Abstract
As a graduate student and later a postdoctoral fellow in Dr. Evans’s lab, I was trying to find a technique to assess the mechanical properties of soft tissue in vivo. I experimented with multiple methods, including the use of torque and tissue displacement, a MTS system, and Myotonometer. It was at this time that Professor Evans introduced me to ultrasound elastography, which we experimented with to evaluate rotator cuff muscle stiffness in cadavers and humans. After ending my tenure in his lab, I was able to take the early concepts and skills that I learned and continued expanding my research as an educator/researcher in Boston. It was a pleasure to be invited by my mentor and colleague Dr. Kevin Evans to share my thoughts on the use of ultrasound elastography in JDMS.
Assessing soft tissue stiffness is an essential part of clinical examinations. A well-trained clinician can use his or her palpation skills to gather sound clinical information; however, palpation can be very subjective. In addition, palpation only provides qualitative information about the tissue being examined, limiting its use in biomechanical research. Ultrasound elastography is a technique that has the ability to provide objective quantitative information about tissue mechanical properties.
There are two approaches to elastography. First, strain elastography (SE), which estimates tissue stiffness by measuring the strain on the tissue using the transducer to compress the tissue (mimicking palpation). The relative strain ratio of the tissue is then calculated relative to acoustic coupler. 1 This “push-push” technique was the process Dr. Evans and I used in our early work.2,3 Using the SE technique has allowed us and other researchers to quantify the strain ratios of the shoulder muscles in both cadavers and healthy shoulders. However, this approach has certain limitations. For example, depending on the mechanical properties of the acoustic coupler, the stiffness values of the target tissue may differ, with no standard values to compare. Furthermore, achieving consistent manual compression and velocity of compression of the transducer can be a challenge. Lastly, muscle tissue is anisotropic, meaning that the orientation of the transducer relative to the muscle fibers will affect the tissue elasticity. This also means that we need to quantify the muscle tissue stiffness in different transducer orientations. The utility of these values is limited for the aforementioned reasons.
The second widely used elastography technique uses shear waves. Shear wave elastography (SWE) uses the ultrasound propagation velocity as a measure of tissue stiffness (sound travels faster in stiff materials). Specifically, the ultrasound transducer sends a low frequency ultrasound pulse called acoustic radiation force impulse (ARFI) in the tissue of interest. This ARFI can make tissue vibrate in a plane perpendicular to the direction of ARFI. In other words, as the transducer propagates acoustic force on the tissue in a perpendicular orientation, the resulting absorbed energy moves horizontally across the tissue. The vibration of that tissue sends a reflected signal back, which is used to calculate stiffness. This velocity can also be converted to strain modulus values, however, this conversion makes certain assumptions for calculating tissue strain modulus. 1 It is important to note that the FDA has not yet approved quantification of shear modulus, though it has approved the use of velocity of propagation.
Another technical issue to consider with the use of elastography is that the strength of the ARFI can affect the stiffness values and also determines the optimal tissue depth for which stiffness can be calculated. Before widespread use of elastography can be recommended, we need consensus on the technical aspects of acquiring an elastogram. This consensus should include factors that might affect the calculation of tissue stiffness. These factors may include but are not limited to patient and transducer positioning, optimal and reproducible locations for making stiffness measurements, condition of the muscle (at rest/contracted/stretched) during elastogram and the specific parameters used (ultrasound machine focus/depth of penetration/wavelength, etc), and return on investment selection.
Some have suggested that commercial vendors may need to have some mandate from NEMA to provide sonographers, sonologists, and researchers a comparable platform to share their elastographic data. Continued publications have provided elastographic values for breast and liver4–6 tissue; however, these velocities and/or kilopascals (kPa) measures remain tied to the equipment used and the conditions under which they were acquired. This may explain the reluctance for some practices to adopt SE or SWE as a diagnostic tool in their clinics and laboratories. 7 There is a need for a common diagnostic platform that can enable clinicians and researchers to share elastographic data given that the American College of Radiology (ACR) has ranked SWE as “usually appropriate”; however, the need to assure reproducibility of these data is important. 8
With all new technology comes a certain level of challenge, which makes the work that much more exciting. I hope to continue my work and collaboration with Dr. Evans as we share our experiences with this new technology.
