Evaluation of Transversely Isotropic Muscle Phantoms Using Ultrasonic Rotational and Rocking Shear Wave Elastography...

Muscle tissue stiffness is closely linked to muscle physiological and pathological states and can be assessed using shear wave elastography (SWE), a noninvasive ultrasound-based technique. However, SWE assumes isotropic, linear elastic material properties, limiting its clinical adoption for skeletal muscles, which are often modeled as transversely isotropic (TI) materials. Full characterization of TI materials requires assessment of both shear horizontal (SH) and shear vertical (SV) wave propagation modes, yet research on SV mode quantification remains limited. This study compares four TI phantoms containing different fiber analogs-polyethylene (PE), nylon, polyester, and ex-vivo tissue-embedded in a polyvinyl alcohol matrix. Using ultrasonic rotational and rocking SWE imaging, this study assessed shear wave speeds of the SH and SV modes, signal quality, shear anisotropy, and the effect of push width (F#). Results showed high variability across phantoms, with the TI phantom made of PE fibers exhibiting the highest shear anisotropy, followed by the TI phantom with ex-vivo tissue. The TI phantom with nylon fibers displayed shear anisotropy comparable to resting muscle, while the TI phantom with polyester fibers exhibited nearly isotropic behavior. The SH mode was quantified in all phantoms at a 0° rocking angle, whereas the SV mode was observed only in the TI phantoms with ex-vivo tissue and nylon fibers (F# 1.2 and 2.5). Push width influenced SV mode visibility but not shear wave speed quantification. Despite practical limitations of phantom fabrication and SV signal quality challenges, this study establishes a foundation for optimizing TI phantom properties and imaging conditions to enable SV mode detection, advancing muscle tissue characterization using SWE.Clinical Relevance- This study developed and evaluated transversely isotropic phantoms that mimic muscle tissues to advance shear wave elastography (SWE) for anisotropic tissues. This advancement could enable greater clinical use of SWE in muscle tissue assessment, enhancing the understanding of muscle function and aiding the study of muscle physiology and pathology.