Abstract
Design and implantation of bionic implants for restoring impaired hair cell function relies on accurate knowledge about the microanatomy and nerve fiber pathways of the human inner ear and its variation. Non-destructive isotropic imaging of soft tissues of the inner ear with lab-based microscopic X-ray computed tomography (microCT) offers high resolution but requires contrast enhancement using compounds with high X-ray attenuation. We evaluated different contrast enhancement techniques in mice, cat, and human temporal bones to differentially visualize the membranous labyrinth, sensory epithelia, and their innervating nerves together with the facial nerve and middle ear. Lugol’s iodine potassium iodine (I2KI) gave high soft tissue contrast in ossified specimens but failed to provide unambiguous identification of smaller nerve fiber bundles inside small bony canals. Fixation or post-fixation with osmium tetroxide followed by decalcification in EDTA provided superior contrast for nerve fibers and membranous structures. We processed 50 human temporal bones and acquired microCT scans with 15 μm voxel size. Subsequently we segmented sensorineural structures and the endolymphatic compartment for 3D representations to serve for morphometric variation analysis. We tested higher resolution image acquisition down to 3.0 μm voxel size in human and 0.5 μm in mice, which provided a unique level of detail and enabled us to visualize single neurons and hair cells in the mouse inner ear, which could offer an alternative quantitative analysis of cell numbers in smaller animals. Bigger ossified human temporal bones comprising the middle ear and mastoid bone can be contrasted with I2KI and imaged in toto at 25 μm voxel size. These data are suitable for surgical planning for electrode prototype placements. A preliminary assessment of geometric changes through tissue processing resulted in 1.6% volume increase caused during decalcification by EDTA and 0.5% volume increase caused by partial dehydration to 70% ethanol, which proved to be the best mounting medium for microCT image acquisition.
Highlights
In the first staining experiment on mouse specimens, I2KI, and Phosphotungstic acid (PTA) provided the highest contrast of soft tissue in ossified inner ears
Non-destructive microscopic X-ray computed tomography (microCT) imaging has been extensively used in the past years to visualize anatomical details of inner ear samples and to assess morphometric variations
Visualization of the delicate membranous labyrinth and main nerve fibers in the inner ear is of great importance for any study to assess geometric variability to design new electrodes for electrical stimulation
Summary
Cochlear implant electrodes are nowadays offered individually tailored to recipient’s needs, as there is a huge anatomical variation of the cochlea length ranging from 28 to 42 mm at the level of the sensory epithelium from base to apex (Hardy, 1938; Spoendlin and Schrott, 1989; Wysocki, 1999; Escude et al, 2006; Erixon et al, 2009; Biedron et al, 2010; Rask-Andersen et al, 2011; Johnson et al, 2014). Designed electrodes for a future vestibular implant aim to restore vestibular function after bilateral loss that cannot be compensated by other sensory inputs This raises the need for a thorough morphometric evaluation of variations in anatomical structures of the inner ear. There is a need to analyze the endolymphatic compartment and nerve fiber pathways within the bony labyrinth in a statistically representative number of human inner ears to assess anatomical variation with high level of detail. Conventional clinical tools such as magnetic resonance imaging (MRI) and X-ray computed tomography (CT) offer only limited contrast and resolution of soft tissues within the temporal bone. The inner ear presents a considerable challenge for all kind of X-ray imaging techniques, since it is situated within the bone with one of highest mineral densities in the mammalian body (high X-ray absorption) and contains delicate membranous structures (very low X-ray absorption)
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