KEYWORDS: Mirrors, Ear, Reflectivity, Cameras, Surgery, Optical coherence tomography, Tissues, Signal processing, Scanning probe lithography, Signal to noise ratio
Sound processing in the inner ear involves separation of the constituent frequencies along the length of the cochlea. Frequencies relevant to human speech (100 to 500 Hz) are processed in the apex region. Among mammals, the guinea pig cochlear apex processes similar frequencies and is thus relevant for the study of speech processing in the cochlea. However, the requirement for extensive surgery has challenged the optical accessibility of this area to investigate cochlear processing of signals without significant intrusion. A simple method is developed to provide optical access to the guinea pig cochlear apex in two directions with minimal surgery. Furthermore, all prior vibration measurements in the guinea pig apex involved opening an observation hole in the otic capsule, which has been questioned on the basis of the resulting changes to cochlear hydrodynamics. Here, this limitation is overcome by measuring the vibrations through the unopened otic capsule using phase-sensitive Fourier domain optical coherence tomography. The optically and surgically advanced method described here lays the foundation to perform minimally invasive investigation of speech-related signal processing in the cochlea.
In this study, we measure the in vivo apical-turn vibrations of the guinea pig organ of Corti in both axial and radial
directions using phase-sensitive Fourier domain optical coherence tomography. The apical turn in guinea pig cochlea has
best frequencies around 100 – 500 Hz which are relevant for human speech. Prior measurements of vibrations in the
guinea pig apex involved opening the otic capsule, which has been questioned on the basis of the resulting changes to
cochlear hydrodynamics. Here this limitation is overcome by measuring the vibrations through bone without opening the
otic capsule. Furthermore, we have significantly reduced the surgery needed to access the guinea pig apex in the axial
direction by introducing a miniature mirror inside the bulla. The method and preliminary data are discussed in this
article.
Optical interferometry using Fourier domain OCT (FD-OCT) can image structures using vibration as the contrast mechanism. An A-scan measurement of light reflected from a tissue at a position x,y yields an intensity spectrum, I(λ), which is sequentially acquired over a short time period, 88 ms, to yield 1000 spectra at equally spaced time points, I(λ,t), while an audio frequency of sound pressure is imparted to the tissue to induce vibration. Analysis converts I(λ,t) into displacement Δz(z,t) of tissue structures along the z axis. Fourier analysis converts Δz(t) at each z into a vibration spectrum, and the amplitude of vibration at the driving audio frequency is specified. This process is repeated for each x position at a given y, to yield an image of vibration amplitude, A(z,x) [nm]. Hence, vibration amplitude becomes the contrast mechanism. This method is not a new idea, but a derivative of Doppler OCT. This report simply provides a concise algorithm in MATLAB for students wishing to implement vibration measurements using FD-OCT. An example shows the vibration of the ossicles of the middle ear viewed through the intact tympanic membrane.
KEYWORDS: Signal to noise ratio, Optical coherence tomography, Ear, Interference (communication), Signal processing, Solids, Data acquisition, Neodymium, Biomedical engineering, Medical research
Using data from our previously described otoscope1 that uses 1310 nm phase-sensitive spectral domain optical coherence tomography (PS-SDOCT), we demonstrate a software technique for improving the signal-to-noise (SNR). This method is a software post-processing algorithm applicable to generic PS-SDOCT data describing phase versus time at a specific depth position. By sub-sampling the time trace and shifting the phase of the subsamples to maximize their correlation, the subsamples can be coherently averaged, which increases the SNR.
KEYWORDS: Optical coherence tomography, Cameras, Scanning probe lithography, In vivo imaging, Ear, Reflectivity, Tissues, Signal to noise ratio, Data acquisition, Mirrors
A major reason we can perceive faint sounds and communicate in noisy environments is that the outer hair cells of the
organ of Corti enhance the sound-evoked motions inside the cochlea. To understand how the organ of Corti works, we
have built and tested the phase-sensitive Fourier domain optical coherence tomography (PSFDOCT) system. This system
has key advantages over our previous time domain OCT system [1]. The PSFDOCT system has better signal to noise and
simultaneously acquires vibration data from all points along the optical-axis [2]. Feasibility of this system to measure in
vitro cochlear vibrations in the apex was demonstrated earlier [3]. In this study, we measure the in vivo vibrations of the
organ of Corti via round window in live anaesthetized guinea pigs using PSFDOCT. This region of the guinea pig
cochlea responds to very high frequencies (10 - 40 kHz). The current vibration noise floor for native organ of Corti
tissue is 0.03 nm in this frequency range. Sound-induced vibrations of the stapes, which delivers input to the cochlea, are
also measured. The measured vibrations of the organ of Corti demonstrate non-linear compression and active
amplification characteristic of sensitive mammalian cochlea.
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