Shear Wave Elastography Applied for the Investigation of Tendon Material Properties.

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 (1). 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 timeof-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. Pulseecho 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, cs, is proportional to the shear modulus of the tissue � = ρcs 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 (2). The preceding relationship is valid in tissues that are infinite (or large in extent), isotropic, homogeneous, linear, and elastic (3). 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 (4‐9). 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 (10). Lastly, tendons also exhibit time-dependent behavior, which is a property called viscoelasticity (10‐13). All of these higherorder 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