Local assessment of shear anisotropy in fiber network phantoms of the levator ani muscle using rotational shear wave elastography

OBJECTIVE: Childbirth is a major cause of levator ani muscle (LAM) injury, affecting over 10% of women after vaginal delivery. Progress in prevention, diagnosis, and treatment is limited by poor understanding of LAM mechanical properties particularly muscle elasticity, which is closely linked to injury mechanisms and can be measured noninvasively using shear wave elastography (SWE). Conventional SWE assumes large, isotropic tissues, making it unsuitable for the small, anisotropic, and complex LAM. This study aimed to develop fiber-network LAM phantoms and to investigate rotational SWE imaging to assess local, direction-dependent shear anisotropy properties of the LAM. Approach: Six LAM phantoms with varying fiber type, fiber density, freeze-thaw cycles, and fiber networks (puborectalis alone or combined puborectalis-pubococcygeus) were constructed by embedding synthetic fibers within a polyvinyl alcohol matrix. A custom rotational SWE imaging setup with a Verasonics V1 system was used. Shear wave velocities were estimated from axial velocity maps using a semi-automatic Radon sum algorithm and fitted to an elliptical model. Based on the theory of shear wave propagation in transversely isotropic (TI) materials, this model enabled the estimation of shear anisotropy metrics, including fiber direction, shear anisotropy, longitudinal and transverse shear moduli, and TI profile fit quality. Main results: Rotational SWE imaging differentiated puborectalis LAM phantoms with shear anisotropy (1.09-4.95), longitudinal shear moduli (8.67-60.41 kPa), and transverse shear moduli (3.09-12.26 kPa), reflecting distinct biomechanical and age-related LAM properties. Different muscle network configurations of the LAM were also distinguished across three probe positions, and local fiber curvature was detected. Significance: This study demonstrates that rotational SWE imaging can characterize local shear anisotropy and fiber-network architecture in anatomically informed LAM phantoms, beyond what can be obtained from conventional SWE. These findings provide a foundation for future studies investigating whether rotational SWE imaging can improve the in vivo assessment of the LAM.