Quantitative anatomy of the round window and cochlear aqueduct in guinea pigs

In order to analyze the entry of solutes through the round window membrane, a quantitative description of round window anatomy in relationship to scala tympani is required. High-resolution magnetic resonance microscopy was used to visualize the fluid spaces and tissues of the inner ear in three dimensions in isolated, fixed specimens from guinea pigs. Each specimen was represented as consecutive serial slices, with a voxel size of approximately 25 microm(3). The round window membrane, and its relationship to the terminal portion of scala tympani in the basal turn, was quantified in six specimens. In each image slice, the round window membrane and scala tympani were identified and segmented. The total surface area of the round window membrane averaged 1.18 mm(2) (S.D. 0.08, n=6). The length and variation of cross-sectional area as a function of distance for the cochlear aqueduct was determined in five specimens. The cochlear aqueduct was shown to enter scala tympani at the medial limit of the round window membrane, which corresponded to a distance of approximately 1 mm from the end of the scala when measured along its mid-point. These data are of value in simulating drug and other solute movements in the cochlear fluids and have been incorporated into a public-domain simulation program available at http://oto.wustl.edu/cochlea/.