3D printing refers to the fabrication of a tangible object from a digital file by a 3D printer. Materials are commonly deposited layer-by-layer and then fused to form the final three dimensional object. Additive Manufacturing (AM), Rapid Prototyping (RP), and Additive Fabrication (AF) are synonyms for 3D printing. According to the most recent classification by American Society of Testing and Materials (ASTM), there are seven major types of 3D printing technology. Although these technologies share similarities, they differ in speed, cost, and resolution of the product. Moreover, a variety of materials can be used to fabricate the model.
A handheld printed model derived from Digital Imaging and Communications in Medicine (DICOM) images represents a natural progression from 3D visualization . DICOM image files cannot be used directly for 3D printing; further steps are necessary to make them readable by 3D printers. The purpose of this hands-on course is to convert a set of DICOM files into a 3D printed model through a series of simple steps. Some of the initial post-processing steps may be familiar to the radiologist, as they share common features with 3D visualization tools that are used for image post-processing tasks such as 3D volume rendering.
Most 3D printed models are derived from DICOM images generated from CT scanners. Images can be reconstructed from isotropic voxels with slice thickness less than or equal to 1.25 mm. For 3D printing, image post-processing has both similarities to and substantial differences from methods used by radiologists for 3D visualization. As in 3D visualization, specific software packages enable segmentation of DICOM images using semi-automated and manual segmentation algorithms, allowing the user to demarcate desired parts. The most commonly used tools are thresholding, region growing, and manual sculpting.
The segmented data are then exported in a file format that can be recognized by 3D printers. In essence, this process is conversion of 2D images to 3D triangular facets that compose a mesh surface. To date, the most widely used format is Standard Tessellation Language (denoted by the file extension “STL”). In most cases, the STL output is not optimized for printing and further refinement is required. This refining step may be unfamiliar even to radiologists versed in 3D visualization; Computer Aided Design (CAD) software is used to perform steps such as “wrapping” and “smoothing” to make the model more homogeneous. A key part of 3D printing is choosing the appropriate hardware technology and material. There are several considerations in choosing which technology to use, such as availability, cost, speed, biocompatibility, and most importantly anticipated usage of the product (e.g., a model for surgical planning versus a custom made implant).
Our ultimate goal is to educate participants about the capabilities of 3D printing and, through this hands-on-exercise, provide an initial working knowledge of how it is performed. This session focuses on image post-processing of DICOM image files generated from a CT scan for 3D printing. Participants will learn to segment simple to moderately complicated structures and prepare them for 3D printing. Using this handout as a guide, we will teach participants to use three software packages, Mimics and 3-matic (Materialise, Leuven, Belgium) and Objet Studio (Stratasys Ltd., MN, USA).
Mimics is an image-processing package that interfaces between 2D image data (e.g., CT, MRI) and 3D engineering applications. Mimics is widely used in academics, hospitals, and industry for 3D printing as well as for anatomical measurements, 3D analysis, Finite Element Analysis, patient-specific implant or device design, and surgical planning or simulation. Within Mimics, users can segment any region of interest that can be seen in the medical data and accurately create a 3D model of patient anatomy. 3-matic is a Computer Aided Design (CAD) package dedicated for use with anatomical data. It can perform common CAD operations directly on triangulated STL files. It can also be used to optimize the triangle mesh so the anatomical models can be used in a finite element package. Objet Studio is a software platform directly connected to the 3D printer that supports STL files from any 3D CAD application. The software offers simple “click & build” preparation and print tray editing. It provides easy, accurate job estimation and full job control.
The patient in this workshop, the 3D Printing Hands-on Course at the 2015 annual meeting of the Radiological Society of North America (RSNA), was born with double outlet right ventricle syndrome (DORV), accompanied by a ventricular septal defect (VSD). DORV is a rare congenital heart disease (CHD) involving approximately 1–1.5 % of all CHD and with an incidence of 3–9 per 100,000 live births . It is defined as the type of ventriculo-arterial connection in which both great vessels (aorta and pulmonary artery) arise entirely from the right ventricle . It is commonly accompanied with a VSD that allows blood to be transferred from the left ventricle to the right ventricle and then to the aorta. DORV is classified based on the location of the VSD in relation to the great vessels: subaortic, subpulmonic, doubly committed, and non-committed . The subpulmonic VSD that is of interest to us in this particular case is a variant of DORV where the pulmonary artery receives oxygenated blood from the left ventricle and into the pulmonary circulation whereas non-oxygenated blood from the RV is streamed to the aorta and thus to the systemic circulation (left to right shunt; Fig. 1).
Treatment requires definite surgical correction and the repair approaches differ on the basis of the subtypes and co-existence of other heart abnormalities. Of importance during repair is the location and the size of the VSD (including the involvement of the conal septum) . For DORV with subpulmonic VSD, the preferred approach is the arterial switch operation along with VSD closure. VSD closure requires a large intra-ventricular baffle/patch sutured into place, closing the ventricular septal defect and redirecting left ventricular outflow to the aorta.
In this course and in order to establish the anatomy of the patient’s heart, we will segment the heart and the great vessels along with the VSD. A custom-made patch, based on the patient’s anatomy and the dimensions of the defect, will be designed. The final output will be 3D-printable STL files of the heart with the defect and the patch. Because time in the Hands-On Session is limited, the RSNA computers have the CT DICOM images pre-loaded, and the software has already been launched. In practice, these simple initial steps require additional understanding of the software.
In summary, we will design a patch for the VSD using contrast-enhanced cardiac CT images, in four tasks: A) segmentation of the heart and the great vessels, B) editing of the heart model in CAD software in order to expose the relevant anatomy, C) designing of a VSD patch and D) preparing the 3D printing of the heart model and the VSD patch. Before we begin with Task A, we will introduce the Mimics software environment, and more specific the menus, the toolbars and the windows (Fig. 2) as well as useful shortcuts (Table 1).