A Numerical-model-based Optimization Strategy for Design and Fabrication of Transversely Isotropic Tissue-mimicking Phantoms

Polyvinyl alcohol (PVA) has been reported for fabrication of transversely isotropic (TI) bulk phantoms via stretch-persistent physical crosslink under freeze/thaw cycles (FTCs) [1]. Such phantom can mimic TI soft tissues, e.g., muscles, for development of ultrasound elasticity imaging methods. The tensile strength and stretch direction applied to the phantom determine the degree of transverse isotropy and the axis of material symmetry, respectively. However, keeping a PVA cuboid with slippery surfaces under a prescribed tensile strain (ε) by a simple clamp throughout FTCs is technically difficult and often leads to a spatially inhomogeneous stretch, thus hardly achieving the expected anisotropy throughout the phantom. This study therefore proposed a strategy that used finite element simulation (FES) to design an optimized phantom geometry to achieve pre-defined transverse isotropy. An exemplary phantom (10% PVA, one stretch-free FTC, followed by two stretching FTCs with 80% ε), was fabricated using the optimized geometry. Ultrasound shear wave imaging (SWI) was used to verify the mechanical anisotropy of the phantom by assessing shear wave speeds (
$V_{g}$
) along multiple propagation directions (θ
g
=0°:2°:90°) in the stretching plane. A uniform
$V_{g}$
distribution in the region of interest was observed across the imaging depth regardless of the propagation direction. The corresponding shear wave speed contour map showed an ellipsoidal shape, confirming the TI mechanical property of the phantom fabricated using our proposed strategy.