A review of physical and engineering factors potentially affecting shear wave elastography

Abstract

It has been recognized that tissue stiffness provides useful diagnostic information, as with palpation as a screening for diseases such as cancer. In recent years, shear wave elastography (SWE), a technique for evaluating and imaging tissue elasticity quantitatively and objectively in diagnostic imaging, has been put into practical use, and the amount of clinical knowledge about SWE has increased. In addition, some guidelines and review papers regarding technology and clinical applications have been published, and the status as a diagnostic technology is in the process of being established. However, there are still unclear points about the interpretation of shear wave speed (SWS) and converted elastic modulus in SWE. To clarify these, it is important to investigate the factors that affect the SWS and elastic modulus. Therefore, physical and engineering factors that potentially affect the SWS and elastic modulus are discussed in this review paper, based on the principles of SWE and a literature review. The physical factors include the propagation properties of shear waves, mechanical properties (viscoelasticity, nonlinearity, and anisotropy), and size and shape of target tissues. The engineering factors include the region of interest depth and signal processing. The aim of this review paper is not to provide an answer to the interpretation of SWS. It is to provide information for readers to formulate and verify the hypothesis for the interpretation. Therefore, methods to verify the hypothesis for the interpretation are also reviewed. Finally, studies on the safety of SWE are discussed.

Keywords: Engineering factors; Interpretation; Physical factors; Shear wave elastography; Shear wave speed.

Fig. 1

Propagation of shear waves generated by push pulse transmission. a Propagation of a shear wave generated by a push pulse with a single focus. b Propagation of a plane shear wave generated by continuously transmitting multiple push pulses with different focal points

Fig. 2

Outline of propagation speed measurement of shear waves generated by push pulse transmission. a Positions of scan lines ($x_1$, $x_2$, $x_3$, $x_4$) in measuring the particle displacement of shear waves. b Transition of particle displacement waveforms and its peak time transition (travel time) ($t_1$, $t_2$, $t_3$, $t_4$) obtained at each scan line. c Relationship between the propagation distance of a shear wave (travel distance) and travel time. The slope of this straight line corresponds to the average value of the SWS in the ROI

Fig. 3

An illustration of a dispersion curve showing the velocity dispersion in which the phase velocity of shear waves differs depending on the frequency of the shear wave propagating in the viscoelastic body. A flat line for no viscosity, a gentle slope for low viscosity, and a steep slope for high viscosity are observed. (Conceptual diagram of [71])

Fig. 4

Typical viscoelastic models [62]. a Maxwell model, b Voigt model, c Kelvin model. Here, $G$, $G_1$, and $G_2$ indicate the shear modulus, and $\mu$ indicates the viscosity coefficient

Fig. 5

Nonlinear property of the measured SWS when load was applied along the direction of the muscle fibers. (Conceptual diagram of [75])

Fig. 6

Anisotropy in which the SWS differs depending on the angle θ formed by the fiber orientation and the direction of shear wave propagation. (Conceptual diagram of [80])