Producing MRI Components with FDM
Magnetic resonance imaging (MRI) is a non–invasive medical imaging technology that produces highly detailed cross–sectional images of internal body structures including organs, soft tissues and bones. Developmental MRI devices are highly engineered, complex systems that are typically produced in very low volumes, often as low as one, to meet the requirements of researchers and special applications. MRI technology is based on the use of powerful magnets and radio frequency signals so the use of metals must be minimized in their construction to avoid interference.
MRI machines typically require large numbers of plastic components with complex geometries produced to very demanding requirements. A key application for plastic components in MRI machines is for specialty coils, the subassembly that interfaces with the part of the body that is to be imaged. The coil emits a radio frequency signal that is absorbed by hydrogen protons, causing them to move to a higher energy state. The signal is turned off and the coil then detects the release by the protons of the energy they had absorbed. Computers process the signal to produce an image of the patient’s body.
A typical specialty coil might have 24 plastic components with geometries ranging from relatively simple to highly complex. The coils have very specific requirements in order to avoid interfering with the operation of the MRI machine. These requirements vary with different applications, but in general an MRI coil component material needs to have low proton signal strength to avoid interfering with the MRI image. The coil material also needs to have a low magnetic field distortion to avoid interfering with the magnets that align the protons in the body. Finally, it requires high radio frequency dielectric strength to safely insulate the patient from electrical shock.
Manufacturers of MRI coils have in the past used conventional processes such as CNC machining, room temperature vulcanization (RTV) molding, and reaction injection molding to make prototypes and finished parts for MRI coils as well as other plastic MRI components. These traditional manufacturing methods each impose specific constraints that complicate the job of designing MRI components. For example, when designing a part that will be machined, an engineer needs to be concerned about whether the right size cutter is available to machine radii on the part. None of these processes is able to create the internal cavities required in some MRI components so it is often necessary to design, build and assemble multiple components to create the void.
Conventional manufacturing processes also have limitations from a cost and lead-time standpoint. The cost of tooling for molding processes can drive up the cost of low–volume MRI components. On the other hand, CNC machining requires programming costs which can also be difficult to justify for low-volume parts. Because it’s a subtractive process, CNC machining also often wastes a considerable amount of expensive material, especially on parts with complex geometries.
By overcoming the limitations of conventional production methods, Fused Deposition Modeling (FDM) enables MRI system and component manufacturers to build better machines at a lower cost in less time. FDM technology is an additive manufacturing process that builds plastic parts layer by layer, using data from computer aided design (CAD) files. It uses real thermoplastics, not thermoplastic-like materials, that are acceptable for use in MRI systems. FDM’s cost advantage in the production of MRI prototypes and low-volume production parts arises from the fact that it does not require tooling or CNC programming and because it eliminates material waste. Eliminating tools and programming also helps reduce lead time. MRI components produced with FDM are free of the design constraints imposed by traditional manufacturing methods and their design for manufacturability rules. FDM parts can be built to virtually any geometry that engineers can envision, which often results in better performing machines that can be produced at a lower cost and are less expensive to maintain.
There are a number of FDM thermoplastics that have been tested and found to meet the general requirements for use in MRI devices, including: polycarbonate (PC), polycarbonate-ISO (PC that meets the International Standards Organization (ISO) 10993-1 and United States Pharmacopeia (USP) Class VI classification), polyphenylsulfone (PPSF) and ULTEM 9085. The selection of the appropriate material is application dependent. For example, one application might require material traceability, a reason to favor PC-ISO, while another might require high heat-deflection temperatures, which would suggest the use of PPSF. Alternatively, PC offers a lower cost option when parts are produced for research and non-clinical applications.
Virtumed LLC produces coils for high magnetic field and research MRI machines. These coils typically require several dozen plastic components with complex geometries. In the past, the company used several different methods to produce these components. Smaller parts and parts with relatively simple geometries were typically machined from plastic. Larger and more complex parts were generally produced by silicon molding. The 24 components required for a typical coil were previously produced using a combination of these methods at a cost of $20,900 and a delivery lead-time of 16 weeks for one set of parts. “We were interested in using rapid prototyping methods to improve our design process and reducing our manufacturing costs and lead-time,” said Brandon Tramm, Mechanical Engineer for Virtumed. “But at the time we started, rapid prototyping materials had not been qualified for use within the intense magnetic field of an MRI machine.”
Tramm supervised the testing of FDM materials for the most important MRI material certifications. “Selection of the right material for a particular application depends upon the specific properties required,” Tramm said. “But in nearly every case at least one FDM material has the right properties.” Virtumed LLC now uses FDM to produce nearly of all of its prototype and production coils. “FDM simplifies the design process by eliminating the constraints of machining and molding,” Tramm said. “I can design the ideal geometry for the application without worrying about whether it can be molded or machined. So we have less engineering time and we can usually reduce the number of parts in the coil which saves on assembly time and maintenance costs.”
The elimination of the tooling costs involved in molding and the reduction of machine time and material waste involved in machining provide a dramatic cost reduction in the typical 24-component coil – down to $4688 per set. Lead time is reduced to only seven days. These prices and lead-times are based on purchasing parts from a service bureau and will be even better when Virtumed LLC has the volume to justify the purchase of their own FDM machine.
How Did FDM Compare to Traditional Methods for Virtumed?
|Machining and silicone molding||$20,900||16 weeks|
|FDM Tooling||$4,700||1 week|
|SAVINGS||$16,200 (78%)||15 weeks (94%)|