Cochlear fluid space dimensions for six species derived from reconstructions of three-dimensional magnetic resonance images

Objectives: To establish the dimensions and volumes of the cochlear fluid s paces. Study Design: Fluid space volumes, lengths, and cross-sectional area s mere derived for the cochleas from six species: human, guinea pig, bat, r at, mouse, and gerbil. Methods: Three-dimensional reconstructions of the fl uid spaces were made from magnetic resonance microscopy (MRM) images. Conse cutive serial slices composed of isotropic voxels (25 mu m(3)) representing the entire volume of fixed, isolated cochleas were obtained. The boundarie s delineating the fluid spaces, including Reissner's membrane, were resolve d for all specimens, except for the human, in which Reissner's membrane was not consistently resolved. Three-dimensional reconstructions of the endoly mphatic and perilymphatic fluid spaces were generated. Fluid space length a nd variation of cross-sectional area with distance were derived by an algor ithm that followed the midpoint of the space along the length of the spiral . The total volume of each fluid space was derived from a voxel count for e ach specimen. Results: Length, volume, and cross-sectional areas are provid ed for six species, Ln all cases, the length of the endolymphatic fluid spa ce was consistently longer than that of either perilymphatic scala, primari ly as a result of a greater radius of curvature, For guinea pig specimens, the measured volumes of the fluid spaces were considerably lower than those suggested by previous reports based on histological data. Conclusions: The quantification of cochlear fluid spaces provided by this study will enable the more accurate calculation of drug and other solute movements in fluids of the inner ear during experimental or clinical manipulations.