Imaging of Endolymphatic Hydrops at 7T MRI

Meniere's Disease is an inner ear disease characterised by progressive, fluctuating hearing loss and dizziness. Pathologically, it is characterised by endolymphatic hydrops. The diagnosis of Meniere's Disease can be elusive since it relies on the subjective reporting of symptoms and there is no definitive diagnostic test. Non-specific symptoms can occur in the early stages of Meniere's Disease and the disease progression can also fluctuate unpredictably. The cochlear and vestibular compartments may be differentially involved so there maybe clinical variability characterised by solely audiological or vestibular symptoms (1).

The structural correlate of MD is endolymphatic hydrops (EH) , in which the central smaller endolymphatic compartment of the inner ear (including the cochlear duct, saccule and utricle) expands into the surrounding peri-lymphatic chambers. Pathological studies of temporal bones in subjects with MD have demonstrated EH in most cases of Meniere's Disease (2) and its presence is required for, the diagnosis of 'certain Meniere's Disease' according to 1995 American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) guidelines (3). An accepted in vivo biological marker for endolymphatic hydrops has yet to be established. Recent developments in MRI technology and techniques have however allowed the demonstration of EH and the clinical application of these imaging studies is now being explored.

High resolution T2-weighted sequences are widely used for the demonstration of labyrinthine anatomy and pathology however they are unable to distinguish the endolymphatic from perilymphatic compartments and hence are generally unable to depict endolymphatic hydrops. The potential role of gadolinium in discriminating the endolymphatic chamber alone first became apparent in animal studies, since gadolinium was seen to accumulate in the perilymph but was excluded from the endolymph by the impermeable tight junctions. Intra-tympanic administration of gadolinium was initially explored, whereby high concentrations could enter the labyrinth through round window diffusion, however the evolution of clinically applicable high resolution sequences at 3T has allowed for gadolinium to be administered by the less invasive intravenous route. Delayed (4 hours post administration) gadolinium enhanced high resolution imaging is now used at a number of centres worldwide for the evaluation of Meniere's Disease patients, although acquisition and analysis methods continue to evolve. This is most frequently performed with 3D Fluid Attenuated Inversion Recovery with variable flip angles turbo spin echo sequence (3D SPACE FLAIR) or 3D real inversion recovery sequence (3D real IR. In order to corroborate the diagnosis of MD, the degree of EH in each cochlea and vestibule is semi-quantitatively graded (4,5) using various scales or may be quantitatively analysed by contouring the size of the two compartments. An ipsilateral increase in the perilymphatic enhancement is an additional MRI feature which is used to help distinguish MD ears.

The identification of cochlear or vestibular hydrops by MRI in vivo may support the diagnosis of Meniere's disease in cases with incomplete phenotypes and where there are other diagnostic considerations such as vestibular migraine or autoimmune ear disease. It may also facilitate early identification, thus guiding future treatment options for patients who have traditionally been required to take a watchful waiting approach to see how their symptoms may evolve or "trial-and-error" approach to management. The identification of bilateral endolymphatic hydrops in a patient with unilateral fluctuating aural symptoms might predict future bilateral disease and potentially influence therapeutic approaches. Establishing imaging as a biomarker in Meniere's disease may allow a contribution to diagnostic criteria and to define homogenous cohorts for longitudinal studies of natural history and treatment response.

However there are deficiencies with this MRI approach which we would like to address with an exploratory study of Meniere's disease patients on 1.5T and 7 Tesla MRI:

1. Due to the small size of the inner ear structures being imaged, and the low concentration of gadolinium in the perilymph, 3T MRI is widely considered as being optimal for the diagnosis of MD. This maximises SNR and improves spatial resolution such that small inner ear structures (e.g. the cochlear duct) maybe delineated, improving precision and inter-observer agreement and diagnostic confidence of EH diagnosis. However there are many institutions which only have access to a 1.5-T system and this potentially limits their investigation of pateints with MD, unless patients are referred to another center. A global market analysis of the distribution of MRI scanners demonstrated that in 2019 there was a 71.3% share of 1.5-T scanners (6). Whilst there are some studies demonstrating feasibility of MD diagnosis on 1.5 T systems (7-9) there has been no study directly comparing the two approaches. It is therefore important to perform a study demonstrating equivalence of diagnostic outcomes at 1.5T and 3T.
2. Gadolinium has an excellent safety record when used at low doses (0.1-0.3 mmol/kg) in patients with normal renal function. However, a causative relationship between gadolinium-based contrast agents and nephrogenic systemic fibrosis is described in patients with renal insufficiency, and there is also increasing evidence that they deposit in the human brain after multiple administrations, although the clinical significance of this remains uncertain (10). Therefore the development of non-contrast enhanced MRI technique is of considerable interest.

Whilst previous MRI approaches have concentrated on the difference in permeability to gadolinium to differentiate the endolymphatic and perilymphatic spaces of the inner ear, the difference in sodium concentrations between the two compartments is another differential feature which has not been explored. The perilymph and endolymph have unique ionic compositions suited to their functions in regulating electrochemical impulses necessary for hearing. The endolymph has a concentration of 1mM/l where the perilymph has a concentration of 140 mmol/l. Sodium imaging with MRI can provide quantitative measures of the sodium concentration in tissue (11,12). It utilizes the signal from the sodium nucleus to acquire images of the sodium biodistribution. Due to the low natural abundance of biological sodium, in comparison to water, as well as a rapid quadrupolar relaxation and lower gyromagnetic ratio the signal available is much lower than that of conventional proton MRI. Together, these properties make sodium very difficult to image with adequate signal-to-noise ratio (SNR) and hence it is most applicable to ultra-high-field MRI (7T) given the significant gain in signal strength.

An initial phase of 7T sodium imaging sequence development has been completed but there remain significant challenges to its application in the inner ear and it remains uncertain whether there will be sufficient spatial resolution and SNR, even at 7T. Therefore, the initially application of Sodium Imaging in MD should be in cases where there is known to be a maximal asymmetry in the size of the endolymphatic compartments on previous imaging studies in order to determine feasibility before determining whether it should be applied to a larger cohort. If Sodium Imaging of the inner ears is considered feasible then correlation of the semiquantitative analysis with the clinical diagnosis and with the size of the endolymphatic structures on reference standard delayed post gadolinium inversion recovery 3T MRI sequences would add validation to this approach.

1. To evaluate whether imaging both 1.5T and 3T have an equivalent performance in terms of diagnosing MD ears with delayed post gadolinium enhanced MRI (applying quantitative and semi-quantitative analysis)
2. To compare the diagnostic performance of Sodium Imaging at 7T (applying semi-quantitative analysis) with that of delayed post gadolinium enhanced 3T MRI in distinguishing symptomatic from asymptomatic Meniere's Disease (MD) ears.