Shear wave elastography (SWE), as a type of elastography, is a non-invasive medical imaging technique used to quantitatively assess the elasticity and stiffness of tissues. The method excites the shear wave in the tissue by ultrasonic wave and captures the propagation speed of the shear wave with ultrasonic imaging equipment. The propagation speed of the shear wave is related to the elastic modulus of the tissue: in the harder tissue, the shear wave propagates faster, while in the softer tissue it propagates slower. SWE is widely used in the assessment of liver diseases (such as liver fibrosis), breast masses, thyroid nodules, and the musculoskeletal system to help diagnose the disease and monitor the effect of treatment. SWE is becoming an important tool in the field of soft tissue elastography because of its objective, quantitative and highly repeatable advantages over traditional manual palpation.
Historical background Ultrasound elastography (USE) is an imaging technique designed to detect and measure tissue stiffness, first introduced in the 1990s. Over the years, it has undergone significant advancements, allowing for quantitative evaluation of tissue elasticity. The ultrasound elastography gradually developed into four main types: compression sonoelastography, transient elastography, tension elastography, and shear wave elastography. Recent studies have highlighted the growing potential of shear wave elastography (SWE) in assessing a wide range of traumatic and pathological conditions affecting musculoskeletal soft tissues. Promising findings have demonstrated its utility in evaluating the mechanical properties of tendons, muscles, nerves, and ligaments. For tendons, SWE has been used to assess stiffness changes associated with injuries, degeneration, and recovery processes, providing insights into conditions such as tendinopathy. In muscle evaluation, SWE has shown the ability to detect alterations in stiffness related to overuse, trauma, and neuromuscular disorders, offering valuable information for both diagnosis and rehabilitation monitoring. Furthermore, SWE has been increasingly applied to peripheral nerves, aiding in the detection of entrapment neuropathies, nerve injuries, and post-surgical changes. For ligaments, SWE provides a non-invasive method to evaluate their biomechanical integrity following injuries or reconstructive surgeries, facilitating better understanding and management of ligament-related disorders. These advancements reflect the ongoing development of SWE as a reliable tool for non-invasive, quantitative assessment, making it a promising addition to medical imaging and diagnostics.
Basic physics
The basic principle of SWE is to generate shear waves in tissues and detect their speed of propagation, so that the shear modulus could be indirectly derived. To better illustrate the basic physics of SWE, the process of it is divided into 3 steps, Acoustic Radiation Force (ARF) generation, shear wave tracing, and shear modulus estimation.
Acoustic Radiation Force (ARF) generation The shear wave is in essence a transverse wave present in solids (such as human tissues) when the solid is subject to a periodic shear force. The generated shear wave will propagate in a direction perpendicular to the vibration. In shear wave elastography, shear waves are generated using focused acoustic radiation force (ARF) from a linear ultrasound array. The Acoustic radiation force is a non-linear acoustical phenomenon. Basically, particles are subject to a net force in a gradient acoustic field. Although the ARF is widely used to manufacture acoustical tweezers and manipulate particles, it also has the capability to remotely generate displacements in tissue. Here, an ultrasound transducer array emits ultrasound pulses which converges at the focus, serving as the source of shear stress. Then the shear stress and strain waves propagate outwards.
Shear wave tracing
Once shear waves are generated, they induce tissue displacement. Another ultrasound linear array is utilized to real-time image the displacement of tissue. Tissue displacement is calculated using a speckle tracking algorithm. The shear wave speed at each pixel in the imaging plane is calculated using a time-of-flight method. This approach assumes that shear waves travel laterally within the plane. By analyzing signals from adjacent lateral positions, their correlation is used to measure the travel time of the shear wave between these points, allowing the determination of the local wave propagation speed.
Shear modulus estimation
The last step is to reconstruct the elasticity map from the collected signal. This shear wave velocity distribution across the imaging plane is closely associated with the shear modulus (G), which quantifies tissue stiffness and elasticity and is typically expressed in kilopascals. The shear modulus is derived using the equation G = ρ c s 2 {\displaystyle G=\rho c_{s}^{2}} , where ρ represents the tissue density and c s {\displaystyle c_{s}} is the shear wave speed calculated from the previous step. In soft tissue, the density is often approximated using values found in literature or assumed to be similar to water (1 g/cm³). For isotropic materials, the relationship between the shear modulus and Young's modulus can also be expressed as E = 2 G ( 1 + ν ) {\displaystyle E=2G(1+\nu )} where ν {\displaystyle \nu } is the Poisson ratio. Soft tissues under small deformations are typically treated as incompressible ( ν = 0.5 {\displaystyle \nu =0.5} ), simplifying the equation to E = 3 G {\displaystyle E=3G} . As a result, some studies report shear wave velocities or G, while others use E based on these relationships.
Classification of SWE There are several different categories of shear wave elastography, grouped based on their historical evolution and technical advancements.
Transient elastography (TE)
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