Laura Chernak

Tendon motion and strain patterns evaluated with two-dimensional ultrasound elastography

The purpose of this study was to evaluate the use of 2D ultrasound elastography to assess tendon tissue motion and strain under axial loading conditions. Four porcine flexor tendons were cyclically loaded to 4% peak strain using a servo hydraulic test system. An ultrasound transducer was positioned to image a longitudinal cross-section of the tendon during loading. Ultrasound radiofrequency (RF) data were collected at 63 frames per second simultaneously with applied force and crosshead displacement. A grid of nodes was manually positioned on an ultrasound image of the unloaded tendon. Small kernels (2×1 mm) centered at each node were then cross-correlated with search regions centered at corresponding nodal locations in the subsequent frame. Frame-to-frame nodal displacements were defined as the values that maximized the normalized cross-correlations. This process was repeated across all frames in the loading cycle, providing a measurement of the 2D trajectories of tissue motion throughout the loading cycle. The high resolution displacement measures along the RF beam direction were spatially differentiated to estimate the transverse (relative to tendon fibers) tissue strains. The nodal displacements obtained using this method were very repeatable, with average along-fiber trajectories that were highly correlated (average r=0.99) with the prescribed crosshead displacements. The elastography transverse strains were also repeatable and were consistent with average transverse strains estimated via changes in tendon width. The apparent Poissons ratios (0.82-1.64) exceeded the incompressibility limit, but are comparable to values found for tendon in prior experimental and computational studies. The results demonstrate that 2D ultrasound elastography is a promising approach for noninvasively assessing localized tissue motion and strain patterns.

The Use of Ultrasound Elastography to Assess Regional Variations in Tendon Strain

Muscle-tendon loading patterns are complex, with computational models suggesting that both muscle and tendinous tissues undergo highly nonuniform deformation patterns [1]. Hence, musculoskeletal tissue injuries may alter both the morphology and mechanical interactions of muscle and tendon, potentially contributing to secondary pathologies. For example, the presence of residual scar tissue following acute strain injury likely alters force transmission across the muscle-tendon junction and contributes to re-injury risk [2]. While visual ultrasonic methods for assessing tendon strain have provided insight into overall tissue mechanics [3], no prior technique has demonstrated the ability to measure strain distributions in vivo. The purpose of the current study was to evaluate the potential use of ultrasound elastography as a tool for measuring in vivo tendon strain patterns. We achieved this purpose by first developing and assessing an elastography-based approach in an ex vivo experimental setup, and then repeating the analysis on pilot ultrasonic data collected in vivo.Copyright