Effect of Implant Position on Magnetic Resonance Image Distortion

Auditory brainstem implants (ABI) are placed against the auditory centres in the brainstem if it is not possible to implant the cochlea or if the cochlear nerve, which conveys neural signals from the cochlea to the brainstem, is damaged or absent. The latter device is usually used in patients with Neurofibromatosis type 2 (NF2) who develop bilateral benign inner ear tumours called vestibular schwannomas as well as other neurological tumours1. These patients often become bilaterally profoundly deaf either as a result of the disease or because of surgery to remove the tumours and require an ABI.

Patients with NF2 require monitoring of their tumours. In the absence of an auditory implant it is possible to use magnetic resonance imaging. This is the gold standard imaging technique as it gives excellent views of the soft tissues of head. However, there has been concern regarding the use of MRI in patients with auditory implants until recently because of potential problems such as demagnetisation of the magnet within the implant, implant magnet displacement and risk to the integrity of the implant electronics. In addition, the magnet field of the implant magnet results in imaging artefact (manifesting as a signal void) around the implant site that limits the information that can be obtained.

Inclusion criteria

• Any patient over the age of 18 with NF2 who has an auditory implant (cochlear or auditory brainstem implant) in situ and requires MRI scanning.

Exclusion criteria

Children under the age of 18 years Claustrophobia Bilateral implants

Contraindications and relative contraindications for MRI (as per the Radiology Department MRI protocol) including:

Pacemaker Certain metallic heart valves Ferromagnetic implanted materials and foreign bodies