3D modeling starts with data acquisition. Every modality of 3D medical imaging such as MRI, CT, CTA, MRA, DSA or PET is suitable for 3D printing. The medical dataset is usually stored as a DICOM (Digital Imaging and Communications in Medicine) file. For cerebral vasculature, the best resolution would be obtained by using 3D reconstructions of cerebral angiographs5. If angiography imaging is used, the disadvantage is that only the supply area of one cerebral vessel is depicted at the same time. Therefore, angiography data sets are not suitable for the reconstruction of the circle of Willis. We decided to select DICOM files of contrast enhanced Time of Flight TOF sequences which are part of our standard MRA protocol.
Next, a 3D reconstruction has to be generated using slice imaging data. Applications like 3D Slicer are able to do this automatically. From the 3D reconstruction of the entire dataset, the structure of interest is selected or so called segmented. This step requires medical knowledge to identify the correct structures. Ideally, the surgeon himself will perform the segmentation which allows him to focus on areas of interest and to prepare for the upcoming surgical procedure. Segmentation is already a field for the application of artificial intelligence AI but the freeware available is scarce and manual modifications are still needed today. 3D Slicer is a freeware that allows the conversion of DICOM data into multiplanar and three-dimensional data sets. With the segmentation tool "Segment Editor", the cerebral vessels can be most easily extracted based on the difference in intensity compared to brain-parenchyma in the TOF gadolinium sequence using the "Threshold tool". The "Island tool" results in the removal of non-connected structures within seconds and cleans the mesh. This completes the segmentation and the 3D model can be exported as a Standard Tessellation Language STL file. The next step is dedicated to geometric surface preparation. The mesh must be closed, overlapping surfaces removed and holes filled. These steps can easily be performed in Meshmixer. The Analysis-Inspector tool automatically corrects the mesh and prepares it for printing. Furthermore, Meshmixer allows to select and delete undesired structures in a very quick manner using the selection tool. Geometric surface preparation can be performed entirely by using Blender, but Mesmixer is far more intuitive to use for the 3D beginner. To smooth the mesh, we used the sophisticated Laplacian smoothing modifier in Blender to get the best results. To simplify further processing and save computing power, the number of vertices of the surface mesh must be reduced without losing detailed information. For this purpose we used the "Decimate Modifier" in Blender. With the preset ratio of 1.0, 100% of vertices will be preserved, but in most cases you will be able to decimate the vertices by half without losing relevant detail. Therefore, we recommend to choose a ratio of 0.5 which will efficiently facilitate further operations. After optimizing the mesh, the 3D reconstruction can be exported again as a new stl file. For 3D printing, the final stl file is imported into a printing application. The print software virtually aligns the model on the corresponding print platform and you are able to set the parameters for printing. We are using the freeware Cura ( https://ultimaker.com/software/ultimaker-cura ) for this purpose. Our model was printed in red polylactic acid PLA with an Ultimaker 3 double extruder filament printer. The printer reaches a maximal resolution of 0.06 mm layer thickness. For our purpose, a layer thickness of 0.15 mm was sufficient. As a support filament we used polyvinyl alcohol PVA which is conveniently water-soluble. PLA is a thermoplastic material with a high-melting point of around 250 °C and is suitable for steam sterilization at 121 °C. The total costs of our model of the Circle of Willis are very low, because we exceptionally use freeware. Therefore only the material has to be paid, the price for PLA and supporting filament was below 10$ in our example. There are some one-time costs for the printer, in our case they amounted to approximately 4000$, whereby there are also some much cheaper models available for a few 100$. Alternatively, printing can be outsourced to a print service.