Visualization of Reissner membrane and the spiral ganglion in human fetal cochlea by micro-computed tomography

Although visualization of fine structures in the cochlea such as Reissner membrane (vestibular membrane) is important for elucidation of the mechanism and the establishment of therapy for inner ear diseases, they cannot be visualized by even the most advanced high-resolution medical computed tomography (CT) and magnetic resonance imaging. Visualization of Reissner membrane in dissected animals by micro-magnetic resonance imaging has been reported, but bone could not be visualized. We attempted to visualize human fetal Reissner membrane and the spiral ganglion by micro-focus x-ray CT (micro-CT), which has a spatial resolution several hundred times greater than the conventional medical CT.

Serial tomograms of a dissected pyramis, including the cochlea of human fetuses (stillborn specimens), were obtained by micro-CT, and 3-dimensional reconstruction was performed by a volume-rendering method.

Clear tomograms (theoretical spatial resolution, 12.2 x 12.2 microm; slice thickness 77.5 microm) and 3-dimensional reconstructed images (theoretical spatial resolution, 6.8 x 6.8 microm; slice thickness, 40.0 microm) of Reissner membrane and the spiral ganglion with a bony labyrinth (cochlear bone) were successfully obtained for the first time. The thickness of Reissner membrane obtained by the tomogram was 12 microm, which corresponds to the optical macroscopic value from resin-embedded histologic sections.

This study showed that micro-CT enables us to visualize the internal fine structure of the human cochlea. As the success rate of the visualization of Reissner membrane is not high, it is necessary to improve the image quality and contrast resolution of micro-CT to enable stable visualization of fine structures. The development of imaging equipment such as micro-CT for medical use should play an important role in the elucidation of the mechanism and the establishment of therapy for inner ear diseases.