Phantom models are important tools, but it is challenging to make them anatomically accurate. This protocol uses CT and ultrasound to create a patient-specific phantom that includes a tumor. We use complex heterogeneous models and 3D printing to achieve a high degree of anatomical realism.
Patient-specific brain phantoms are highly useful as they allow for surgical planning and clinical training, and they allow surgeons to practice new techniques and to test new instruments and hardware. After obtaining preoperative contrast-enhanced T1-weighted MRI and volumetric CT data, load the images into a 3D modeling software program and use the plain cut tool to split the brain segmentation into two hemispheres. Save each hemisphere as separate right and left brain STL files and import the files into an appropriate computer-aided design software program.
In the mesh workspace, use the reduce function to decrease the size of each model as much as possible so that it can be handled by the program while still retaining all the necessary detail. In the solid workspace, use the mesh to BRep tool to convert the imported mesh to a body that can be manipulated. Click create and box to draw a box around the tumor, rotating the view to ensure that the box completely encloses the tumor on all sides.
In the operation dropdown menu, designate the box as a new body. Click the modify tab and use the combine tool to cut the tumor from the box to create a box with a hollow shape of the tumor inside of it. To create planes through the box in the places that the mold will be cut, click construct and midplane to create a plane through the center of the box.
Right-click on the midplane and select offset plane to position the plane more precisely. Under the modify tab, use the split body function to split the mold along the created planes and click create sketch and center diameter circle to add small circular rivets to the face of each piece of the mold. Right-click to extrude the circles outward a few millimeters on one face and extrude them inward on the corresponding face.
Then save each piece of the mold as a separate STL file. To print the 3D brain and tumor molds, for faster printing, select a large layer height of about 0.2 millimeters and a low infill value of 20%in the 3D printing software. If the molds are positioned appropriately on the base plate, support materials should not be necessary.
Print the molds using a rigid material such as polylactic acid. Before printing the skull mold, select add support in the software to use PVA as the support material. To prepare the PVA for the models, heat 1, 800 grams of deionized water to 90 degrees Celsius in a two liter conical flask and measure out 200 grams of PVA powder.
Position an electronic stirrer in the flask, taking care that it doesn't touch the bottom or sides. Set the speed to 1, 500 revolutions per minute. Add the PVA powder to the flask over a period of about 30 minutes.
When all of the PVA has been added, stir the solution for an additional 90 minutes. When a sticky gel has been obtained, remove the flask from the water bath and cover the top with plastic wrap to prevent the formation of a skin on top of the material. Once cool, the PVA will appear transparent.
Pour it into a beaker. Tiny white crystals may be observed, but any bubbles on the surface must be gently scraped off. Then add 0.5%potassium sorbate to the PVA as a preservative and thoroughly stir the solution.
To prepare the phantoms, measure out enough PVA to fill the tumor mold into a beaker and pour the rest into a separate beaker. To prepare the PVA for the tumor, add 1%glass microspheres for ultrasound contrast and 5%barium sulfate for x-ray contrast to the first beaker. Stir the resulting solution by hand.
Sonicate the beaker to ensure homogenous mixing of the additives and allow the solution to cool for about 10 minutes, removing any bubbles by scraping as necessary. Secure the tumor mold together and pour the PVA through the hole in the top of the mold. Allow the PVA to rest for a few minutes to allow any bubbles formed in the pouring process to escape through the hole, before placing the mold into a minus 20 degrees Celsius freezer.
After six hours, thaw the mold for six hours at room temperature. Repeat the 12-hour freeze-thaw cycle, then carefully remove the mold from the model. Create the cerebellum and brain hemisphere models in the appropriate molds as just demonstrated.
After the second freeze-thaw cycle for each phantom model piece, carefully remove the models from the molds and place the cerebellum tumor phantom on the spike at the base of the skull bottom model. Place the two brain hemispheres into the uppermost part of the cerebellum tumor piece and place the four dowels in each space on the skull bottom model. Then place the skull top model on top.
If required, the model may be maneuvered into the desired position to simulate its intraoperative use in surgery. Following the protocol as demonstrated, an anatomically realistic phantom consisting of a patient-specific skull, brain, and tumor can be fabricated. The relevant anatomical structures for the phantom are segmented using patient MRI and CT data.
Meshes can then be created for each piece of the model to manufacture the 3D molds as demonstrated. The cerebellum mold is the most complex to design and assemble. The skull is the most difficult part to print as it requires support material.
The completed phantom offers a realistic model of the patient's skull, brain, and tumor. The two brain hemispheres are produced separately and have a realistic appearance featuring the gyri and sulci of the brain. The cerebellum fits comfortably into the base of the printed skull and the brain hemispheres sit on top of this structure.
The tumor is easily visible in the cerebellum as the extra contrast added to the tumor results in an off-white color that separates it from the surrounding material to which it is attached. The tumor can be visualized in the phantom with both CT and ultrasound imaging. The patient intraoperative ultrasound data can then be used to compare the phantom images to the real patient images.
When creating the molds, pay particular attention to the position of the cuts to ensure that the phantom will be able to be removed when it has solidified. By creating a patient-specific brain phantom, we've been able to test a novel neural navigation system that can also be used to train neurosurgeons in the use of intraoperative ultrasound.
