To produce the plastic parts used in magnetic resonance imaging (MRI) scanners, manufacturers have used conventional manufacturing techniques such as CNC machining, reaction injection molding (RIV), and room temperature vulcanization (RTV) molding. To accelerate design and manufacturing, reduce development and production costs, and make production more efficient for the parts' low production volumes, MRI OEMs and suppliers are increasingly turning to direct digital manufacturing (DDM). Here, different kinds of additive technologies build parts directly from CAD data.

One of the technologies — fused deposition modeling (FDM) — is particularly suitable for developing MRI components. FDM uses production-grade thermoplastics, so parts withstand high heat, caustic chemicals, sterilization, and high-impact applications. The use of resins such as polycarbonate (PC) and polyphenylsulfone (PPSF) can provide time, cost, and efficiency advantages in the fabrication of MRI prototypes and the DDM of end-use parts.

FDM machines dispense the thermoplastic for the part and a different material for the disposable support structure. The material feeds from a spool of filament into an extrusion head, which heats it to a semiliquid state. Directed by the CAD data, an extrusion nozzle moves in several axes, depositing the material, layer-by-layer. Layers can be as fine as 0.005 in. FDM requires no special facilities or ventilation and involves no harmful chemicals or by-products.

For functional prototypes, FDM produces models with mechanical, thermal, chemical, and electrical properties suited for test units and pre-production scanners. And for finished parts, the process can produce the first article on the same day a design is completed. This rapid delivery comes for a fraction of the cost of machining or molding. Better yet, MRI components produced with FDM are free of design constraints imposed by traditional design-for-manufacturability rules. However, FDM materials must be qualified for use in the intense magnetic field of an MRI machine.

Qualifying FDM materials

The list of material certifications for an MRI system is extensive and varies by application. Also, each manufacturer may have its own material standards. However, for any MRI research, non-clinical, or clinical application, the most important properties are proton signal strength, magnetic field (B₀) distortion, and RF dielectric strength.

RF coil-development company MR Instruments Inc., Minneapolis, (mrinstruments.com), and Stratasys worked together to test the complete line of FDM materials. The group first tested materials to determine if a material has an allowable proton signal emission for use in an MRI scanner. Excessively high signals degrade the scanner's image.

Pigments, additives, and residues can affect proton strength, so the group tested each color of material separately. It also tested unfinished materials and materials that had undergone post-processing such as chemical finishing and chemical finishing followed by a soda blast. An acceptable level of proton signal strength is less than 15% variance of the sample signal versus the background.

Results show that PC (white), PC-ISO (translucent), and PPSF are acceptable materials for 3 Telsa (3T) MRI scanners. (High-power 3Ts are replacing the 1.5T scanners commonly found in clinical facilities.) The materials did not exceed the 15% threshold for any of the finishing conditions.

The group then evaluated the three materials for B₀ distortion to quantify the effect of each on the magnetic field. With an allowable limit of less than one part per million, all three passed.

Finally, the group tested for RF dielectric strength to ensure that patients are safely insulated from electric shock. To be used in 3T MRIs, materials must withstand a 500 V (RMS) charge without voltage breakdown. All three materials exceeded the acceptable requirement by more than double the limit.

Since the three materials are suitable for construction of end-use parts in a 3T MRI scanner, the appropriate choice depends on the application. For example, PC-ISO is ISO certified for material traceability, while PPSF has high heat deflection temperatures suited to autoclaving and a V-0 flammability rating. Alternatively, PC is a lower-cost option when manufacturing parts for research and non-clinical applications.

Comparing manufacturing techniques

A DDM application of FDM is being used for appliances that steady the patient during an MRI. While the machine is capturing images, the patient must lie still, but the procedure can take an hour or more. One company is evaluating FDM to manufacture fixtures for restraining a patient's head, arm, or leg during an MRI. The parts are in the imaging field and so must satisfy proton strength and field distortion requirements.

In another example, MR Instruments uses FDM to make both functional prototypes and end-use parts for its line of research and development RF coils. These supplement base MRI systems to produce higher-resolution images in localized areas. Much of the company's use of FDM is with the plastic housings that surround the target area and contain the electronic circuitry.

In its justification process, MR Instruments evaluated the cost and time to produce a 32-channel head coil housing with FDM, CNC machining, and RTV molding. The plastic housing contains seven components.

Due to the complexity of the design, CNC machining was appropriate for only two of the seven parts. The estimate for these was $1,800 and two weeks. In contrast, FDM parts were made for $975 in two days.

For the whole assembly, the company looked at RTV molding to produce cast urethane parts. The cost for patterns, molds, and the first set of cast parts was $15,900, with a lead time of five weeks. Alternatively, the cost for FDM parts was $2,515 and the lead time was nine days.

MORE ABOUT MRI

Magnetic resonance imaging (MRI) scanners produce images of internal body structures such as bone, organs, and soft tissues. The machines produce highly detailed, cross-sectional images using powerful magnets and radio waves. The current 3 Telsa scanners are complex and highly engineered systems, which accounts for their $3 million price tag. According to Global Industry Analysts Inc., GE Healthcare, Siemens Medical Solutions, and Philips Medical Systems top the global MRI equipment market, projected to reach $5.5 billion by 2010.

MORE RECENT TESTING

The testing group recently did proton image testing on PC, PCiso, and PPSF materials in a 9.4T strength research coil and found them to still have acceptable limits of signal. This is good news to research coil manufacturers working on higher strength coils. The group also recently tested the materials' carbon signal and found that ULTEM 9085 and PPSF might be suitable for specific low carbon applications.