Fusion of Preoperative MRI and Postoperative FD-CT for Direct Evaluation of Cochlear Implants : An Analysis at 1.5 T and 3 T

Auditory outcomes after scala vestibuli array insertion are similar to those after scala tympani insertion 1 year after cochlear implantation

PURPOSE

In cochlear implantation, a scala vestibuli (SV) insertion of an electrode array is a rare occurrence and is reported to be linked to poor hearing outcomes. Using the same electrode array, the auditory performance of patients with a complete SV location was compared with that of patients having a complete scala tympani (ST) location 1 year after implantation.

METHODS

Thirty-three patients were included in this retrospective case-control study (SV, n = 12; ST, n = 21). The matching criteria were electrode array type, age at implantation, and duration of severe or profound deafness. The array location was analyzed using 3D reconstruction of postoperative CT scans. Postoperative audiological evaluation of the implanted ear was performed using pure-tone audiometry, speech recognition of monosyllabic words in quiet, and words and sentences in noise.

RESULTS

On the preoperative CT scan, six patients in the SV group presented with both round window (RW) and ST ossification, three with RW ossification alone, and three with no RW ossification. Auditory performance did not differ between SV and ST groups 1 year after cochlear implantation. Speech recognition of words was 49 ± 7.6% and 56 ± 5.0% in quiet and 75 ± 9.5% and 66 ± 6.0% in noise in SV and ST groups, respectively.

CONCLUSION

ST insertion is the gold standard that allows the three cochlear scalae to preserve scalar cochlear integrity. However, 1 year after implantation, a planned or unexpected SV insertion is not detrimental to hearing outcomes, providing similar auditory performance in quiet and noise to ST insertion.

Co-Registration of pre- and post-Operative images after cochlear Implantation: A proposed technique to Improve cochlear visualization and localization of cochlear electrodes

BACKGROUND AND PURPOSE

After cochlear implantation, metallic artifact can obscure nearby structures on CT images, which is problematic in patients with facial nerve stimulation (FNS). This study evaluated the usefulness of co-registered pre- and post-operative examinations to evaluate the cochlear implant and adjacent structures.

MATERIALS AND METHODS

A retrospective review was completed of consecutive patients that underwent CT imaging of the temporal bone before and after placement of a cochlear implant. Two blinded neuroradiologists independently reviewed all available examinations. All examinations were assessed for the presence or absence of dehiscence of the osseous ridge between the cochlea and facial nerve canal (FNC). Pre-operative and fused pre- and post-operative examinations were compared in their ability to visualize the osseous ridge using a 5-point Likert scale (ranging from 1 = unfused images were substantially superior to 5 = fused images were substantially superior). The electrode closest to the FNC were noted.

RESULTS

Of 34 included patients, 13 (38.2%) were female and 21 (61.8%) were male; average age was 72.2. Seven patients (20.6%) had frank dehiscence between the cochlea and FNC. Fused images were superior to the post-operative study alone for assessing the integrity of the osseous partition between the cochlea and FNC and for reducing artifact from the electrode array (average Likert scores for both reviewers were 4.4 and 4.7). There was good agreement between reviewers in noting electrode closest to the FNC (concordance correlation coefficient=0.82).

CONCLUSIONS

Following cochlear implantation, co-registered pre- and post-operative CT images are superior to conventional images in assessing the anatomic relationship between the cochlea and FNC.

Improved cochlear implant electrode localization using coregistration of pre- and postoperative CT

BACKGROUND AND PURPOSE

Artifact from cochlear implant electrodes degrades image resolution on CT. Here, we describe the use of coregistered pre- and postoperative CT images to reduce metallic artifact from the electrodes to assess its position more accurately within the cochlear lumen.

METHODS

Pre- and postoperative CTs were reviewed after coregistration/overlay of both exams. Images were evaluated by two neuroradiologists for scalar location of electrodes tip (± scalar translocation), tip fold over, and angular depth of insertion.

RESULTS

Thirty-four patients were included in the final cohort. Transscalar migration was present in three (8.8%) cases (one case demonstrated tip fold over), with initial disagreement regarding transscalar migration in 1 out of 34 patients (2.9%). Agreement regarding depth of insertion was present in 31 (91.1%) cases. Five-point Likert scales were used to compare the ability to resolve the proximity of electrodes to the lateral/outer cochlear wall without and with overlay, which is a qualitative measure of artifact from the array. Likert scores showed definitive benefit of metal artifact reduction using overlayed images with an average score of 4.34.

CONCLUSION

This study demonstrates a novel technique of using fused coregistration of pre- and postoperative CTs for the purpose of artifact reduction/electrode localization. It is anticipated that this technique will permit more accurate localization of the electrodes for improvement in surgical technique and electrode array design.

High-speed flat-detector computed tomography for temporal bone imaging and postoperative control of cochlear implants

PURPOSE

Flat-detector computed tomography (FD-CT) is the standard for cochlear implant (CI) imaging. FD-CT systems differ in technical characteristics. Our aim was an evaluation of two different FD-CT generations with different protocols and hardware regarding image quality, radiation dose, and scan time.

METHODS

Two temporal bone specimens (- / + CI = TB₀/TB₁) were scanned using three different scanners: two FD-CT systems with different scanning protocols (standard FD-CT: 20 s 70 kV, 20 s 109 kV; high-speed FD-CT [HS-FD-CT]: 7 s 109 kV, 9 s 109 kV, 14 s 72 kV) and MS-CT (5 s 120 kV). Acquired datasets were evaluated in consensus reading regarding qualitative and quantitative parameters: addressing CI- and cochlea-specific parameters, cochlea delineation, lamina spiralis ossea visibility, distinction of single CI electrodes, determination of intracochlear implant position, stapes delineation, and mastoidal septation were assessed. Addressing protocol-specific parameters, radiation dose (dose-length-product/DLP), and scan time were assessed.

RESULTS

Two HS-FD-CT protocols (14 s/9 s) provide higher or equivalent diagnostic information regarding CI- and cochlea-specific parameters compared to both standard FD-CT protocols. The fastest HS-FD-CT protocol (7 s)-providing inferior diagnostic information compared to all other FD-CT protocols-still exceeds MS-CT. The highest DLP was recorded for the 14 s HS-FD-CT protocol (TB₁ = 956 mGycm); the lowest DLPs were recorded for the 7 s HS-FD-CT protocol (TB₀ = 188 mGycm) and for MS-CT (TB₀ = 138 mGycm), respectively. HS-FD-CT allows a significant reduction of scan time compared to standard FD-CT.

CONCLUSION

High-speed FD-CT improves visualization of temporal bone anatomy and postoperative assessment of CIs by combining excellent image quality, fast scan time, and reasonable radiation exposure.