14Living With the Device: MRI, Magnets and External Hazards

CThe implant in the magnet: what an MRI does to it

The internal magnet that holds the external coil in place is the source of nearly every MRI problem: it creates a large image artifact, it experiences torque that can hurt and that can dislodge or flip the magnet, and the static field can partially demagnetise it. The artifact is a signal void roughly centred on the magnet that can obscure the ipsilateral temporal lobe, cerebellum and posterior fossa — a real limitation when the brain is the reason for the scan. Reported adverse events at 1.5T even with a head wrap include discomfort or pain and magnet displacement; pain is what most often forces a scan to be abandoned. Most current implants carry MRI-conditional approval at 1.5T, and many newer systems at 3T, only under specified conditions. Conditions typically include a tight compressive head bandage over the magnet, specific positioning, and sequence/scan-time limits.[2015][2018]

CManaging the magnet for MRI: wrap, remove, or self-align

Three strategies exist: a compressive head wrap with the magnet left in, surgical removal of the magnet before the scan (and replacement after), or a newer self-aligning magnet that rotates freely to follow the field. Under controlled conditions a 1.5T scan can be completed in most patients without removing the magnet, but a minority abandon the scan for pain. Magnet removal eliminates torque and dislocation but requires a minor procedure each time and still leaves an artifact from residual ferromagnetic components. Self-aligning (diametric) bipolar magnets rotate to align with the static field, markedly reducing torque, pain and dislocation, and often allowing scans without a wrap; series report scans completed without dislodgement. Whatever the route, the radiology and CI teams should plan the scan together, know the exact device model and its field-strength limit, and have the patient report pain immediately so the scan can be paused.[2018][2015]

COther energies: ESD, cautery, radiotherapy, trauma and lightning

Electrostatic discharge (ESD) can corrupt the processor's program or damage internal electronics, and has been reported to trigger facial-nerve stimulation; the everyday culprits are plastic playground equipment, slides, synthetic clothing and dry climates. Surgical diathermy is the highest-yield hospital hazard: monopolar electrocautery must never be used on the head or neck of an implanted patient because induced current can destroy the receiver-stimulator and injure the cochlea; bipolar cautery used away from the device is the safe alternative. Therapeutic ionising radiation (radiotherapy) directed near the implant can degrade the electronics; the device should be shielded or, if in the field, considered for repositioning, with manufacturer dose limits respected. Direct blunt trauma over the receiver-stimulator — a fall, a blow, contact sport — can fracture the device or shear the lead; helmets and avoidance of high-impact contact reduce risk. Lightning and other very high-energy electrical exposures are a theoretical risk to the electronics; removing the external processor during electrical storms is a simple precaution.[2022][1991]

CWhat to tell the patient: everyday life is mostly safe

Remove the external processor before any procedure involving monopolar diathermy and before high-static activities (plastic slides, removing synthetic clothing in a dry room); touch a grounded surface or the child first to discharge static. Carry the implant identification card; tell every clinician (especially in radiology and theatre) which device is implanted and its MRI field-strength limit before any scan. Airport security walk-through and hand-wand screening are safe, though the device may trigger the alarm — show the ID card. Mobile phones, microwaves, household appliances, anti-theft gates and routine dental equipment are safe in normal use. Resume normal sport and swimming once healed, but counsel on helmet use and avoiding direct blows over the implant site.[2022][2015]