A Primer on 3D Printing

rfs 3d printing radiology

The potential for three-dimensional printing (3DP) models in medicine is virtually infinite. Radiologists are in a unique position to be the leaders in an emerging field, owing to our combined anatomical and cross-sectional imaging expertise.

3DP also affords us an opportunity for cross-collaboration with other subspecialists, from interventional radiology to cardiology to surgery. You may be wondering what the buzz about 3DP is all about. Let’s start with some basics.

 

What is 3DP?

3D constructs are fabricated from volumetric image data, initially from DICOM images. After the raw data is acquired, it is transferred to free or commercial post-processing software that allows for reconstruction of the volumetric data into a printable 3D model file format (i.e., STL, Standard Tessellation Language). The key step here is segmentation, which involves drawing regions of interest on target tissue to then create an STL model. These models are composed of facets, or triangular shapes. More numerous and smaller facets yield a finer resolution.

What are the applications of 3DP?

Medical applications of 3DP are broad and encompass aspects of patient care, device design, and medical education.

  • Pre-interventional / pre-surgical planning by promoting understanding of patient anatomy and improved intra-operative accuracy. The 3D models can also be used for simulation. Common examples include repair of congenital heart defects and craniofacial reconstruction.
  • Patient and trainee education allows for individualized anatomic study and explanation of procedural steps in consultation, in the clinic, or in the reading room.
  • Implant and prosthesis fabrication is beneficial in order to custom-tailor a device to an individual patient’s anatomic needs. One implementation is a rapid-prototype biocompatible prosthesis such as artificial bone, sparing the patient the morbidity associated with a bone graft.
  • Tissue engineering results from the formation of a 3D scaffold composed of biodegradable materials onto which cells can adhere, migrate, and proliferate.

How do I 3DP?

The variability in 3D models comes in the methodology utilized to construct such models and the materials from which they are formed. Here’s a breakdown of the four most common techniques.

  • Stereolithography, the first additive manufacturing process, remains the most popular rapid prototyping technology available. Substrates include liquid polymers, composites, ceramics, and hydrogels that harden by crosslinking when exposed to UV light. The advantage of stereolithography lies is its resolution and accuracy, but at the cost of a high-temperature post-processing step.
  • Selective Laser Sintering (SLS) produces 3D shapes from plastic, ceramic, polymer and metal powders via a robust laser. The laser is passed over a thin layer of powder, which causes the powder particles to fuse into a rigid layer. Individual layers are stacked to fabricate a 3D shape. Hydroxyapatite can be used to create bone scaffolds. Compared to stereolithography, SLS results in porous models, but does not necessitate a high temperature post-processing step. SLS models are durable but costly, owing to the materials used.
  • Fused Deposition Modeling (FDM), also known as material extrusion, is simple, rapid, and less expensive. In this technique, a heat-controlled nozzle extrudes a semisolid material onto a base, where the material hardens as it cools. The base is mobile so that the nozzle position moves and subsequent layers can be deposited. FDM can use biodegradable polymers that harden rapidly after extrusion, owing to their high melting points. One advantage of FDM is that the resultant scaffolds are highly uniform and can form complex geometric structures, but with lower resolution.
  • Inkjet Printing (IJP) uses hardware analogous to off-the-shelf home inkjet printers, some of which can be converted to perform this task. The printer can form a model in one of two ways, indirect or direct: droplets of a binder solution are placed on a bed of powder resulting in congealing or the printer directly places curable droplets to conform to the model design. This technique relies on piezoelectric crystals to deposit the droplets. After mechanical deformation of the crystals, the material is deposited. This process is repeated layer-by-layer until a complete model is formed. The excess powder is washed away. A clear advantage of IJPs is the capability of color printing.

In summary, 3DP promises to revolutionize individualized medicine through its ability to rapidly and accurately reconstruct anatomic models from volumetric data that was initially acquired from cross-sectional imaging. These models can be used for a variety of applications, both relevant to direct patient care and trainee education in anticipation of invasive procedures. For more information, check out some of the resources listed below. Happy printing!

Sources Referenced:

1. Sheth R, Zhang Y, Khademhosseini A, Balesh E, Hirsch J, Oklu R. Three-Dimensional Printing: An Enabling Technology for Interventional Radiology. J Vasc Interv Radiol. 2016. 27(6):859-865. Available at bit.ly/2nzJzvT.

2. Mitsouras D, et al. Medical 3D Printing for the Radiologist. Radiographics. 2015. 35:1965-1988. Available at bit.ly/2oeUyOl.

3. Trace A, Ortiz D, Deal A, Retrouvey M, Elzie C, Goodmurphy C, Morey J, Hawkins C.M. Radiology’s Emerging Role in 3-D Printing Applications in Health Care. J Am Coll Radiology. 2016;13:856-862. Available at bit.ly/2nE7f3O.


 By Courtney Tomblinson, MD (@cmtomblinson), Rahmi Oklu, MD (@rahmioklu), Mayo Clinic Arizona

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