An Approach for Individualized Cochlear Frequency Mapping Determined From 3D Synchrotron Radiation Phase-Contrast Imaging.

Hanif M Ladak,Helge Rask-Andersen,Luke Helpard,Hao Li,Ning Zhu,Seyed A Rohani,Sumit Agrawal

doi:10.1109/tbme.2021.3080116

Abstract

Cochlear implants are traditionally programmed to stimulate according to a generalized frequency map, where individual anatomic variability is not considered when selecting the centre frequency of stimulation of each implant electrode. However, high variability in cochlear size and spatial frequency distributions exist among individuals. Generalized cochlear implant frequency maps can result in large pitch perception errors and reduced hearing outcomes for cochlear implant recipients. The objective of this work was to develop an individualized frequency mapping technique for the human cochlea to allow for patient-specific cochlear implant stimulation. Ten cadaveric human cochleae were scanned using synchrotron radiation phase-contrast imaging (SR-PCI) combined with computed tomography (CT). For each cochlea, ground truth angle-frequency measurements were obtained in three-dimensions using the SR-PCI CT data. Using an approach designed to minimize perceptual error in frequency estimation, an individualized frequency function was determined to relate angular depth to frequency within the cochlea. The individualized frequency mapping function significantly reduced pitch errors in comparison to the current gold standard generalized approach. This paper presents for the first time a cochlear frequency map which can be individualized using only the angular length of cochleae. This approach can be applied in the clinical setting and has the potential to revolutionize cochlear implant programming for patients worldwide.

Highlights

T HE cochlea is a small, helical-shaped structure within the inner ear responsible for hearing by transducing mechanical vibrations into electrical impulses for neural interpretation
Three-dimensional reconstructions and frequency coordinates were obtained of the OC and spiral ganglion (SG), which allowed for accurate frequency mapping of both structures
The first OC anglefrequency function, which can be individualized based on basilar membrane (BM) angular length, significantly outperformed the current generalized approaches derived from histological data

Summary

Introduction

T HE cochlea is a small, helical-shaped structure within the inner ear responsible for hearing by transducing mechanical vibrations into electrical impulses for neural interpretation. The BM has a variable stiffness along the cochlear spiral which results in a spatial separation of audio frequencies and is the primary mechanism which allows for the perception of different pitches in sound [1]. The outer hair cell based amplification contributes to the sensitivity and frequency selectivity of sound perception [2], [3]. This spatial distribution of frequencies within the cochlea is known as cochlear tonotopy. The vibration of the BM, and the OC hair cells, results in the generation of electrical impulses These electrical impulses are conducted through peripheral axons (dendrites) from each tonotopic specific location on the BM to the spiral ganglion (SG) and the auditory nerve [4], [5]. The SG maintains the spatial separation of frequencies, but with a different spatial relationship compared to the BM since the peripheral axons do not consistently follow a radial trajectory toward the central axis of cochleae (Fig. 1)

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