Is Mirena MRI Safe?
Yes, performing a pelvic MRI in a woman with a Mirena (levonorgestrel-releasing intrauterine system) is safe at both 1.5T and 3.0T field strengths.
MRI Safety Profile
The Mirena IUD is MR conditional and poses no significant risk during standard pelvic MRI examinations. Multiple studies have demonstrated:
- No heating risk: Temperature increases remain within physiological range (≤4.8°C at whole-body SAR up to 4 W/kg) 1, 2
- No displacement: The fully metal-free levonorgestrel IUS shows no magnetic force, torque effects, or movement in magnetic fields up to 3.0T 3, 1
- Minimal artifacts: Signal voids are limited to the immediate vicinity of the device (approximately 4mm diameter at 3T), which do not significantly impair diagnostic quality 1, 2
Key Distinction from Other IUDs
It's critical to differentiate the Mirena from other IUD types:
- Copper/gold IUDs: Show minimal deflection (7° ± 7°) and are MR conditional at 1.5T and 3.0T 1
- Stainless steel IUDs (used in China): Are MR unsafe due to severe magnetic forces (>7.5N), 90° deflection angles, and massive artifacts (200mm diameter) 1
The Mirena, being a hormone-releasing system without ferromagnetic components, falls into the safest category for MRI compatibility 3.
Clinical Imaging Considerations
For diagnostic purposes: While the Mirena is MRI-safe, it may not be visible on MRI sequences, which can be problematic if you're trying to locate a displaced device. One case report demonstrated that a migrated Mirena into the abdominal cavity was not detected by MRI but was clearly visible on CT 4. This is due to the device's material composition lacking sufficient contrast on standard MRI sequences 5.
Guideline support: Recent ACR Appropriateness Criteria acknowledge MRI pelvis without IV contrast as appropriate for troubleshooting when ultrasound is limited, though they don't specifically address IUD safety concerns 6. The absence of contraindications in current guidelines implicitly supports safe use.
Common Pitfalls to Avoid
- Don't confuse with copper IUDs: While both are generally MR conditional, copper devices may produce slightly larger artifacts
- Don't rely on MRI to locate the device: If you need to confirm IUD position or rule out perforation/migration, ultrasound or CT are superior modalities 4
- Don't scan at field strengths >3.0T: Safety data only exists for 1.5T and 3.0T systems 3, 1
Bottom Line
Proceed with pelvic MRI in Mirena users without hesitation at standard field strengths (1.5T or 3.0T). The device will not heat, move, or cause significant artifacts that would compromise diagnostic quality 1, 2.