High-Resolution Secondary Reconstructions with the Use of Flat Panel CT in the Clinical Assessment of Patients with Cochlear Implants

DISCUSSION

Advances in CI design and implantation have been limited by the inability to assess individual electrode contact placement, reliably calculate angular insertion depth, and accurately determine the relationship of the electrode array to adjacent labyrinthine structures. These data provide important prognostic information and may play a critical role in CI outcomes.⁸

Imaging evaluation of postoperative CI placement was initially performed with plain radiographs in the Stenvers projection.^9,10 MSCT replaced this method and has since become the reference standard imaging technique for assessing the electrode array location relative to intracochlear scalae, the electrode-modiolar interval, and the proximity of the electrode to the fallopian canal.⁶ CT offers the ability to provide 3D positional information and excellent contrast for different tissue types.¹¹ Despite these advantages, however, attenuated bone structures within the acquisition field¹² and metallic artifacts severely limit the visibility of electrode contacts and adjacent structures.^10,13,14

FPCT is a relatively new imaging technique that creates less streak artifacts and offers higher spatial resolution than MSCT (cubic voxel size of approximately 0.07 mm versus 0.4–0.6 mm, respectively).⁷ These CT-like images can discern individual electrode contacts, often not possible on MSCT.^2,7 In addition, FPCT has demonstrated improved imaging of the cochlea and facial nerve canal, enabling identification of electrode contacts in close proximity to the fallopian canal and therefore most prone to induce inadvertent stimulation of the facial nerve.^2,15

Initial studies with the use of FPCT in the evaluation of CIs were performed with the use of cadaveric-implanted temporal bones.^2,6,7 The authors noted potential limitations caused by the use of temporal bone specimens rather than whole skulls, which were thought to be more challenging for imaging purposes.² Only recently have patients with CIs been evaluated by FPCT.^3,16 Our study represents one of the larger series to date of CI assessment with FPCT (14 patients, 18 electrode arrays), and the only one to explore optimal reconstruction parameters, including secondary reconstruction techniques. In our series, FPCT was able to identify all 18 insertion points and all 216 individual electrode contacts as well as to identify surrounding labyrinthine structures that were not degraded by streak artifact. Secondary reconstructions of the initial FPCT dataset by use of a VOI to include only the electrode array created voxel sizes of 0.07–0.08 mm, allowing for higher-resolution images than initial reconstructions (Fig 1B–D). These datasets were optimized by use of an HU kernel type and sharp image characteristic, which enabled the distinct identification of individual electrode contacts and surrounding osseous and soft tissue structures. Figure 4 highlights the range of image quality obtained with different postprocessing algorithms and shows the advantages of the “sharp” algorithm, with better delineation of osseous structures and less blurring of the image.

FPCT could not clearly detect the basilar membrane to designate exact scalar location, which is consistent with previous reports.² CT methods for this designation use oblique coronal images reconstructed perpendicular to the basal turn and parallel to the modiolus. The scalar position, however, is inferred in relation to the cochlear walls¹⁷ because the image resolution is insufficient to visualize the osseous spiral lamina and basilar membrane.¹⁸ Although we could infer the scalar position on the basis of the relative location of the CI in the cochlear chamber, this was not specifically attempted because definitive location could not be made or compared with any other standard. Additionally, given that an electrode may penetrate the basilar membrane and traverse >1 scalae, it is difficult to assume true location without visualizing distinct landmarks. Scalar localization will be the subject of future research as reconstruction algorithms and other advanced postprocessing techniques continue to improve. Whereas some reports cite the ability to clearly detect scalar position of the CI, the assessment of positioning within the scala tympani was primarily demonstrated in ex vivo specimens.¹⁶

The limitations of our report include the evaluation of only 1 CI manufacturer (Med-El; chosen for consistency during development of the imaging protocol), and the predominant assessment of CIs with electrode contacts spaced 2.4 mm apart (Med-El standard array), which are farther apart than some arrays currently available through other manufacturers. The distinct identification of individual electrode contacts was easily achievable, suggesting that this method should also distinguish more closely spaced contacts. Patient 14 (Fig 5I), implanted with the Med-El medium 12-electrode contact array, demonstrates the feasibility of visualizing electrode contacts spaced 1.9 mm apart. Currently planned studies will apply the FPCT technology with high-resolution secondary reconstruction algorithms to investigate other electrode designs and CI manufacturers.