Objective: This study aims to identify electrode contact location and to assess frequency deviation between predicted and actual frequency allocation maps in cochlear implant (CI) users.
Study design and methods: This is a retrospective clinical study. Flat-panel computed tomography (FPCT) scans were collected for 17 CI users. Cochlear length was measured using three-dimensional curved multiplanar reconstruction on high-resolution secondary reconstructions. Each electrode's percentage of distance from the base of the helicotrema was measured, and a modified Greenwood's function was applied. The patients' frequency allocation maps were retrieved from electronic medical records and compared with their calculated characteristic frequencies.
Results: Our results revealed that reprogramming based on FPCT imaging findings might improve 83% (n = 216) of 260 electrode contacts. The most basal and apical electrodes (12, 11, 10, 5, 4, 3, 2, and 1) most consistently deviated (>83% of the time) from their theoretical characteristic frequencies; the basal electrodes undershot and the apical electrodes overshot their theoretical values. Frequency mismatch between the characteristic frequencies of auditory neurons and programmed center frequencies ranges from 0.41 to 1.51 in octave bands.
Conclusions: Using FPCT imaging and a modified Greenwood's function, we identify a mathematical discrepancy between theoretical and actual CI placement with respect to frequency-place mapping. We demonstrate a clinically reproducible and direct assessment of frequency-place mismatch. Our individualized calculations account for inter-individual variability in cochlear lengths, operative differences in insertion depths, and electrode array kinking within the cochlea. The benefits of allocating electrode contact frequencies to their tonotopy-derived locations in the cochlea were not investigated in this study, and future prospective trials are needed to demonstrate the consequences of personalized pitch mapping for CI users with respect to speech and pitch perception.