Yuta Mochizuki 1 , Hirofumi Taki 1,2 , and Hiroshi Kanai 2,1 * 1 Graduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan 2 Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan *E-mail: kanai@ecei.tohoku.ac.jp Received November 14, 2015; revised March 2, 2016; accepted March 27, 2016; published online June 16, 2016 An elastic property of biological soft tissue is an important indicator of the tissue status. Therefore, quantitative and noninvasive methods for elasticity evaluation have been proposed. Our group previously proposed a method using acoustic radiation pressure irradiated from two directions for elastic property evaluation, in which by measuring the propagation velocity of the shear wave generated by the acoustic radiation pressure inside the object, the elastic properties of the object were successfully evaluated. In the present study, we visualized the propagation of the shear wave in a three-dimensional space by the synchronization of signals received at various probe positions. The proposed method succeeded in visualizing the shear wave propagation clearly in the three-dimensional space of 35 ' 41 ' 4 mm 3 . These results show the high potential of the proposed method to estimate the elastic properties of the object in the three-dimensional space. © 2016 The Japan Society of Applied Physics 1. Introduction It is well known that the progression of a lesion is accompanied by changes in the hardness of biological tissue. For example, owing to pyramidal tract disorders or peripheral neuropathy, the elastic modulus of muscle decreases. In amyloidosis, atrophy and the elevation of muscle hardness occur. Also, polymyositis and myoglobinuria lead to muscle weakness or a decrease in muscle elasticity. 1,2) Therefore, it would be valuable to measure the elasticity of muscle for the early detection and quantitative diagnosis of muscle disorder. Ultrasound imaging is a noninvasive test with high performance in depicting soft tissue. Several methods have been reported for the improvement in image quality, 3–5) tissue characterization, 6–8) and the estimation of tissue displace- ment. 9–11) Many methods that can be used to noninvasively measure the hardness of soft biological tissue have been proposed; in these method, external pressure is applied to biological tissue and the resulting deformation and movement of the tissue are observed to obtain the parameters regarding the hardness of the tissue. Recently, some methods have been developed for the noninvasive evaluation of the elasticity of soft tissue. 12,13) Other research groups have reported several methods based on the hysteresis property between force and displacement, 14) and based on the dispersion of shear wave propagation velocity. 15) Ultrasound-based measurement methods can be classified into two categories according to the methods used to apply pressure. One is the application of static pressure and the other is the application of dynamic pressure. In the former method, by measuring the deformation of the tissue with ultrasound before and after the application of static pressure, the elasticity of the biological tissue is estimated. However, the elasticity of the tissue cannot be evaluated quantita- tively. 16) Therefore, our group has applied dynamic pressure produced by acoustic radiation to quantitatively evaluate the elastic properties of the tissue by the ultrasound measurement of the propagation of the generated shear wave. Over the past decade, remote actuation methods based on acoustic radiation pressure have been reported. Fatemi and coworkers proposed an imaging modality that uses the acoustic response of an object, which is closely related to its mechanical properties. By measuring the acoustic emission with a hydrophone, hard inclusions, such as calcified tissues in soft materials, were detected experimentally. 17,18) How- ever, the amplitude of the radiated acoustic emission signal was very small for soft tissue and the spatial resolution was limited by the size of the intersectional area of ultrasound beams at two slightly different frequencies. Nightingale and coworkers proposed an alternative imag- ing method (acoustic radiation pressure impulse: ARFI), in which focused ultrasound is employed to apply radiation pressure to soft tissue for a short duration (less than 1 ms). The elastic properties of the tissue were investigated on the basis of the magnitude of the transient response, which was measured as the displacement of tissue with ultrasound. 19–21) However, in order to generate a measurable displacement by several successive ultrasound pulses, high-intensity pulsed ultrasound at 1,000 W=cm 2 was required. According to safety guidelines for the use of diagnostic ultrasound, it is recommended that the intensity be below 240 mW=cm 2 (ISPTA) for pulsed waves and 1 W=cm 2 for continuous waves. 22) The intensity of the pulsed ultrasound employed by Nightingale et al. was, therefore, far greater than that recommended in the safety guidelines. 19) Subsequently, many groups studied methods for actuation using high-intensity pulsed ultrasound to measure elastic properties of biological tissue. 23,24) To decrease the ultra- sound intensity for the actuation of soft tissue, our group chose continuous-wave ultrasound, as did Fatemi et al., 17) in which the maximum intensity of 1 W=cm 2 for continuous waves given by safety guidelines generates an acoustic radiation pressure of 6.67 Pa, which is very small. Therefore, to generate a measurable displacement by acoustic actuation, an effective method of applying acoustic radiation pressure should be developed. A single acoustic radiation pressure does not generate deformation effectively because it primarily changes the position of the object. Thus, our group has developed a method in which two cyclical radiation pressures are simu- ltaneously applied to a phantom from two opposite horizontal directions to cyclically compress the object along the hori- zontal direction. Furthermore, the resultant regional displace- Japanese Journal of Applied Physics 55, 07KF13 (2016) http://doi.org/10.7567/JJAP.55.07KF13 REGULAR PAPER 07KF13-1 © 2016 The Japan Society of Applied Physics