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3-D Print Advance Creates More Accurate Medical Models

3-D print foot model and CT image

3-D printed foot model, left, and its cross section CT image (Steven Keating and Ahmed Hosny, Wyss Institute, Harvard University)

30 May 2018. A new three-dimensional modeling and printing process can produce more accurate models of human anatomy in less time and with less effort than current techniques. A team from the Wyss Institute, a biomedical engineering center at Harvard University and the Media Lab at Massachusetts Institute of Technology describe the process in yesterday’s issue of the journal 3D Printing and Additive Manufacturing.

Researchers from the labs of Wyss Institute engineering professor James Weaver and Media Lab computational design professor Neri Oxman are seeking more detailed and accurate 3-D physical models from medical imaging data. With today’s techniques, say the authors, detailed images from magnetic resonance imaging (MRI) and computed tomography (CT) scans cannot be easily modeled on 3-D printers, requiring compromises in the amount of detail produced, or taking extended periods of time.

While using different imaging technologies, both MRI and CT scans produce highly detailed images of the anatomy, providing cross-sectional views that in principle can be represented in 3-D models. Many of these images, while detailed, also have many irregular shapes and do not provide well-defined borders between objects. Most of today’s 3-D printing techniques, say the authors, rely on thresholding, a process for partitioning images into foreground and background sections, that isolates and converts objects from grayscale pixels into solid black or white.

Current processes also use file formats, such as STereo Lithography or *.stl, to create mesh frameworks that convert quickly and easily into 3-D printer commands. While these methods are fast, they can result in models that exaggerate or underestimate the size of objects in images, or wash out important details.

The researchers, which include radiologists and other medical practitioners in the U.S. and Germany, offer an alternative process for interpreting and translating medical images for 3-D printing. Their new approach uses bitmaps, where each pixel from a grayscale image is assigned a value representing a mix of black and white pixels, with the more black content in the mix, the darker the shade of gray. This process makes it possible to convert volumetric data for visualization, such as those produced by highly detailed MRI and CT scans, into commands for producing highly detailed 3-D models.

This process, say the authors, enables modelers to bypass creation of mesh diagrams and extraction of isosurfaces — a representation of points with equal values in a 3-D data distribution — required with current 3-D printing methods, making the new process faster. The bitmap-generated commands can also drive printing with multiple types of materials. As a result, 3-D printing with this process can represent properties such as stiffness in the models that would not be possible using current techniques.

The team demonstrated their process with 3-D printing of brain tumor, heart, and foot models from MRI and CT scans, and their volumetric data sets. The heart model also used different materials to show variations in stiffness in heart valves.

“Our approach not only allows for high levels of detail to be preserved and printed into medical models, but it also saves a tremendous amount of time and money,” notes Weaver in a Wyss Institute statement. “Manually segmenting a CT scan of a healthy human foot, with all its internal bone structure, bone marrow, tendons, muscles, soft tissue, and skin, for example, can take more than 30 hours, even by a trained professional. We were able to do it in less than an hour.”

The project is more than an academic exercise to Media Lab researcher and co-author Steven Keating, who was diagnosed with a brain tumor while a graduate student. Keating tried to make a 3-D printed model of his tumor and discovered the limitations of current 3-D printing technology. His efforts to create a better modeling system for doctors and patients, says Wyss Institute, energized the project, with his brain tumor as one of its first models.

The process still needs to overcome other limitations before becoming routine practice. CT and MRI scans are usually compressed when stored in today’s medical records systems, and will need to revert back to their raw states for this process. In addition, 3-D bitmap printing software will need to be upgraded to produce the kind of detailed models needed. Nonetheless, MIT filed a patent application for the process.

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