Some Remarks on Imaging of the Inner Ear: Options and Limitations

Abstract

The temporal bone has a highly complex anatomical structure, in which the sensory organs of the cochlea and the vestibular system are contained within a small space together with the sound-conducting system of the middle ear. Detailed imaging is thus required in this anatomical area. There are a great many clinical aims for which the highest-possible spatial resolution is required. These include the localization of cerebrospinal fluid fistulas, the detection of malformations of the middle and inner ear and the vestibulocochlear nerve, an aberrant course of the facial nerve and anomalies of the arterial and venous structures, the confirmation of dehiscence of the semicircular canals and finally, the verification of endolymphatic hydrops in cases of Ménière's disease. However, the term 'high resolution' is very time dependent. Two milestones in this respect have been (in 1991) the 3D visualization of the inner ear by means of maximum-intensity projection (MIP) of a T2-weighted constructive interference in steady state (CISS) sequence of a 1.5-tesla magnetic resonance imaging (MRI) scanner (Tanioka et al., Radiology 178:141-144, 1991) and (in 1997) imaging of the vestibulocochlear nerve for the diagnosis of hypoplasia inside the internal auditory canal using the same sequence (Casselman et al., Radiology 202:773-781, 1997).The objective of this article is to highlight the options for, and the challenges of, contemporary imaging with regard to some clinical issues relating to the inner ear.

Cerebrospinal Fluid Leak in Cochlear Implantation: Enlarged Cochlear versus Enlarged Vestibular Aqueduct (Common Cavity Excluded)

“…The pathogenesis of perilymph fistulas is thought to be the result of a direct communication between the subarachnoid space and inner ear caused by a defect in the bony partition of the fundus of the IAC [14, 15] (not identifiable with studies of images or other methods) or enlarged cochlear or vestibular aqueduct [13, 16, 32]. For a long time, EVA was considered as the main cause of the presence of intraoperative CSF leak [63, 64].…”

“…It is, however, different for the ECA and EVA studies; these can easily be conducted radiologically by means of current CT methods. Evaluating the conditions of these aqueducts which can provoke CSF leak may be particularly useful in planning cochlear implantation surgery, stapedectomy, and others (Figure 1) [16]. …”

Imaging of Temporal Bone

“…CT can detect signs of perilymphatic fistulae (i.e. pneumolabyrinth) but fails to detect subtle traumatic lesions within the inner ear, such as labyrinthine haemorrhage or axonal damage along central auditory pathways (9). Many anatomic structures of the middle and inner ears are not optimally depicted using conventional CT with image reconstruction in the standard axial and coronal planes.…”

“…Rated as: -= not visible, += poorly, ++= moderately and +++= good visible. (6) (54) visualization of the cochlear partitioning (9). A recent paper by Maillot et al (12) indicated that MDCT allows radiologists to examine the complex anatomy of the temporal bone with sub-millimeter resolution and is the first modality of choice.…”

“…In detecting a thin bony coverage of a superior semicircular canal, digital volume tomography (DVT) scans seem to be superior to MDCT scans (13). Giesemann and Hofmann (9) indicated that CT is the gold standard in imaging diagnosis of semicircular canal dehiscence syndrome (SCDS). ; however, it has a high false-positive rate and may be misleading in terms of diagnosis because it overestimates the size of the dehiscence and the prevalence (17).…”