Ultrasound-based shear wave elastography (SWE) is emerging as a complementary imaging modality that can add information about the mechanical properties of tissue. In this issue Dirrichs, et al., present a study using SWE to evaluate various tendinopathies and investigate the additive value that SWE provides over other conventional ultrasound imaging modes .
In conventional ultrasound imaging longitudinal waves are used to make the B-mode images. Longitudinal waves are emitted from the piezoelectric elements and are reflected back to the transducer after encountering different tissue interfaces or being scattered by microscopic features. The time-of-flight principle is used to reconstruct the images. In SWE the same longitudinal waves are transmitted but with much longer duration and higher intensity (50–800 µs compared to 1–2 µs for a B-mode image). This long pulse generates an acoustic radiation force where the beam is focused in the tissue and pushes on the tissue in a local sense. When the push ceases and the tissue relaxes, a shear wave is generated that travels perpendicular to the imaging lines of the B-mode image. Pulse-echo methods used to create B-mode images are then used with very high frame rate acquisitions (~2–20 kHz) to obtain data to measure the motion of the shear wave propagation. The velocity of the shear wave, c s , is proportional to the shear modulus of the tissue µ = ρc s 2 where ρ is the mass density which is typically assumed to be that of water (1000 kg/m 3 ). The shear modulus is often referred to in terms of the “stiffness” of the material . The preceding relationship is valid in tissues that are infinite (or large in extent), isotropic, homogeneous, linear, and elastic . Most tissues do not have these characteristics and therefore the “stiffness” is reported in terms of the shear wave velocity (meters/second) which requires few assumptions, or it is reported as a modulus (Pascal).
SWE has been applied in many different applications such as liver, breast, thyroid, musculoskeletal, kidney, and bladder, and others are being added at a rapid pace . A significant advantage of shear wave elastography is that it provides an objective, quantitative characterization of soft tissues. Because ultrasound imaging is increasingly being used in musculoskeletal applications, the increased use of this new modality, SWE, which can be done with the same ultrasound instrument, can add to the diagnostic impression.
The frequency content of the shear waves typically ranges from 50 to 1000 Hz, and their wavelengths are typically greater than the thickness of the tendon (0.5–2.0 cm). This causes the waves to interact with the boundaries of the tendon and become “guided waves.” The relationship of the guided wave velocity to the modulus of the tendon is very complex, especially because the tendon is anisotropic. Orientation of the measurements becomes very important because it is known that waves travel faster along the fibers making up the tendon compared to when the waves propagate across the fibers . Lastly, tendons also exhibit time-dependent behavior, which is a property called viscoelasticity . All of these higher-order mechanical properties have been examined in fundamental studies to investigate the wave propagation in tendons. Even with these complex caveats, measurement of modulus or shear wave velocity under controlled conditions can be used to detect or diagnose tendinopathies as described in multiple studies .
Clinical implementations of SWE in modern ultrasound instruments use the elastic shear wave velocity to report the results and may have some bias associated with them because of the complicated architecture of tendons. But as shown in previously reported studies and in the study in this issue by Dirrichs et al., SWE measurements have clinical value for evaluating tendinopathies . In a fundamental study, Helfenstein-Didier, et al., performed an elegant analysis on data obtained from experiments in the Achilles tendon with varied angle of plantar flexion. They used the shear modulus reported by the clinical scanner and the modulus obtained using phase velocity analysis and found that they were well correlated ( r = 0.84) . This further emphasizes that the measurements reported by clinical instruments have the potential to be used across populations and for monitoring a given patient.
Various methods have been developed using the principles of SWE to make measurements of elastic and viscoelastic material properties in tendons . Because some tendons such as the patellar and Achilles tendon are superficial, external mechanical vibration with a shaker has been used to generate propagating waves at a given frequency or set of frequencies . However, the majority of methods have used acoustic radiation force to generate the waves because of its ease of use in current ultrasound instruments . Most studies in human subjects have examined the Achilles tendon, but other studies have also examined the patellar, quadriceps, and flexor pollicis longus tendons. Some studies have examined regional variation of modulus or speed, repeatability of measurements, and the effects on the measurements with application of pressure and loading . The variation of modulus or speed with age has also been investigated, though with conflicting results . In studies involving the Achilles tendon, it has been shown that the presence of tendinopathy decreases the measured wave velocity or modulus . A few studies have also incorporated examination of the anisotropic characteristics of the Achilles tendon .
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