First-Order Speckle Statistics for the Detection of Microstructural Anisotropy

Speckle statistics estimation is a useful quantitative ultrasound tool for characterizing tissue microstructure. However, because of their elongated geometry, fibrillar tissue components like collagen may not be described well by speckle statistics models. The purpose of this study is to perform a systematic analysis of the effects of microstructural anisotropy on speckle statistics estimation. We created phantoms made of wool fibers to correlate speckle statistics estimates to elongated scatterer geometries. Phantoms were attached to a calibrated spring to induce fiber alignment by applying a known tension. Ultrasonic beams were steered to 0°, ± 5°, and ± 10°. Nakagami and homodyned K distribution parameters were calculated from each steered acquisition. Applying tension (0- $3~\pm ~0.2$ N) induced alignment in the wool fibers such that speckle statistics estimates exhibited increased dependence on beam steering angle. Whereas an isotropically scattering phantom exhibited 4%, 55%, 25%, and 17% total changes in Nakagami m, Nakagami $\Omega $ , homodyned K $\alpha $ , and homodyned K k metrics (respectively) over all steering angles with reference to the 0° estimate, changes of 35%, 177%, 151%, and 23% were observed in wool fiber phantoms. The same experiment was repeated in the Achilles tendon of a human subject (28%, 190%, 140%, and 53%) and in the cervix of a Rhesus macaque (9%, 76%, 58%, and 11%) to demonstrate sensitivity in vivo. This study demonstrates how speckle statistics parameters can be used to measure the degree of alignment of anisotropic acoustic scatterers separately from spatial density.