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Mid-field MRI with cochlear implants: A healthy volunteer study of magnet displacement, pressure perception, and temporal bone imaging

This study demonstrates that 0.55T mid-field MRI significantly reduces magnet displacement and torque risks for patients with axial magnet cochlear implants compared to 1.5T MRI, while maintaining diagnostic image quality for temporal bone imaging.

Original authors: Steve E.J. Connor, Anthony Price, Irumee Pai, Philip Touska, Cristina Dudau, Philippa Bridgen, Pierluigi Cio, Lucy Billimoria, Daniel West, Oscar Lally, Sebastien Ourselin, Joseph V Hajnal

Published 2026-08-14

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Steve E.J. Connor, Anthony Price, Irumee Pai, Philip Touska, Cristina Dudau, Philippa Bridgen, Pierluigi Cio, Lucy Billimoria, Daniel West, Oscar Lally, Sebastien Ourselin, Joseph V Hajnal

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Magnetic Tug-of-War: Why Some Implants Hate Strong Magnets

Imagine your body is a quiet, peaceful town, and inside that town, some people have tiny, powerful magnets implanted behind their ears to help them hear. These are cochlear implants, and they are miracles of modern engineering. But there's a catch: the world of medical imaging often uses giant magnets to take pictures of our insides. When a person with a cochlear implant walks into a standard MRI machine, it's like bringing a compass into a tornado. The giant magnet in the machine tries to grab the tiny magnet in the ear, pulling and twisting it. This can hurt the patient, damage the device, or even rip the magnet out of its housing.

For years, doctors have had to be very careful, often avoiding these scans or using heavy bandages to hold the magnets in place. But what if the "tornado" wasn't quite so strong? Scientists have been developing "mid-field" MRI machines that are powerful enough to see inside the head but not quite as intense as the old giants. The big question is: do these gentler machines actually stop the magnets from getting twisted around, and can they still take clear enough pictures to be useful? This study dives into that exact problem, testing whether a "medium-strength" magnet is the sweet spot for safety and clarity.

The Experiment: A Race Between Two Magnets

In this study, a team of researchers set up a fascinating test using ten healthy volunteers. Since you can't ethically implant magnets into healthy people just to test them, the team strapped non-functioning cochlear implants onto the volunteers' heads, wrapping them securely with bandages. They used two different types of "dummy" implants: five had the old-school style magnets (called axial magnets) that stick straight out like a flagpole, and five had the newer, smarter style (rotating diametric magnets) that can spin to align themselves with the magnetic field.

The volunteers then went through a "magnetic obstacle course." They were scanned twice: once in a standard, high-power 1.5 Tesla (1.5T) MRI machine (the "tornado"), and once in a newer, mid-power 0.55 Tesla (0.55T) machine (the "gentle breeze"). The order was random, and the volunteers didn't know which machine they were in, keeping the test fair.

The Twist: The Old Magnets Got Spun

The results were dramatic for the old-school magnets. When the volunteers with the axial magnets went into the powerful 1.5T machine, the magnetic force was so strong that in 4 out of 5 cases, the magnet inside the housing actually twisted or shifted out of place. It was like a strong wind blowing a flag so hard it snapped the pole. The researchers measured the twisting force (torque) and found it was 0.133 Newton-meters in the big machine.

However, when those same volunteers went into the 0.55T machine, the story changed completely. Zero of the axial magnets moved. The twisting force dropped to 0.062 Newton-meters. The "gentle breeze" wasn't strong enough to knock the flag over.

The newer, rotating diametric magnets were the superheroes of the group. They didn't budge at all, whether in the 1.5T tornado or the 0.55T breeze. Their torque stayed almost identical at both strengths (0.046 Nm vs 0.044 Nm), proving that their self-aligning design works perfectly to neutralize the magnetic pull.

The Feeling: Did It Hurt?

You might think that if a magnet is twisting, it would hurt. The researchers asked the volunteers to rate the pressure on a scale from 0 to 10. Surprisingly, the volunteers didn't report a significant difference in pain or pressure between the two machines. Whether the magnet was twisting or not, the bandages and the scalp seemed to cushion the sensation enough that the volunteers couldn't tell the difference. This suggests that while the magnets were physically moving in the big machine, the feeling of that movement was masked by the bandage.

The Picture: Can We Still See the Details?

The most important part of an MRI is the picture. If the machine is too weak, the image might be too grainy to see the tiny bones of the inner ear. The team took detailed pictures of the temporal bone (the skull area around the ear) using two different scanning techniques: a 2D T2-weighted scan and a 3D CISS scan.

They found that the 0.55T machine produced images that were just as clear and useful as the 1.5T machine for looking at the inner ear structures. The "hard" shadows caused by the metal implants (artifacts) were present in both, but the distance between the shadow and the important anatomy was similar. In fact, the 0.55T machine was a bit more flexible; it didn't hit the safety limits for heating (SAR) that the 1.5T machine did, which meant the scans could run smoothly without needing to slow down or stop.

The Verdict

This study suggests that for people with older-style cochlear implants that use axial magnets, the 0.55T mid-field MRI is a game-changer. It acts like a safety net, stopping the magnets from twisting and potentially causing damage, while still taking pictures clear enough for doctors to diagnose problems.

However, the researchers are careful to note that this doesn't mean the old machines are useless. For the newer implants with rotating magnets, the high-power machines are still safe. But for the thousands of people with older implants who have had bad experiences with pain or magnet shifting in strong MRI machines, this "gentle breeze" machine offers a promising new path forward. It's not a magic fix for everything, but it's a solid, safer option for a specific group of patients who need to see inside their heads without the fear of a magnetic tug-of-war.

Technical Summary: Mid-field MRI with Cochlear Implants

Problem Statement
The increasing prevalence of cochlear implant (CI) recipients and the rising utilization of magnetic resonance imaging (MRI) have heightened concerns regarding device safety and image quality. The primary safety risks involve translational forces and torque exerted on the internal CI magnet by the static magnetic field ( $B_0$ ), which can cause magnet displacement, device malfunction, or significant patient pain leading to premature scan termination. Traditional axial magnet designs are particularly susceptible to torque perpendicular to the skull, whereas newer self-aligning diametric magnets aim to mitigate these risks. However, adverse events persist, especially with legacy axial magnet devices. Furthermore, high-field (1.5T) MRI imposes strict Specific Absorption Rate (SAR) limits to prevent heating, often necessitating sequence modifications that compromise scan time or image quality. While lower field strengths theoretically reduce induced forces and heating (scaling with $B_0$ and $B_0^2$ , respectively), the specific impact of mid-field (0.55T) MRI on magnet displacement risk, patient tolerance, and the diagnostic utility of temporal bone imaging remains to be established across different magnet designs.

Methodology
This study employed a healthy volunteer model where non-functioning, MRI-conditional Advanced Bionics cochlear implants were bandaged to the head to simulate the clinical scenario. The study utilized two distinct implant models:

HiRes™ Ultra 3D: Featuring a self-aligning diametric magnet.
HiRes 90K™: Featuring a traditional axial magnet.

Experimental Design:

Participants: Ten healthy volunteers underwent consecutive MRI sessions on both 0.55T (Siemens MAGNETOM Free.Max) and 1.5T (Siemens MAGNETOM Sola) scanners. The order was randomized and blinded.
Physical Force Testing: Translational forces were estimated using the deflection angle method (ASTM F2052-21), and torque was measured at the isocenter using a custom torsion spring rig (ASTM F2213-17).
Clinical Simulation: Participants were bandaged with the CIs at standardized angles. Pressure perception was assessed using a Visual Analogue Scale (VAS) at baseline, upon entering the bore, and immediately post-scan. Magnet displacement (tilting or dislocation) was evaluated by inspecting the magnet position within the silicone housing after scans.
Imaging Protocols: Optimized 2D T2-weighted fast spin echo (2D T2-FSE) and 3D constructive interference in steady state (CISS) sequences were acquired for seven participants. Parameters were adjusted to comply with manufacturer SAR limits and to minimize CI-related artifacts (e.g., increased receiver bandwidth, AI-based reconstruction at 0.55T).
Image Analysis: Three radiologists independently reviewed images for diagnostic confidence, artifact severity, and the distance between CI-related artifacts and anatomical structures (cerebellopontine angle, internal auditory meatus, inner ear). Statistical analysis included Wilcoxon signed-rank tests for ordinal data and Bland–Altman analysis for distance measurements.

Key Results

Magnet Displacement and Torque:
- Axial Magnets (HiRes 90K™): Significant displacement occurred in 4 out of 5 participants after 1.5T MRI (2 tilted, 2 dislocated), whereas no displacement occurred at 0.55T ( $p=0.04$ ). Torque measurements confirmed a ~2.2-fold increase at 1.5T (0.133 Nm) compared to 0.55T (0.062 Nm).
- Diametric Magnets (HiRes Ultra 3D™): No displacement was observed at either field strength. Torque measurements were low and relatively field-independent (0.046 Nm at 0.55T vs. 0.044 Nm at 1.5T).
Patient Tolerance: Pressure perception scores did not differ significantly between the 0.55T and 1.5T systems, nor between the two magnet types, suggesting that external bandaging may not fully replicate the pain mechanisms of implanted devices or that the sample size was insufficient to detect subtle differences.
Image Quality and Artifacts:
- Both 0.55T and 1.5T provided adequate diagnostic confidence for temporal bone structures (CPA, IAM, inner ear) using optimized sequences.
- The 0.55T 2D T2-FSE sequence achieved diagnostic confidence comparable to 1.5T and successfully operated within SAR limits for the axial magnet CI without the extensive parameter compromises required at 1.5T.
- Quantitative analysis of artifact-to-structure distances showed no clear systematic bias between field strengths, though variability was high. The extent of "hard" artifact was similar between 0.55T and 1.5T.

Significance and Claims
The authors conclude that mid-field 0.55T MRI offers a distinct safety advantage for patients with axial magnet cochlear implants by significantly reducing the risk of magnet displacement and the associated torque forces, while maintaining clinically acceptable image quality for temporal bone evaluation.

The study posits that 0.55T MRI could serve as a valuable alternative pathway for patients with legacy axial magnet CIs who have previously experienced magnet complications or pain during high-field MRI. Unlike the 1.5T environment, the 0.55T system allowed for the use of 2D T2-FSE sequences within SAR limits without the need for severe parameter reductions that lead to motion artifacts. The authors note that while diametric magnets appear safe at both field strengths, the mid-field option provides a critical safety margin for older device models. The paper frames this as an "exploratory" finding, emphasizing that the external bandaging model has limitations in mimicking the pain response of implanted patients, and calls for further investigation into wider applicability across other auditory implants.