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

Summary

This protocol describes a robust, reproducible method for fabricating anisotropic polyvinyl alcohol (PVA) phantoms—essential tools for validating ultrasound elastography techniques targeting mechanically anisotropic soft tissues (e.g., skeletal muscle).

Abstract

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.

Introduction

Certain biological soft tissues (e.g., skeletal muscle) exhibit macroscopic mechanical anisotropy, which originates from their aligned microstructural architecture¹. Specifically, connective tissues organize muscle fibers into a hierarchical structure: endomysium surrounds individual fibers, perimysium bundles fibers into fascicles, and epimysium encapsulates the entire muscle, supporting force transmission and nutrient supply². Ultrasound-based elastography techniques, such as shear wave imaging (SWI), are widely used for muscle assessment^3,4. However, the aniso....

Protocol

NOTE: The following steps are illustrated in Figure 1.

1. 3D printing of phantom mold

1. Using 3D modeling software, design the phantom geometry to incorporate two holding rings (one at each end) for maintaining stretch, with an effective central region of 35 mm × 60 mm × 5 mm (length × width × height) to ensure uniform anisotropy induction.
2. Print the mold using a 3D printer.
   NOTE: The holding rings prevent slippage during stretching, ensuring consistent strain distribution. All 3D printing files (.stl) are provid....

Results

Figure 2A presents a schematic of the complete fabrication process, including PVA solution preparation, molding, initial stretch-free FTC, demolding, and subsequent stretched FTCs. Using the proposed protocol, we fabricated two anisotropic PVA phantoms for SWI and uniaxial tensile testing, respectively. SWI was performed using a research ultrasound system equipped with an L7-4 probe (center frequency: 5.2 MHz), employing supersonic mode (3 foci, 100 µs push)⁴ and cohe.......

Discussion

The success of this protocol relies on achieving complete, uniform PVA dissolution and maintaining sustained stretching during FTCs—two critical steps for inducing stable, homogeneous mechanical anisotropy. The recipe used herein (i.e., PVA concentration, FTC number, and applied tensile strain) was selected to fabricate anisotropic PVA phantoms with stiffness within the physiological range of muscle tissue. Notably, the stiffness and anisotropy ratio of the PVA phantom can be readily tuned by adjusting these recipe param.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Analytical balance	OHAUS	AX224	Analytical balance with internal calibration, capacity 220 g, readability 0.1 mg
Bidirectional, opposite-thread linear stage	Taobao	N/A	Travel range 250 mm for each thread, used for constructing the custom-made stretching device
FDM 3D Printer	Bambu Lab	X1 Carbon	Used for printing the phantom mold, as well as the phantom?holding mounts for the custom-made stretching device
FreeCAD	FreeCAD Open-Source Community	N/A	Version 0.22.0, used for designing the phantom mold and the phantom-holding mounts of the custom-made stretching device
Graphite particles	MREDA	M151649	Graphite particles, 99.95% purity, 2000 mesh(~6.5 um), used as acoustic scatterers, 1% by weight
Magnetic stir bar	Corning	401435	PTFE-coated magnetic stir bar, 1 x 3/8 inch, for stirring PVA solution
Magnetic stirrer	IKA	25005927	RCT basic safety control magnetic stirrer with heating function
Polyvinyl alcohol	MACKLIN	P81584	Polyvinyl alcohol, Mw 89000-98000, 95% hydrolyzed, used as phantom matrix, 10% by weight
Potassium sorbate	Sigma-Aldrich	85520	Potassium sorbate,≥99.0% purity, used as preservative, 1% by weight
Rotation stage	Newport	URS50BPP	Used for rotating the ultrasound probe in shear wave imaging test for multipe probe angles (0°:2°:90°)
Single column universal testing machine	Instron	Model 5944	Used for uniaxial tensile testing, 50 N force sensor
Research Ultrasound System	Verasonics	Vantage 256	Used for conducting shear wave imaging test, L7-4 probe (center frequency: 5.2 MHz)
Water bath	LICHEN	HH-2	Digital display constant temperature water bath, 2 holes single row, for heating and dissolving PVA solution