Anisotropic Polyvinyl Alcohol Phantom Fabrication for Ultrasound Elastography: Procedure and Quality Controls

Polyvinyl alcohol (PVA) phantoms are typically isotropic and are widely used to validate ultrasound elastography techniques for soft-tissue evaluation. However, some biological soft tissues (e.g., skeletal muscle) exhibit distinct mechanical anisotropy, which necessitates the use of anisotropic PVA phantoms for the rigorous testing of elastography methods targeting these tissues. While prior studies have noted that in-house anisotropic PVA phantoms can be fabricated via stretch-integrated freeze/thaw cycles (FTCs), critical technical details (e.g., fabrication process, quality control) remain insufficiently documented. This work presents a visually detailed, reproducible protocol for fabricating anisotropic PVA phantoms, focusing on key materials, stepwise processes, and quality controls to induce stable, uniform anisotropy. Key materials include PVA as the phantom matrix, potassium sorbate as a preservative, graphite particles as acoustic scatterers, and pure water (or deionized water) as the solvent. The fabrication process comprises three core stages: 1) Preparation of a homogeneous PVA-based solution through controlled thermal conditions to ensure complete PVA dissolution; 2) Solidification via FTCs: the cooled solution is poured into 3D-printed molds, followed by stretch-free FTCs to form a preliminary structure; 3) Inducing anisotropy via stretched FTCs: additional FTCs are performed under controlled stretching to induce directional anisotropy. Quality-control measures (e.g., avoiding air bubbles during PVA dissolution) are described in detail. After fabrication, ultrasound shear wave imaging (SWI) and uniaxial tensile testing are employed to confirm the phantom's mechanical anisotropy. This paper provides a standardized approach for fabricating anisotropic tissue-mimicking phantoms to validate ultrasound elastography techniques with enhanced accuracy and consistency.