MR Imaging and Cochlear Implants with Retained Internal Magnets: Reducing Artifacts near Highly Inhomogeneous Magnetic Fields

Abstract

The number of patients receiving cochlear implants and auditory brainstem implants for severe to profound sensorineural hearing loss has rapidly increased. These implants consist of an internal component implanted between the skull and the temporal scalp and an external removable speech processor unit. A small magnet within the internal component is commonly used to hold the external speech processor unit in place. Several cochlear implant models have recently received U.S. Food and Drug Administration and European Economic Area regulatory approval to allow magnetic resonance (MR) imaging examinations to be performed under certain specified conditions. The small internal magnet presents a challenge for imaging of the head and neck near the implant, creating a nonlinear magnetic field inhomogeneity and significant MR imaging artifacts. Fat-saturation failures and susceptibility artifacts severely degrade image quality. Typical artifacts at diffusion-weighted imaging and accelerated imaging are exacerbated. Each examination may require impromptu adjustments to allow visualization of the tissue or contrast of interest. Patients may also be quite uncomfortable during the examination, as a result of either imposed magnetic forces or a tight head wrap that is often applied to minimize internal magnet movement. Translational forces and torque sometimes displace the implanted magnet even when a head wrap is used. Diseases such as neurofibromatosis type 2 that are associated with bilateral vestibular schwannomas and hearing loss often require lifelong tumor surveillance with MR imaging. A collaborative team of radiologists, technologists, and/or medical physicists or MR imaging scientists, armed with strategies to mitigate artifacts near implanted magnets, can customize the examination for better visualization of tissue and consistent comparison examinations over time. RSNA, 2018.

MRI Observation After Intralabyrinthine and Vestibular Schwannoma Resection and Cochlear Implantation

“…Besides the right choice of MRI sequence, the position of the implant 7-9 cm from the external auditory canal at an angle of 90 or 160 • was shown to enable access to the cochlea and IAC (19) (Figure 6). SEMAC-VAT (Siemens), MAVRIC (General Electric), and O-MAR (Philips) are metal artifact reduction sequences (MARS) and should be further investigated to evidence their value for cochlear implant patients (21). The disadvantages of this sequence are their costs and a limitation in terms of combination with other sequences.…”

Cochlear Implantation in Sporadic Vestibular Schwannoma: A Systematic Literature Review

“…There are several unique challenges to using CIs with sporadic VS. First, the use of MRI in a patient status post CI is possible with appropriate precautions; however, imaging artifact along the posterior fossa with a CI magnet in place is inevitable and may limit the ability to closely follow a VS. 16,17 The patient must also be comfortable in accepting the challenges of undergoing MRI if deemed necessary. For these reasons, the practitioner should choose to implant sporadic VS patients with tumor deemed low risk for requiring future intervention and counsel the patient appropriately.…”

“…However, we now understand that with few modifications MRI can still be effective with a CI in place. 16,17 The objective of this study is to perform a systematic review examining outcomes in patients with sporadic VS undergoing ipsilateral CI.…”

Cochlear Implantation: An Overview

“…Metal artifact reduction scan techniques are commercially available as well, which may help reducing artifact but are unlikely to fully compensate for magnetic material. 29 On FIESTA-C/CISS sequences, band artifacts may interfere with visualization, which is particularly problematic in NF2 patients, who are annually monitored for tumors within their cerebellopontine angle and internal auditory canal. Echo-planar imaging and fluid-attenuated inversion recovery imaging are notoriously challenging due to the gross geometric distortion and signal loss that results.…”

“…Echo-planar imaging and fluid-attenuated inversion recovery imaging are notoriously challenging due to the gross geometric distortion and signal loss that results. 29 Ultimately, since different planar views and different sequences exhibit varying degrees of artifact severity, alternating between coronal and axial images, and evaluating multiple sequences is beneficial. Usually, even with the magnet in place, the ipsilateral internal auditory canal and cerebellopontine angle can be adequately examined in at least 1 plane.…”