The State of High-Resolution Imaging of the Human Inner Ear: A Look Into the Black Box.

Unlike most medical fields, otology has not benefited from the transformative impact of high-resolution, cellular-level imaging. The sensorineural cells required for human hearing-located within the cochlea-are just 10-50 µm, placing them outside the resolution of magnetic resonance imaging, computed tomography, and positron emission tomography. These cells are highly mechano- and chemo-sensitive, and their death or dysfunction underlie the vast majority of hearing loss. Further, the cochlea is only 4-7 mm in diameter, has complex anatomy, and is deeply embedded in bone. Cochlear blood flow is partially separated by a blood barrier, limiting access to radiotracers or fluorophores. These and other features have left the human cochlea as a "black box" that cannot be assessed with high precision in vivo, limiting the development of novel hearing loss therapies. The benefits and drawbacks of existing medical imaging techniques used to diagnose disorders of the human inner ear are discussed, as well as those of emerging technologies that may help overcome challenges to access, resolution, and functional detail. A comprehensive and up-to-date discussion is provided on research efforts to improve and adapt current clinical imaging methods and introduce recent innovations that have shown exciting promise for deriving both structural and metabolic information from cochlear cells.