Post mortem cadaveric and imaging mapping analysis of the influence of cochlear implants on cMRI assessment regarding implant positioning and artifact formation

Abstract

Objectives: In times of an aging society and considering the escalating health economic costs, the indications for imaging, particularly magnetic resonance imaging (MRI), must be carefully considered and strictly adhered to. This cadaver study aims to examine the influence of cochlear implant (CI) on the assessment of intracranial structures, artifact formation, and size in cranial MRI (cMRI). Furthermore, it seeks to evaluate the potential limitations in the interpretability and diagnostic value of cMRI in CI patients. Additionally, the study investigates the imaging of the brain stem and the internal ear canal and the feasibility of excluding cholesteatomas in cMRI for CI patients.

Materials and methods: Two cadaveric specimens were implanted with cochlear implants at varying angular positions (90°, 120°, and 135°), both unilaterally and bilaterally, with and without magnet in situ. MRI acquisition consisted of sequences commonly used in brain MRI scans (T₁-MP-RAGE, T₂-TSE, T₁-TIRM, DWI, CISS). Subsequently, the obtained MRI images were manually juxtaposed with a reference brain from the Computational Anatomy Toolbox CAT12. The size and formation of artifacts were scrutinized to ascertain the assessability of 22 predefined intracranial structures. Furthermore, the internal auditory canal, middle ear and mastoid were evaluated.

Results: The cadaveric head mapping facilitated the analysis of all 22 predefined intracranial structures. Artifacts were assessed in terms of their minimum and maximum impact on image comparability. Image quality and assessability were stratified into four categories (0-25%, 25-50%, 50-75%, and 75-100% of assessability restriction). The visualization of the central, temporal, parietal, and frontal lobes was contingent upon CI positioning and the choice of imaging sequence. Diffusion-weighted cMRI proved inadequate for monitoring cholesteatoma recurrence in ipsilateral CI patients, regardless of magnet presence. The ipsilateral internal auditory canal was inadequately visualized in both magnet-present and magnet-absent conditions. We divided our results into four categories. Category 3 (orange) indicates considerable limitations, while category 4 (red) indicates no interpretability, as the image is entirely obscured by artifacts.

Conclusion: This study provides detailed predictive power for the assessability and therefore the relevance of performing cMRIs in CI patients. We advocate consulting the relevant CI center if artifact overlay exceeds 50% (categories 3 and 4), to evaluate magnet explantation and reassess the necessity of cMRI. When suspecting cholesteatoma or cholesteatoma recurrences in patients with ipsilateral cochlear implants, diagnostic investigation should preferably be pursued surgically, as the necessary MRI sequences are prone to artifact interference, even in the absence of a magnet. The ipsilateral internal auditory canal remains inadequately evaluable with a magnet in situ, while without the magnet, only rudimentary assessments can be made across most sequences.

Keywords: Artifact; Cochlear implant; Cranial imaging; MRI; Magnet.

Fig. 1

Illustration of the three different evaluated implant positions in 90°, 120° and 135° for both cadaveric heads

Fig. 2

Unilateral 135° CI position with magnet in situ demonstrating the definition of minimal (yellow) and maximal (red) artifact. We defined the cutoff between minimal and maximal artifact regions as follows: the minimal artifact region does not allow for assessment of imaging interpretability, whereas the maximal artifact region may still depict large tumors, hemorrhages, or malformations larger than 1 cm

Fig. 3

Mapping imaging analysis of cadaveric heads, with modeled intracranial structures. In red the maximal artifact regions, that cannot be interpreted, here for the examined region of the left cerebellum (demonstrated in orange) (condition of CI = bilateral, 135° with magnet in situ) (MRI, sequence: CISS, in three dimensional reconstruction)

Fig. 4

Mapping imaging analysis of cadaveric heads, with modeled intracranial structures. In red the maximal artifact regions, that can not be interpreted, here for the examined region of the brain stem (condition of CI = bilateral, 135° with magnet in situ) (MRI, sequence: CISS, in three dimensional reconstruction)