The study protocol was approved by our institutional review board (EK 1486/2019) and the study procedure was performed in accordance with the Declaration of Helsinki. Due to the retrospective nature of this study, informed consent for study participation was waived.

For this retrospective study, 15 MR (magnetic resonance) examinations of 13 patients after translabyrinthine vestibular schwannoma resection and ipsilateral cochlear implantation were included. Vestibular schwannoma was removed via a translabyrinthine approach and ipsilateral cochlear implantation was performed simultaneously. All patients were implanted with a MED-EL SYNCHRONY Implant (MED-EL Corporation, Innsbruck, Austria). Whether cochlear implantation was carried out after translabyrinthine tumor removal was decided after performing electrically evoked auditory brainstem response measurements with an intracochlear test electrode as described in the previous studies [11, 12]. The electrode array was 28 mm in six patients and 31.5 mm in seven patients. The distance between the cochlear internal magnet and the external auditory canal was 9–10 cm. Follow-up MR examinations were performed to rule out tumor recurrence. Eleven patients underwent one follow-up MR examination, and two patients had two MRI scans, respectively. MR examinations were performed at an average of 16 months after cochlear implantation. Demographic details are shown in Table 1.

The MR examinations of the cerebellopontine angle (CPA) were performed on a 1.5T (tesla) MRI unit (Magnetom, Vida, Siemens Healthineers AG, Erlangen, Germany) using a 20-channel head/neck coil. Prior to the examination, all external components of the cochlear implant were removed, and patients received a head wrap according to the recommendations of the manufacturer. Head wrap was carried out by an ENT specialist familiar with implant position and head wrap necessities. Surgical magnet removal was not carried out in any case.

The standardized MR imaging protocol included the following sequences relevant for this study: (a) a transverse turbo spin echo (TSE) T1-weighted (w) sequence with WARP metal artifact reduction with and without contrast media application [repetition time (TR), 500–600 ms; echo time (TE), 11 ms; flip angle 150°; field of view 256 × 256 mm; bandwidth, 540 Hz/Px; in-plane resolution, 0,9 × 0,9 mm; slice thickness, 2 mm; interslice gap 0,2 mm; view angle tilting, 80%]; (b) a coronal TSE T2-w sequence with WARP metal artifact reduction [repetition time (TR), 2500–3500 ms; echo time (TE), 80 ms; flip angle 150°; field of view 432 × 512 mm; bandwidth, 540 Hz/Px; in-plane resolution, 0,9 × 0,9 mm; slice thickness, 2 mm; interslice gap 0,2 mm; view angle tilting, 80%]; (c) a heavily T2-weighted three-dimensional sequence [constructive interference in steady state (CISS)] [repetition time (TR), 5,5 ms; echo time (TE), 2.8 ms; flip angle 80°; field of view 380 × 512 mm; bandwidth, 540 Hz/Px; in-plane resolution, 0.9 × 0.9 mm; slice thickness, 0.7 mm].

All examinations were anonymized and stored on a picture archiving and communication system (IMPAX, AGFA HealthCare) and randomly presented to a board-certified radiologist (U.S-N., 6 years of experience in head and neck imaging), who was not aware of any patient data.

A subjective visual grading system was used to assess image quality of three anatomical regions, namely the CPA, the IAC, and the labyrinth on the ipsilateral and contralateral side of the CI, respectively (Table 2). The labyrinth on the ipsilateral side of the CI only consisted of the cochlea due to the previous translabyrinthine surgery. It is important to note that this grading system is not validated. However, similar grading scales have been used in the previous studies [10, 13, 14].

For the assessment of the interreader agreement, a board-certified radiologist (MAA, 5 years of experience in MRI) evaluated the same images in an independent reading session. For the assessment of the intrareader agreement, the images were re-evaluated by the board-certified radiologist 5 weeks after the first evaluation.

All statistical analyses and graphs were performed using commercially available software (STATA 12.0, StatCorp, College Station, TX, USA). Cross-tabulation was used to demonstrate the absolute frequencies and percentages of visually graded anatomical regions. To assess differences of visibility of anatomical structures between the ipsilateral and contralateral side of the device, a visual grading characteristics analysis was performed. Weighted K statistics were used for the assessment of the interreader and intrareader agreement.