Challenges and limitations of patient-specific vascular phantom fabrication using 3D Polyjet printing

Abstract

Additive manufacturing (3D printing) technology offers a great opportunity towards development of patient-specific vascular anatomic models, for medical device testing and physiological condition evaluation. However, the development process is not yet well established and there are various limitations depending on the printing materials, the technology and the printer resolution. Patient-specific neuro-vascular anatomy was acquired from computed tomography angiography and rotational digital subtraction angiography (DSA). The volumes were imported into a Vitrea 3D workstation (Vital Images Inc.) and the vascular lumen of various vessels and pathologies were segmented using a "marching cubes" algorithm. The results were exported as Stereo Lithographic (STL) files and were further processed by smoothing, trimming, and wall extrusion (to add a custom wall to the model). The models were printed using a Polyjet printer, Eden 260V (Objet-Stratasys). To verify the phantom geometry accuracy, the phantom was reimaged using rotational DSA, and the new data was compared with the initial patient data. The most challenging part of the phantom manufacturing was removal of support material. This aspect could be a serious hurdle in building very tortuous phantoms or small vessels. The accuracy of the printed models was very good: distance analysis showed average differences of 120 μm between the patient and the phantom reconstructed volume dimensions. Most errors were due to residual support material left in the lumen of the phantom. Despite the post-printing challenges experienced during the support cleaning, this technology could be a tremendous benefit to medical research such as in device development and testing.

Keywords: 3D printing; CT; Cone-Beam CT; Vascular phantoms; additive manufacturing; patient specific phantoms.

Figure 1

Flow diagram showing the steps performed in order to obtain a STL file for a simple phantom of a right internal carotid needed for the 3D printer.

Figure 2

Flow diagram showing the steps performed in order to obtain a STL file for a complex phantom needed for the 3D printer (ICA-internal carotid, BA- Basilar Artery, MCA- Middle Cerebral Artery, ACA- Anterior Communicating Artery)

Figure 3

Inlet and outlet design: We show the manipulation of the outlets corresponding to the right Middle Cerebral Artery Region (dotted circle). Top row shows creation of closed loops to reduce the number of outlets. Bottom row shows an extension example of one of the outlet (black arrow on last view top row) for practical reasons. The model orientation in the lower row was changed to give a better view of the intermediate steps required to extend this particular outlet (black arrow).

Figure 4

Extrusion and errors associated with this process. Mesh errors are shown for the area outlined by the dashed circle. In the solid model the errors location is indicated by the black arrows, however in this view the errors are not visible. In the x-ray view of the model the errors due to the mesh self-intersection after extrusion are automatically identified and shown as blue curves.

Figure 5

Support addition details. Mesh errors (black arrows) are not recognized by the mesh manipulating software. In the x-ray view it can be seen that the outer mesh of the phantom is intersecting the outer mesh of the support structure. The STL for the printer requires a continuous surface (if you start from one point one could reach any other point on the model, including vessel lumen). In the final view, at the location of the white arrows, the redundant surfaces were fixed, the outer wall of the mesh is merged with the support wall.

Figure 6

Patient specific vascular phantoms: (a) carotid phantom using elastic material, (b) carotid phantom using hard transparent material, (c) carotid syphon aneurysm phantom (hard material), (d) internal carotid artery aneurysm phantom (hard material), (e) basilar aneurysm phantom embedded as a hollow structure in a hard material.

Figure 7

Complex phantom: (a) phantom embedded in support material indicated with white arrows, (b) phantom after the material has been removed, (c) phantom in a flow loop, black arrow shows location of a blood clot.

Figure 8

DSA runs of complex neuro-phantom using a bi-plane C-arm angiographic system. All small vessels are patent.

Figure 9

Clot retrieving procedure: Large black solid arrows indicate the steps in which the procedure was performed.

Figure 10

Accuracy of phantom fabrication for a carotid syphon aneurysm: (a) Overlain images of reference model (yellow) and reconstructed comparison model (red); (b) Comparison of differences between two models as shown on the reconstructed model with light green representing zero difference; (c) Histogram of differences between reference and reconstructed models (abscissa in mm).

Figure 11

Accuracy of phantom fabrication for an internal carotid artery aneurysm: (a) Overlain images of reference model (yellow) and reconstructed comparison model (red); (b) Comparison of differences between two models as shown on the reconstructed model with yellow representing zero difference; (c) Histogram of differences between reference and reconstructed models (abscissa in mm).