Effect of Implant Position on Magnetic Resonance Image Distortion

Overview

Patients with Neurofibromatosis type 2 (NF2) almost always develop acoustic neuromas in both ears. As a result of the tumours themselves or the treatment of the tumours, many patients become profoundly deaf in both ears. It is possible to restore a degree of hearing by placing a hearing implant in the brainstem adjacent to the hearing pathways within the brain. This is termed an auditory brainstem implant (ABI). The aim of this study is to find the optimum head position in relation to the MRI magnet field, which would produce least area of image distortion in patients with ABIs. This will be of benefit in optimising the MR imaging in future patients with implants.

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.