Manufacturing Abdominal Aorta Hydrogel Tissue-Mimicking Phantoms for Ultrasound Elastography Validation

Here we describe a method to manufacture aneurysmal, aortic tissue-mimicking phantoms for the use in testing ultrasound elastography. The combined use of computer-aided design (CAD) and 3-dimensional (3D) printing techniques produce aortic phantoms with predictable, complex geometries to validate the elastographic imaging algorithms with controlled experiments.

This method can help evaluate the algorithms inherent to the field of vascular ultrasound elasticity imaging, by noninvasively mimicking clinically relevant vessels with complex geometries and spatially varying material properties. The main advantage of this technique is that it establishes a methodology to easily manufacture tissue mimicking phantoms with tuneable geometric parameters as well as spatially varying mechanical properties. This manufacturing technique may advance our ability to diagnose abdominally aortic aneurysms by validating a method for measuring the changing mechanical properties inherent to the disease.

Demonstrating the procedure, will be Luke Cybulski, a technician from my laboratory. To prepare the hydrogels, mix 22.2 grams of PVA-c powder in 200 milliliters of tap water in a glass beaker, and microwave the solution to a boil. Then stir and boil the solution again until all of the PVA is dissolved.

Next, suspend 0.4 grams of calcium carbonate powder in 10 milliliters of water and mix the calcium carbonate suspension thoroughly into the PVA solution as ultrasound scatterers. Then, cover the solution for cooling to room temperature. Make another PVA-c solution with 17.6 grams of PVA-c powder in 100 milliliters of tap water in another glass beaker with boiling and stirring until the solution is clear.

Then add 0.4 grams of calcium carbonate powder in five milliliters of water ultrasound scatterer suspension to the second PVA-c solution. Mix a final PVA-c solution with 193.7 grams of PVA-c powder and 3.5 liters of tap water in a large pot and bring the solution to a boil, removing the pot from the heat once the PVA is fully suspended in solution. Then add 7.4 grams of calcium carbonate powder in 10 milliliters of water suspension to the pot and allow the solution to cool to room temperature.

To assemble the molds, attach one end of approximately 100 millimeter piece of flexible tubing to the injection port of an outer lumen mold and attach a stop cock with syringe connections to the other end. Using deformable wax, align the registration pins of the inner lumen mold and adhere the bulging vessel part of the inner lumen mold to the straight vessel part of the inner lumen mold. In a well ventilated area, apply a spray on flexible rubber coating the aneurysmal end of the inner lumen mold to prevent the hydrogel from dissolving the PVA mold part during the molding process.

With the larger side of the aneurysmal part of the outer mold facing down, fill the bulge with 15 milliliters of 17.6 grams PVA-c solution and place the assembled inner mold parts in the front outer mold part using rubber bands to hold the inner lumen part in place. Then place the mold assembly in a minus 20 degree celsius freezer for 12 hours. Meanwhile, apply a generous amount of deformable wax to the back surface of a printed sample mold and clamp the sample mold to a flat plastic sheet cut to a minimum size of approximately 100 by 60 by 10 millimeters.

Then, fill the space between the mold and the plastic sheet with the 17.6 grams PVA solution and place the mold in the minus 20 degrees celsius freezer. Without letting the solution in the first mold assembly thaw assemble and clamp together the entire vessel mold lining the seams of the outer lumen mold with deformable wax to ensure that the hydrogel does not leak during the injection. Fill a 60 milliliter syringe with the 22.2 gram PVC solution and holding the bifurcation end of the mold up inject the PVA-c solution into the mold assembly.

Allow the newly constructed mold to set for 30 minutes with gentle tapping every 10 minutes to allow any air bubble to rise to the top of the mold. Then, freeze the entire mold assembly for 12 hours. Meanwhile, assemble and clamp another sample mold and flat plastic sheet cut as demonstrated, fill the space between the mold and the plastic sheet with the 22.2 gram PVA solution, and freeze the mold for 12 hours.

At the end of the freezing incubation, thaw both molds for 12 hours at room temperature, followed by four more 24 hour freeze and thaw cycles. After the fifth freeze-thaw cycle, remove the PVA-c testing samples from their molds and trim any excess cryogel from the samples for storage in a sealed container of a five percent by volume bleach water solution at room temperature. Next, remove the PVA-c vessel from the outer lumen mold, carefully separating the straight vessel part of the inner lumen mold from the aneurysmal part.

Then cut the registration spacers from the bifurcated end of the aneurysmal part of the inner lumen mold to expose the printed PVA filament and place the PVA printed part in a water bath at room temperature to dissolve the PVA aneurysmal part. After dissolving and removing the PVA printed part from the inside of the vessel phantom store the phantom in sealed container of a five percent by volume bleach water solution at room temperature. Fill the background mold with approximately 3.3 liters of the 183.7 gram PVA-c solution and fill a sample mold with 183.7 gram PVA-c solution and freeze and thaw the background and sample molds for two 24 hour cycles.

After the second thaw, remove the background sample and background phantom from their molds and store them in a sealed container of fresh five percent by volume bleach water solution at room temperature. For testing of the phantoms and samples, place the vessel and background phantoms into a large water bath and use tubing clamps to attach the larger vessel end to the output of the hemodynamic water pump. Place the vessel phantom in the background phantom and use tubing clamps to attach the bifurcated ends of the phantom to the inlet to the hemodynamic pump.

Place a solid state pressure sensor catheter in the system of the vessel end pump near the inlet of the hemodynamic pump and run the hemodynamic pump such that the pressures of the Walde formations are between a minimum of zero kilopascals and a maximum of 7.5 kilopascals. Then use an ultrasound system and a convex transducer with a center frequency of approximately five megahertz to collect ultrasound images of the background and vessel phantoms in cross section at the location of the maximum vessel diameter recording the pressure data using a digital acquisition system. Here, represented as B-mode images of the vessel mimicking phantoms for the minimum and maximum pressures measured by the catheter are shown.

In this manufactured phantom the ratio of the average pressure normalized strain measured within the posterior quarter of the phantom to the average strain in the anterior quarter was 0.92. In this manufactured phantom, the aneurysmal section of the phantom was made with a 15 percent by mass PVA-c solution and the remainder of the phantom was made using the 10 percent by mass PVA-c and the ratio of the posterior and to anterior strain was determined to be 1.87. Here, the heterogeneous phantom was generated with a 20 percent by mass PVA-c with a posterior to anterior strain ratio of 4.23 and this 25 percent by mass PVA-c heterogeneous phantom demonstrated a posterior to anterior strain of 7.37.

As demonstrated, the final vessel phantoms can be dynamically pressured and are stable under large loads. While attempting this manufacturing process, it's important to maintain the registration of all the mold sections. If the mold parts are not properly fitted thin sections of the vessel wall may result.

The use of polyvinyl alcohol cryogel allows for a wide range of stiffness values to mimic the changing material properties of blood vessels. Following the technique shown in this procedure, other phantom geometries can be created and tested by using similar CAD molds or patient specific molds from contrast enhanced CT images of vessels. The cryogel phantoms developed here were specifically designed for ultrasound imaging, however, they are also compatible with magnetic residence and computer tomogrophy imaging systems and may be used to validate a wide range of imaging techniques.