The Challenge
Our customer, a medical device manufacturer, was developing a new patient monitoring system for use in MRI environments. The device required multiple wire harnesses connecting sensors, displays, and power systems—all while operating within the intense electromagnetic field of an MRI scanner.
Key Technical Challenges
MRI Compatibility
All materials must be non-ferromagnetic
EMI Immunity
Function reliably in extreme RF environment
Patient Safety
No heating, no induced currents near patient
Biocompatibility
Patient-contact cables require ISO 10993
Cleanability
Withstand hospital disinfection protocols
Documentation
Full DHF for FDA 510(k) submission
Our Approach
1. Material Selection
Standard copper and steel components were out. We worked with our materials team to identify MRI-safe alternatives for every component:
| Component | Standard Material | MRI-Safe Alternative |
|---|---|---|
| Conductors | Tinned copper | Copper (acceptable) or carbon fiber (near patient) |
| Shielding | Braided steel | Copper braid with non-magnetic drain |
| Connectors | Nickel-plated brass | Plastic housings, gold-plated copper contacts |
| Fasteners | Steel screws | Titanium or PEEK plastic |
2. EMI/RF Filtering
MRI scanners generate extremely powerful RF pulses. Cables act as antennas, potentially corrupting signals or inducing dangerous currents. Our solution:
- RF filters at every cable entry point to the device enclosure
- Twisted pair construction for all signal cables (common mode rejection)
- Double-shielded cables for analog sensor signals
- Fiber optic links for longest runs (complete galvanic isolation)
3. Safety Engineering
Patient safety is paramount. We implemented multiple safeguards:
Heating Prevention
Looped cables can act as inductors in RF fields, generating heat. We specified cable lengths to avoid resonance at MRI frequencies and added thermal fuses.
Current Limiting
High-impedance designs and current-limiting resistors prevent induced currents from reaching dangerous levels even under fault conditions.
Regulatory Compliance
Medical device harnesses aren't just about electrical function—they require comprehensive documentation for regulatory approval.
| Standard | Scope | Our Deliverables |
|---|---|---|
| IEC 60601-1 | Electrical safety | Insulation testing, creepage/clearance analysis |
| IEC 60601-1-2 | EMC requirements | Shielding specs, filter design documentation |
| ISO 10993 | Biocompatibility | Material certs for patient-contact cables |
| ISO 13485 | Quality management | Full production documentation, traceability |
| 21 CFR 820 | FDA QSR | DHF contribution, production records |
"The documentation took as long as the engineering. But that's medical devices—you're not just proving the harness works, you're proving you have a controlled process that will produce identical harnesses every time. We built the quality system alongside the product."
Hommer Zhao
Project Manager
Results
Project Outcomes
- FDA 510(k) clearance achieved first submission
- Zero field failures in first 18 months of production
- Full MRI compatibility verified at 1.5T and 3T
- Production ramped from prototype to 500/year on schedule
Technical Achievements
- Signal integrity maintained in 3T MRI environment
- Patient-contact cables passed ISO 10993 cytotoxicity
- 100% lot traceability for all components
- Cleaning validation for hospital disinfectants
Lessons Learned
Start material qualification early
MRI-compatible alternatives have longer lead times. We lost 6 weeks waiting for specialty connectors that could have been ordered during design.
Test in the real environment
Bench EMC testing didn't reveal all the issues we found in actual MRI scanners. Budget for scanner access early in the project.
Document as you go
Trying to reconstruct design decisions for the DHF after the fact is painful. Capture rationale in real-time.
Engage regulatory early
The FDA predicate device search and gap analysis should happen before detailed design, not after.
Related Resources
About the Author
Hommer Zhao led this medical device project from initial consultation through FDA clearance. He specializes in regulated industry harness development, having supported medical, aerospace, and automotive programs requiring comprehensive documentation and quality systems.
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