3.2.1 Cochlea

In modern-day practice the inner ear structures are evaluated with sectional imaging using computed tomography (CT) and magnetic resonance imaging (MRI). With either imaging technique, the cochlea appears as a spiral-shaped structure with 2.5 turns and a normal measured height of 5.1 mm (with a range of 4.4 to 5.9 mm). ¹ The cochlear turns (basal, middle, and apical) are separated by interscalar septae, a bony plate radiating from the modiolus that forms the base of the cochlea (Fig. 3.1). The spiral lamina that also projects from the modiolus is a microanatomical osseous structure that separates the spiral of the cochlea into scala tympani (inferiorly), scala media, and scala vestibuli (superiorly). The spiral lamina can be seen with difficulty with conventional CT; however, it is easily appreciable with thin section MRI ² (Fig. 3.2a). The scala tympani and scala vestibuli that are filled with fluid can be separated with high-resolution clinical 1.5 and 3 Tesla MRI; however, the scala media can only be visualized with high Tesla imaging. ^{3 , 4} The cochlear nerve passes from the internal auditory canal (IAC) to the modiolus through a bony canal called the cochlear aperture (or bony cochlear nerve canal) (Fig. 3.1). This “neck of the cochlea” has a normal measurable width of 1.9 mm (±0.24 mm). ^{2 , 5}

3.2.2 IAC and the Vestibulocochlear Nerve

The size of IAC varies but the mean canal diameter is 4.21 ± 0.79 mm (with a range of 2–8 mm), and the two ears have almost symmetric size with a difference of up to 2 mm. ^{6 , 7} The vestibulocochlear and the facial nerve can only be well appreciated with high-resolution, high T2-weighted imaging (Fig. 3.2a). Although the axial slices can demonstrate the size of the IAC and help to evaluate the course of the vestibulocochlear nerve (VCN), the sagittal oblique images obtained perpendicular to the long axis of the IAC are best to distinguish each of the individual components of the VCN as the nerves are visualized in cross-section. ⁸ At the medial aspect of the IAC the VCN is seen as a crescent-shaped structure (Fig. 3.2b); however, more laterally, in the fundus, the three components of the VCN can be seen separately and the cochlear nerve lies in the anteroinferior aspect (Fig. 3.2c). ⁹ The normal size of the cochlear nerve on MRI measures 1.8 ± 0.2 mm at the porus acousticus and 1.2 ± 0.2 mm in the mid to distal IAC. ¹⁰ In the majority of cases, the cochlear nerve is larger than both the superior or inferior vestibular nerves. ¹¹ The cochlear nerve is of similar size or larger than the facial nerve in more than half of the cases. ⁹

3.2.3 Cochlear Nucleus

MRI is better in delineating details of the brain anatomy but it is somewhat limited in the brainstem. ¹² The difficulties in assessing the brainstem by using MRI arises not only from the small size of various brainstem structures, but also from the fact that those anatomical components do not exhibit enough contrast to enable their individual identification. ¹³ Therefore, when relaxation-based MR image contrast is used, despite high resolution, conspicuity of those structures such as cranial nerve nuclei cannot be achieved in clinical field strengths. ¹⁴ Nevertheless, the bulge of the medulla into the lateral recess of the fourth ventricle and to the foramen of Luschka caused by the cochlear nuclear complex can be easily identified by MRI (Fig. 3.2a). ¹⁵

3.2.4 Auditory Pathway

Morphologic Imaging Anatomy

Similar to the cochlear nuclear complex, the ascending fibers of the auditory pathway are not visible with normal visual inspection of routine MR sequences. The auditory radiation can only be demonstrated with dedicated fiber tracking obtained from the diffusion tensor imaging. ¹⁶ Functional MRI (fMRI) studies that allow noninvasive assessment of brain function can localize the auditory cortex (Fig. 3.3 ).

3.3.1 Imaging Techniques

For the preoperative imaging of ABI candidates, high-resolution CT and MRI provide complementary information.

3.3.2 Radiological Evaluation of the Auditory Brainstem Implant Candidates

Evaluation of Eligibility Criteria for Implantation in Congenital SNHL

The initial radiological assessment of an ABI candidate with congenital SNHL is indeed one of a cochlear implant (CI) candidate with ineligibility for the latter determining indication for the former. ^{22 , 23} The selected congenital inner ear anomalies that result in an ABI candidacy will be discussed in detail in another chapter, but from an imaging point of view not only the specific anomaly has to be detected and correctly labeled but also the size of the cochlear aperture and the presence of the cochlear nerve and its size should be evaluated. The structures that need to be carefully evaluated in a pre-implant imaging study are the following.

Cochlea

Cochlear hypoplasia represents a group of cochlear malformations which have dimensions less than normal cochlea with decreased number or height of the cochlear turns. ^{24 , 25} When the cochlea is hypoplastic the nerve may be also be hypoplastic or aplastic and needs to be assessed with MRI. ²⁶ Similarly, common cavity anomaly, and type 1 and 2 incomplete partition anomalies have also been reported to have accompanying cochlear deficiencies including aplasia and need to be evaluated with MRI. ^{7 , 27 , 28}

Cochlear Aperture

Atresia of the cochlear aperture is a strong indicator of underlying cochlear nerve anomaly. ^{7 , 29} Tahir et al reported that all 21 cases with cochlear aperture atresia in their series had accompanying cochlear nerve deficiency (either aplasia or hypoplasia). ⁷ The dimension of a patient’s cochlear aperture also needs to be assessed, as its diameter is a marker of the cochlear nerve status. ^{7 , 30} The cochlear aperture is considered stenotic when it is narrower than 1.4 mm (Fig. 3.4a). ^{5 , 31 , 32 , 33 , 34} In the series reported by Tahir et al, most of the stenotic cochlear aperture cases had accompanying hypoplasia/aplasia of the cochlear nerve with a normal size nerve in only 15% of the cases. ⁷ It is critical to realize that the aperture can be stenotic in the presence of a normal appearing and normal size cochlea; thus, a normal cochlear shape does not always indicate normal cochlear nerve structure and further imaging with MRI is required to assess the cochlear nerve status. ⁷

Internal Auditory Canal

The IAC is considered stenotic when the diameter at its midpoint is smaller than 2 mm. ³⁵ The IAC stenosis or atresia may be easily demonstrated by CT. Although the exact measurement of the IAC diameter is more difficult with MRI, the high-resolution T2-weighted images can still demonstrate IAC hypoplasia or atresia with deceased or absence of high T2 signal of cerebrospinal fluid (CSF) within the IAC. Again the finding of a narrow or aplastic IAC raises concern for a deficiency of the cochlear nerve. ^{29 , 36} However, the IAC morphology is an unreliable surrogate marker of cochlear nerve integrity and as reported by Adunka et al, a normal IAC diameter can be seen in up to half of cochlear nerve aplasia patients. ^{7 , 37}

Cochlear Nerve

The evaluation of the VCN and especially its cochlear branch is of extreme importance prior to cochlear implantation. In a normal-sized IAC the diagnosis of cochlear nerve aplasia is relatively straightforward with dedicated sagittal oblique high-resolution images (Fig. 3.4b). ³⁸ However, in a very stenotic IAC, the diagnosis may be difficult because of the inability to separate the nerves. ^{35 , 37} Again the visualized inner ear may be normal or have subtle abnormality despite severe deficiency of the cochlear nerve. ^{38 , 39} Differentiation between hypoplasia and a normal size of the cochlear branch can also be challenging and require the highest possible resolution. ³⁹ There isn’t a well-defined consensus regarding the definition cochlear nerve hypoplasia. Li et al defined the cochlear nerve hypoplasia as a cochlear nerve with a diameter smaller than that of the facial nerve, seen on the oblique sagittal images. ²⁹ Similarly, Glastonbury et al designated the cochlear nerve as small when it appeared decreased in size compared with the other nerves of the IAC. ³⁹ It is critical to recognize that there might be occasional discrepancy between the imaging and audiological findings regarding the presence/functionality of the cochlear nerve. ^{40 , 41} Several studies have shown that subsets of patients with cochlear nerve aplasia have positive audiological responses and might derive benefit from cochlear implantation. ^{40 , 41 , 42} Anatomical connections between the cochlear nerve and other branches of the vestibulocochlear complex that are below the resolution of the current MRI might be responsible for this radiological–audiological inconsistency. ⁴³ Imaging with ultra-high field magnets with diffusion tensor imaging (DTI) fiber tractography might solve this problem in the future. ^{3 , 44}

Brainstem and Supratentorial Brain

In every patient who is a candidate for an ABI placement, the imaging of the brainstem and supratentorial brain structures with MRI is crucial not only to verify the integrity of the auditory pathways up to the temporal cortex but also to determine possible underlying congenital or acquired malformations that might hinder post implant rehabilitation. ^{22 , 45} There is significant variability of the anatomy of the lateral recess in children with congenital deafness due to abnormalities of embryonic and fetal development. ⁴⁶ It has been previously reported that congenital developmental abnormalities of the brain are more common in patients with auditory neuropathy spectrum disorder. ^{21 , 46 , 47} In patients with bilateral cochlear nerve deficiency, hindbrain anomalies such as pontine hypoplasia were reported to be the most common abnormal intracranial finding. ⁴⁶ Additionally, there might be evidence of central pathologies such as chronic changes of hypoxic-ischemic injury, kernicterus, and chronic changes of congenital central nervous system (CNS) infections. ^{48 , 49} White matter lesions are also common findings in the preimplant imaging of the CI/ABI candidates. ^{21 , 50} These lesions are nonspecific, and more diffuse and prominent parenchymal changes were found to represent negative prognostic factors for speech and language development. ^{20 , 49 , 50} It is therefore critical to make a comprehensive evaluation of the brainstem and cerebrum in each ABI candidate. Additionally, with new developing technologies, MRI also has the potential to study the anatomical and functional organization of the auditory cortex through voxel-based morphometry and fMRI. ⁵¹ DTI metrics, such as fractional anisotropy, may prove important in selecting patients and predicting outcomes after the implantation. ⁵²

Evaluation of Eligibility Criteria for Implantation in Acquired SNHL

The radiological evaluation of an ABI candidate with acquired SNHL is different from the cases with congenital SNHL as the imaging is more focused on the detection of possible surgical challenges and the following structures should be individually assessed.

The Brainstem

The posterolateral medulla where the cochlear nuclear complex is located should have no appreciable signal changes. Post-traumatic or postoperative encephalomalacic changes as well as potential injury from prior radiation therapy are important considerations for this location. ^{53 , 54} Similarly, ischemic changes involving the brainstem and especially posterolateral medulla should be absent. ⁵⁵ Although currently T2-weighted imaging is the recommended method of assessment for pontomedullary junction, in the future DTI may enable depiction of more subtle anomalies such as early detection of radiation-induced changes on auditory pathways in patients with vestibular schwannomas and might predict the success of the implantation preoperatively. ^{44 , 56}

The Lateral Recess

The site of electrode placement should be normal in size without asymmetric widening as an enlargement at this location might contribute to the migration or rotation of the electrode array (Fig. 3.5 ). ⁵⁷ Additionally, in NF2 patients, the presence or previous resection of tumors may result in distortion of the brainstem and the lateral recess may be difficult to identify. ⁵⁸ In the setting of scar tissue from previous surgery, visible as obliteration of normal CSF signal, accompanying enhancement might be seen at the site of contemplated implantation and may result in distortion of the normal anatomy.

Basal Cisterns

Anomalous lower cranial nerves and vascular anomalies around the lateral recess are potential variations that need to be looked for as those findings might prevent successful ABI placement or might cause unexpected surgical difficulties. ⁵⁹ This assessment is best done with high-resolution heavily T2-weighted sequences. It has also been reported that previous meningitis might lead to excessive bleeding and increased surgical difficulties. It is thus important to evaluate postcontrast T1-weighted images for possible increased leptomeningeal enhancement. ⁶⁰

Supratentorial Brain

In NF2 patients additional supratentorial masses such as meningiomas or meningiomatosis as well as parenchymal signal abnormalities should beinvestigated. ⁶¹ Similarly, possible accompanying traumatic encephalomalacia of the auditory cortex or sequel of shearing injury should be looked for in the setting of an ABI candidate with bilateral post-traumatic deafness. ^{60 , 62}

3.4.1 Imaging Issues after ABI

Following implantation, imaging is usually performed to determine the location of the electrode array to check for appropriate electrode array placement. ⁶³ Additionally, a brain imaging might be required to investigate immediate postoperative complications or for the follow-up of an NF2 patient. ⁶⁴

Radiographic Evaluation

Plain X-ray films in anteroposterior (AP) and modified Stenvers’ views are the standard methods of imaging following ABI surgery and are sufficient in a majority of patients to depict the integrity and positioning of electrode array and to detect electrode kinking (Fig. 3.6 ). The evaluation on those plain films were standardized by Cerini et al on lateral and AP views, where angles rather than distances from specific landmarks were used to establish correct electrode positioning. ⁶⁵ However, only limited information concerning the exact positioning of the electrode array can be extrapolated from plain films due to the innate planar representation. ⁶⁶

Stay updated, free articles. Join our Telegram channel

Join