Effect of collagen on the anisotropy of quasi-longitudinal mode ultrasonic velocity in fibrous soft tissues: a comparison of fixed tendon and fixed myocardium.

The widespread use of echocardiography has generated considerable interest in the ultrasonic properties of myocardial collagen. This study was designed to investigate the effect of collagen on the propagation of ultrasound by measuring the anisotropy of ultrasonic velocity through formalin fixed specimens of bovine Achilles tendon. Tendon was chosen for this study because it possesses a high content of collagen and a well-defined unidirectional arrangement of fibers. Ultrasonic velocity data were acquired from nine samples of fixed tendon that were each insonified at multiple angles relative to the fibers in 2 degrees increments for a full 360 degrees. Analysis of the data revealed a substantial angular dependence of velocity qualitatively similar to that reported for formalin fixed specimens of normal human myocardium, but approximately 17 times larger in magnitude. Together with measured values of density, these results were used to compute the elastic stiffness coefficients C11 (corresponding to propagation perpendicular to the fibers) and C33 (corresponding to propagation parallel to the fibers) of fixed tendon, yielding 3.08 and 4.51 GPa, respectively.