Fluid Attenuation Inversion Recovery sequences (FLAIR) that will demonstrate minute amounts of contrast agent in the inner ear [56, 58]. The use of MRI in temporal bone imaging is dependent on the area to be visualized, the patient’s age, the pathology involved, and its severity level. MRI is the gold standard in radiologic evaluation of soft tissue changes in the temporal bone and may serve as a complementary method when CT is used to characterize the bony structures.
MRI diagnosis of MD has been challenging until recent years [59]. The first efforts to demonstrate visualization of fluid spaces in the inner ear with gadolinium chelate (GdC) were carried out in animal studies by using animal MRI equipment of 4.7 T scanner [60]. After demonstrating the contrast of perilymph, Zou et al. [61, 62] were the first to demonstrate that endolymphatic hydrops could be visualized accurately in the guinea pig and that the changes were in accordance with the histological verification of the degree of endolymphatic hydrops. These findings were followed by Niyazov et al. [63] who showed similar results using a clinical 1.5 T machine. In humans using 1.5 T MRI, the passage of GdC delivered transtympanically was shown to accumulate in the inner ear after 12 h post injection and fully contrasted the labyrinth after 24 h post injection. However, 1.5T MRI equipment was not sensitive enough to demonstrate the delicate details of the perilymph and endolymph borders [64]. Figure 3 demonstrates the cochlear fluid spaces and endolymphatic hydrops [59].
The recent development of 3T MRI provides a tool for visualizing endolymphatic hydrops with GdC as the contrast agent [65–67] (Fig. 3). MRI, especially in Japan, Germany and more recently in USA has become a clinically useful tool for the diagnosis of atypical and typical cases of MD. Methodological development in imaging techniques and increase of the magnetic field strength have allowed separation of bone from fluid and contrast agent, and have improved spectral resolution, signal-to-noise ratio and contrast intensity, and reduced scan acquisition times [55, 56, 68]. These properties are particularly helpful in resolving details between the minute fluid-filled spaces within the inner ear (approximately 50 µL for endolymph and 150 µL for perilymph!).
A grading scale for the degree of endolymphatic hydrops has been proposed for use in research settings that was validated using identical histologic criteria and has also been applied for clinical evaluations [61, 69, 70]. The normal limit of ratio of the endolymphatic area over the vestibular fluid space (sum of the endolymphatic and perilymphatic area) is 33% and any increase in the ratio would be indicative of endolymphatic hydrops [70, 71]. According to the criteria, mild endolymphatic hydrops in the vestibule cover the ratio of 34–50% and significant endolymphatic hydrops cover the ratio of more than 50% in the vestibule [70]. The respective evaluation of the ratio of the endolymphatic area over the total fluid space in the cochlea is correlated to the displacement of Reissner’s membrane. Normally, the Reissner’s membrane remains in situ and is shown as a straight border between the endolymph-containing scala media and the perilymph-containing scala vestibuli. Mild endolymphatic hydrops display an extrusion of the Reissner’s membrane towards the scala vestibuli and result in an enlargement of the scala media with an area of less than that of the scala vestibuli. Severe endolymphatic hydrops cause an increase of the scala media with an area larger than that of the scala vestibule [70]. A similar grading system on the ordinal level, with three degrees of severity for cochlear hydrops (mild, marked, extreme), has also been proposed [72]. In cadavers without symptom history, the ratio of the endolymphatic space to the total vestibular fluid space ranged from 26.5 to 39.4% [70, 73].
The perilymphatic space facing the vestibule is sealed by the annular stapedial ligament and the perilymphatic space of scala tympani is sealed by the round window membrane. Animal and human experiments indicate that on MRI the perilymphatic space in the vestibule is filled with GdC earlier and more intensively than the perilymphatic space of scala tympani [74, 75]. Thus, the cochlear perilymph space was often poorly filled with GdC than the vestibular part. Zou et al. [76–78] performed a series of experiments by sealing either the round or oval windows and demonstrated that the permeability of the round window was poorer than that of the oval window. This also explains why the treatment of severe MD with low dose gentamicin infrequently causes deafness (less than 5% with two gentamicin injections) [79, 80] but is effective in ablation of vestibular complaints. For the visualization of inner ear membranes, therefore, it is important to fill the upper posterior part of the middle ear cavity with GdC so that the contrast agent has the possibility to be transported also via the oval window as the annular ligament is quite porous. Intratympanic administration of GdC provided efficient loading of the contrast agent in the inner ear perilymph and reduced the risk of systemic toxicity but raised concerns of local toxicity, as it is off label and requires puncture of the tympanic membrane. Such local toxicity was not observed during short, medium or long-term follow-up [81–83]. In addition, image quality might be compromised owing to impaired GdC penetration of the round and oval window membranes [78, 84] and only the injected side of the inner ear can be evaluated [58]. To evaluate both ears simultaneously, it is necessary to inject GdC into both sides [68, 85, 86]. These drawbacks hinder the widespread use of this procedure [87]. The development of more sensitive MRI techniques facilitates endolymphatic hydrops imaging using a single dose of intravenous GdC [56, 88];
