MRI Safety Considerations for Permanent Magnet Implants in Muscle.

Background: Permanent magnet implants are used with several medical and assistive devices, such as cochlear implants, dental attachments, and prosthetic control, but raise caution for MR imaging. Previous work has evaluated several magnet implants for position and magnetization stability, as well as for image artifacts under MRI. Yet, the intramuscular magnets used for prosthetic control still require evaluation for potential MRI conditionality.

Purpose: To investigate the position and magnetization stability of and image artifacts from 3-mm-diameter spherical permanent magnets (B_r = 1.393 T, H_ci = 1.637 MA/m) implanted within muscle.

Study type: Prospective longitudinal study.

Animal model: Porcine; one animal, eight muscles.

Field strength/sequence: 0.55-T, 1.5-T/SE, GRE.

Assessment: Permanent magnets and nonmagnetic controls were implanted into eight muscles and exposed to 1.5-T MRI 36 days post-implantation. All sites were examined histologically for evidence of implant migration (acute fibrotic response or fibrotic capsule disruption). Benchtop studies evaluated worst-case demagnetization and image artifacts (artifact radius minus implant radius). The primary measure of position stability was histological examination interpreting characteristics of progressive skeletal muscle healing. Secondary position stability analysis was performed via CT imaging.

Statistical tests: Unpaired one-sided sign test with a significance level of 0.05. Demagnetization and imaging artifacts were summarized as maximums.

Results: Fibrotic capsules were similarly intact at permanent magnet and control sites (fibrotic capsule thicknesses: 20-550 μm [magnets], 20-220 μm [controls]). No effect of MRI exposure on implant migration was observed via secondary analysis (p = 0.965 [0.55-T], p = 0.996 [1.5-T]). Maximum demagnetization was 2.1% under 0.55-T exposure and 13.5% under 1.5-T exposure, and maximum image artifact was 71 mm at both imaging strengths.

Data conclusion: The permanent magnet implants used in this study were resistant to migration and substantial demagnetization under 0.55-T and 1.5-T MRI exposure and resulted in negligible image artifacts for critical organ imaging, suggesting that the presence of these implants does not preclude a patient from receiving MR imaging up to 1.5T.

Evidence level: N/A.

Technical efficacy: Stage 5: Improvements in patient care.