Shear wave elastography (SWE) is widely used to evaluate breast lesions [1,2]. The addition of SWE to B-mode ultrasound (US) can improve the diagnostic performance of US in differentiating benign and malignant breast lesions [3-5]. SWE is a US technique that quantifies tissue elasticity by using acoustic radiation forces to generate laterally propagating shear waves. It can measure the propagating shear wave velocity, which differs depending on tissue stiffness [1-5]. Most current SWE approaches use a linear elastic model that only quantifies elasticity, assuming that the tissue is purely elastic. However, human soft tissues show viscoelastic properties.

Shear wave dispersion (SWD) imaging is a newly developed US technique that evaluates the dispersion slope of shear waves, which is related to the viscosity of biological tissues [6-8]. Dispersion refers to the frequency dependence of both the speed and attenuation of shear waves in the viscous component. As the shear wave speed depends on the frequency of shear waves, the dispersion curve can be obtained from an analysis of shear wave speed according to frequency [9]. The dispersion slope is correlated with viscosity. Therefore, an analysis of SWD characteristics can measure tissue viscosity indirectly.

Several recent studies have evaluated the usefulness of SWD imaging in human and rat liver tissues [6-8,10,11]. SWD can be used for evaluating necroinflammation in the liver [6,7]. Lee et al. [7] investigated the role of SWD imaging in detecting allograft damage after liver transplantation. The SWD slope determined using SWE was related to both liver fibrosis and the degree of hepatic necroinflammatory changes, and the SWD slope showed better diagnostic performance than the SWE value in detecting allograft damage after liver transplantation. However, few studies have used SWD imaging to evaluate breast lesions [11,12]. Ormachea et al. [11] obtained SWD images to evaluate breast phantoms and two women (one with a fibroadenoma and one with dense breast tissue). Kumar et al. [12] found a significant difference in shear viscosity between malignant and benign breast masses, with a higher median value for the malignant masses.

This pictorial review aims to assess the feasibility of breast viscosity evaluation using SWD imaging in patients with benign and malignant breast lesions. All images were obtained from women who underwent breast imaging at Korea University Ansan Hospital.

In 1951, long before the advent of clinical elastography, Hans Oestreicher proposed that tissues behave as linear and isotropic viscoelastic media, with corresponding stiffness and viscosity [13]. Currently, SWE can measure the group velocity of a collection of shear waves with reasonable diagnostic performance. Tissue stiffness, measured with two-dimensional (2D) SWE, is a widely recommended parameter. However, stiffness and viscosity may provide distinct sets of clinical data because viscosity is a qualitatively different parameter.

SWD maps can be created using an imaging technique incorporated into a US system (Aplio i-series, Canon Medical Systems, Tokyo, Japan) [6-8]. This technique can estimate the dispersion slope of shear wave speed versus frequency to evaluate changes in tissue viscosity. SWD processing involves four main steps (Fig. 1). In the first step, the displacement induced by shear waves is obtained using a technique based on color Doppler scanning. Second, displacement at each location is transformed from the time to the frequency domain through the Fourier transform algorithm to estimate the phase changes in shear waves at different frequencies. Third, the shear wave speed is calculated using the phase-difference method between displacements at each frequency from the following equation:

where π is the circular constant, f is the frequency, ∆L is the displacement between the frequencies at each point, ∆ø is the difference between the phases at each point, C_s(nf) is the shear speed at each frequency, and ° defines each element in the equation [7]. Finally, the slope of the shear wave speed is calculated based on the distribution of shear wave speed versus frequency. The shear wave viscosity value can be estimated from the slope over the shear wave frequency bandwidth (Fig. 2). The slope of the dispersion slope is then superimposed on each measurement location.

SWD imaging can be activated automatically in SWE mode using the Aplio i-series (Canon Medical Systems). In SWE quad-view mode, it is possible to simultaneously view the shear wave elasticity (elasticity map), SWE arrival time contours (propagation map), grayscale US, and SWD slope (dispersion map) (Fig. 3). The dispersion slope value ([m/s]/kHz), elasticity value (kPa), and their standard deviations are displayed for quantitative assessment.

A radiologist experienced in breast imaging (24 years of experience in breast US) performed 2D SWE and SWD imaging examinations using an Aplio i800 system with a linear probe (US frequency range, 5-18 MHz; PLI-1205BX, Canon Medical Systems). The SWD imaging technology for breast examination was a prototype. For quantitative measurements, the radiologist placed a 2-mm circular region of interest on the fibroglandular breast tissue in ultrasonographic negative patients and the areas with the highest SWE and SWD responses in patients with breast lesions [1]. Subcutaneous fat was selected as the reference area for both the elasticity and dispersion ratios. All SWE and SWD measurements were repeated at least three times per patient [14].

In normal breast tissue, tissue elasticity and viscosity tended to be similar to each other in the fibroglandular tissue, and were slightly higher than the corresponding values in the subcutaneous fat tissue (Fig. 4). In cases of benign breast lesions, the elasticity value was in the range of 10-20 kPa, while the dispersion slope value was approximately 10 (m/s)/kHz, which is similar to that of fibroglandular tissue (Figs. 5, 6). However, the elasticity values of malignant breast lesions notably exceeded 50 kPa, and the dispersion slope values were above 20 (m/s)/kHz in most malignant lesions (Figs. 7-14), which may help distinguish malignant lesions from both benign lesions and normal fibroglandular tissue. Several prior studies have evaluated the SWE characteristics among different histologic types or molecular subtypes of invasive breast cancers [15-18]. However, no studies have assessed these features in SWD imaging.

This pictorial essay shows the SWE and SWD images of benign and malignant lesions on breast US. The SWD slope determined by 2D SWE may provide more detailed and useful information regarding the characterization of breast lesions. We suggest that SWD imaging can help differentiate malignant from benign breast lesions. This possibility needs to be verified in further studies with larger numbers of participants.