3     Radiology in Auditory Implantation

Imaging is only a part of the work-up in cochlear and auditory brainstem implantation as these patients are thoroughly evaluated by a multidisciplinary team before the decision for implantation is made (see Chapter 5). Nevertheless, imaging remains indispensable in this process for judging the feasibility of surgery and may influence the side of implantation, the surgical approach, the type of device, and the type of electrode; at the same time it might give information on the cause of hearing loss.

Contraindications for cochlear implantation can result in no intervention, a staged procedure, or an indication for implantation of an auditory brainstem implant (ABI). It might additionally alter the counseling of the patient or parents regarding the outcomes of implantation, the possible alternative surgical strategies, and the additional risks of surgery. Sometimes it might be necessary to refer to another, more experienced cochlear implant team.

Appropriate training for systematically judging and interpreting the different structures on the CT and MRI scans is necessary in the work-up for implantation. We advise that you develop your own standard routine and review all structures: this is the best form of training. In our experience, blind confidence in radiology reports is unwise; it is especially the combination of surgical and anatomical knowledge with radiologic information wherein lies the additional value of the ENT surgeon. Nonetheless, proper collaboration or consultation with an experienced (neuro-) radiologist is deemed essential.

In this chapter the general radiology of the temporal bone and the cerebellopontine angle is discussed, with a special focus on radiologic evaluation prior to auditory implantation. Particular radiologic topics (such as meningitis or otosclerosis) are addressed in the special indications section of this book (see Chapters 12 to 17).

3.1 General Radiology of the Temporal Bone

3.1.1 Computed Tomography

Preoperative identification of the ossicular chain, the labyrinth, the middle and posterior fossa dura with sigmoid sinus, the facial nerve canal, and the pneumatization of the mastoid helps to plan a surgical strategy and to reduce surgical risks. A CT scan can be seen as a geographical map guiding one toward the cochlea. It also gives the surgeon a chance to recognize anatomical variations such as a low middle cranial fossa dura, an anterior sigmoid sinus, a high jugular bulb, or a poorly pneumatized mastoid. Table 3.1 gives an overview of the important structures to be checked prior to cochlear implant (CI) surgery.

3.1.2 Magnetic Resonance Imaging

Table 3.1 Checklist for preoperative CT examination for cochlear implantation

Source: adapted from Lo 1998.2

These two modalities, CT and MRI, are complementary, and preferably both should be employed in implant surgery. CT imaging, however, represents the geographical map and is of overriding importance. The accessibility and cost of MRI can be of concern, but MRI is necessary when there is doubt about cochlear patency or cochlear nerve integrity or to evaluate possible intracranial lesions, see Table 3.2. Some surgeons report having made MRI the preferred modality in preoperative imaging for cochlear implantation.¹

3.2 Radiology in Cochlear Implantation

In standard cochlear implantation the cochlear implant (CI) electrode is introduced through the transmastoid posterior tympanotomy, inserted through or near the round window, and advanced in the scala tympani of the basal turn toward the apical turn. Hence, the radiologic anatomy of the mastoid cavity, the facial recess, the tympanic cavity, the course of the facial nerve in all segments, the chorda tympani, the cochlea, the round window niche, the scalae, and the internal auditory canal with its nerves must be carefully evaluated by preoperative imaging. A radiology checklist prior to CI surgery can be seen in Table 3.1 Malformations or other causes of hearing loss can also be revealed. Also other possible pathology must be excluded. Cochlear and internal auditory canal (IAC) examination is particularly important and a helpful overview of the distinguishing features of different cochlear and IAC pathologies is given in Table 3.2.

3.3 Radiology in Auditory Brainstem Implantation

3.4 Axial and Coronal CT Sections

3.4.1 CT Mastoid Axial Images (Fig. 3.1–Fig. 3.9)

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