Abstract
We developed a spectral domain optical coherence tomography (SDOCT) fiber optic probe for imaging and sub-nanometer displacement measurements inside the mammalian cochlea. The probe, 140 μm in diameter, can scan laterally up to 400 μm by means of a piezoelectric bender. Two different sampling rates are used, 10 kHz for high-resolution B-scan imaging, and 100 kHz for displacement measurements in order to span the auditory frequency range of gerbil (~50 kHz). Once the cochlear structures are recognized, the scanning range is gradually decreased and ultimately stopped with the probe pointing at the selected angle to measure the simultaneous displacements of multiple structures inside the organ of Corti (OC). The displacement measurement is based on spectral domain phase microscopy. The displacement noise level depends on the A-scan signal of the structure within the OC and we have attained levels as low as ~0.02 nm in in vivo measurements. The system's broadband infrared light source allows for an imaging depth of ~2.7 mm, and axial resolution of ~3 μm. In future development, the probe can be coupled with an electrode for time-locked voltage and displacement measurements in order to explore the electro-mechanical feedback loop that is key to cochlear processing. Here, we describe the fabrication of the laterally-scanning optical probe, and demonstrate its functionality with in vivo experiments.
Highlights
Spectral Domain Optical Coherence Tomography (SDOCT) is a low-coherence interferometric system developed mainly for imaging, and capable of displacement measurements, using Spectral Domain Phase Microscopy (SDPM) [1]
A summing amplifier is first used to increase the amplitude of the driving waveform by a factor of 10, and to add a DC voltage that shifts the spike out of the power supply range. (Alternatively, the spike can be eliminated with software configuration.) The modified waveform (Fig. 3(B)) is low-pass filtered and AC coupled, and sent to a potentiometer-controlled voltage divider to adjust the waveform’s amplitude, which controls the lateral Field of View (FOV) of the bender
Displacement measurement As we have described previously, the Telesto system is synchronized to the Tucker-Davis Technologies (TDT) data acquisition hardware, to acquire time-locked displacement and ear canal pressure data [21]
Summary
Spectral Domain Optical Coherence Tomography (SDOCT) is a low-coherence interferometric system developed mainly for imaging, and capable of displacement measurements, using Spectral Domain Phase Microscopy (SDPM) [1]. Its steep optical sectioning curve, based on its broadband light source, results in ~3 μm axial resolution [2] This resolution is adequate for displacement measurements in the sensory tissue of the cochlea, whose different structures are separated by distances on the order of 10 μm. We have used the Telesto as a bulk-optics SDOCT system to measure displacements of the cochlea’s sensory tissue through the transparent round window membrane (RWM), at the base of the cochlea [2,6]. The access locations are limited because the cochlea is surrounded by bone, and damage will modify the measured displacements, in particular by reducing the active outer-hair-cell-based process termed cochlear amplification. Two in vivo experiments were done to validate the probe’s usage
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