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

Ultrasound elastography is a powerful and increasingly vital tool for non-invasively assessing the mechanical properties of soft tissues crucial for diagnosing various medical conditions. For accurate validation and development of these techniques, reliable tissue-mimicking phantoms are indispensable. Traditionally, Polyvinyl Alcohol or PVA phantoms have been widely used due to their tissue-like acoustic and mechanical properties.

Most PVA phantoms are inherently isotropic. Meaning their mechanical properties are uniform in all directions. However, many crucial biological tissues, such as skeletal muscle exhibit distinct mechanical anisotropy with varying stiffness along different orientations.

Using isotropic phantoms to validate elastography methods for these anisotropic tissues can lead to misinterpretations and limit clinical applicability. To overcome this, there's a significant need for standardized anisotropic tissue-mimicking tools. While previous studies have shown the potential for fabricating anisotropic PVA phantoms, critical technical details, particularly regarding their reproducible fabrication and quality control have often been underspecified.

In this video, we present a visually detailed and highly reproducible protocol for fabricating robust inotropic PVA Phantoms. Let's begin. Before starting, ensure you have all necessary materials and equipment ready, and always prioritize safety by wearing appropriate personal protective equipment like lab coats, gloves, and safety glasses.

For the phantom matrix, we use Polyvinyl Alcohol powder, specifically a high molecular weight grade to ensure appropriate stiffness. As a preservative, we use potassium sorbate to prevent microbial growth. Graphite particles are incorporated as acoustic scatterers, making the phantoms visible and trackable under ultrasound.

And finally, pure water serves as our solvent. Critical to inducing anisotropy are the custom-designed 3D-printed molds. These are specifically designed to hold the phantoms during our fabrication process.

And this custom-built stretching apparatus allows us to apply precise and controlled strain during specific freeze-thaw cycles. First, accurately weigh your dry ingredients, 20 grams of PVA powder, 2 grams of potassium sorbate, and 2 grams of graphite particles. Precision here is key for consistent phantom properties.

Next, measure 176 milliliters of pure water. Add the pure water to a clean beaker. Place the beaker on a magnetic stirrer, and then carefully add the weighed PVA powder to the pure water.

Ensure the stirring bar is added. After the solution is stirred uniformly seal the beaker with aluminum foil. The aluminum foil cover minimizes water evaporation, thereby preventing changes in solution concentration.

And then place the beaker into the water bath. Set the temperature of the water bath to 100 degrees Celsius. This is to enable the water bath to heat up more rapidly with our target temperature set at 80 degrees Celsius.

After sufficient heating, the PVA will have completely dissolved in the solution. At this point, remove the beaker and place it back on the magnetic stirrer to continue stirring slowly, allowing it to cool down to about 40 degrees Celsius. This process takes approximately 15 minutes.

Next, add potassium sorbate and graphite particles to the beaker. Seal the beaker with aluminum foil and keep stirring slowly. After the solution has cooled, carefully pour it into the 3D-printed mold.

Pour slowly and keep the pouring spout close to the mold surface to prevent the formation of new air bubbles, ensuring the mold is evenly filled. Place the mold into a freezer set at minus 20 degrees Celsius and freeze it for 12 hours. This initiates the first freezing step.

After the freezing process is complete, transfer the mold to room temperature and allow it to thaw completely. This process takes approximately 12 hours. Once the initial stretch-free cycles are complete, carefully demold the base phantom.

At this stage, they are still largely isotropic, but possess a stable structure. Now, mount the phantom securely into the stretching apparatus, then apply a precise and controllable uniaxial strain to the phantom, stretching it to 180%of its original length. Uniform stretching along a specific axis is crucial for inducing directional anisotropy as the polymer chains will realign along the stretching direction.

While maintaining this precise stretch, transfer the entire apparatus with the stretched phantom back into the freezer for 12 hours. Following this, move it to the thawing environment for another 12 hours still under continuous tension. The freezing locks in the molecular realignment induced by stretching.

Repeat these stretched freeze-thaw cycles another times. Each cycle further enhances the anisotropic properties resulting in a robust, directionally dependent phantom. After the final stretched cycle, carefully release the tension from the apparatus, then gently remove the fully formed anisotropic PVA phantom.

You will notice that compared to its original preliminary form, there is no obvious change in the phantom's appearance. Yet in reality, significant and reproducible changes have occurred in its microstructure and mechanical properties at this point. After fabrication, it's crucial to validate the induced anisotropy.

We employ two primary methods:ultrasound shear wave imaging and uniaxial tensile testing. For ultrasound shear wave imaging, we position the phantom and acquire shear wave speed measurements. When the shear waves are propagated parallel to the direction of induced anisotropy, we observe a higher shear wave speed, indicating greater stiffness.

Conversely, when the waves are propagated in the perpendicular direction, we measure a significantly lower sheer wave speed, confirming the directional dependence of the phantom's mechanical properties. This quantifiable difference visually demonstrates the successful anisotropy. Also, similar shear wave speed distribution patterns along the depth confirm the phantom's homogeneity.

Uniaxial tensile testing provides a more direct mechanical assessment. We prepare phantom samples cut both parallel and perpendicular to the induced anisotropic axis. As we perform the tensile test, the stress-strain curves clearly illustrate the difference.

The phantom sample in the parallel direction to the anisotropy exhibit a higher Young's modulus, indicating greater stiffness and resistance to deformation in that direction. These results unequivocally confirm the successful fabrication of anisotropic PVA phantoms. In summary, we have presented a detailed, reproducible protocol for fabricating anisotropic Polyvinyl Alcohol phantoms.

By carefully controlling material preparation, freeze-thaw cycles, and crucial stretching steps, researchers can create high quality tissue-mimicking tools with tunable anisotropic properties. These phantoms address a critical gap in elastography research, offering a more physiologically relevant model for validating techniques aimed at anisotropic biological tissues. Thank you for watching.