Most of the optical coherence tomographic (OCT) systems for high resolution imaging of biological specimens are
based on refractive type microscope objectives, which are optimized for specific wave length of the optical source. In
this study, we present the feasibility of using commercially available reflective type objective for high sensitive and
high resolution structural and functional imaging of cochlear microstructures of an excised guinea pig through intact
temporal bone. Unlike conventional refractive type microscopic objective, reflective objective are free from chromatic
aberrations due to their all-reflecting nature and can support a broadband of spectrum with very high light collection
efficiency.
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.
Outer hair cell (OHC) is widely accepted as the origin of cochlear amplification, a mechanism that accounts for the
extreme sensitivity of the mammalian hearing. The key process of cochlear amplification is the reverse transduction,
where the OHC changes its length under electrical stimulation. In this study, we developed a method to modulate
electro-mechanical transduction with an optogenetic approach based on channelrhodopsin 2 (ChR2), a direct lightactivated
non-selective cation channel (NSCC). We specifically expressed ChR2 in mouse cochlea OHCs through in
uterus injection of adenovirus vector with ChR2 in fusion with the fluorescent marker tdTomato. We also transfected
ChR2(H134R), a point mutant of ChR2, with plasmid to an auditory cell line (HEI-OC1). With whole cell recording, we
found that blue light (470 nm) elicited a current with a reversal potential around zero in both mouse OHCs and HEI-OC1
cells and generated depolarization in both cell types.
We present an optical vibrometer based on delay-encoded, dual-beamlet phase-sensitive Fourier domain interferometric system to provide depth-resolved subnanometer scale vibration information from scattering biological specimens. System characterization, calibration, and preliminary vibrometry with biological specimens were performed. The proposed system has the potential to provide both amplitude and direction of vibration of tissue microstructures on a single two-dimensional plane.
Direct measurement of absolute vibration parameters from different locations within the mammalian organ of Corti is
crucial for understanding the hearing mechanics such as how sound propagates through the cochlea and how sound
stimulates the vibration of various structures of the cochlea, namely, basilar membrane (BM), recticular lamina, outer
hair cells and tectorial membrane (TM). In this study we demonstrate the feasibility a modified phase-sensitive spectral
domain optical coherence tomography system to provide subnanometer scale vibration information from multiple angles
within the imaging beam. The system has the potential to provide depth resolved absolute vibration measurement of
tissue microstructures from each of the delay-encoded vibration images with a noise floor of ~0.3nm at 200Hz.
The differential motion of the organ of Corti has been expected as a result of the outer hair cell force,
believed to be necessary for the cochlear amplifier. In vitro experiments have been performed to demonstrate
this motion but the in vivo data was unavailable due to the technical difficulties. Using a specially-designed
time-domain optical coherence tomography system, we performed in vivo imaging and vibration measurement
at the sensitive base of the guinea pig cochlea. This technique, for the first time, provides in vivo information
about the internal vibration of the organ of Corti. At low sound level, when the cochlea is more sensitive, top
surface of the organ of Corti, the reticular lamina (RL) showed tuning at a higher frequency than of the bottom
surface, basilar membrane (BM) and its vibration amplitude is 2-3 times of that of the BM. Corresponding to the
frequency difference, the phase of RL vibration is lead to that of the BM. Both the amplitude gain and the phase
lead on RL is level dependent. This suggests that they are related to the cochlear amplification. The amplitude
gain at the RL is an enhancement of the BM motion for stimulating the stereocillia. The advance in time of RL
vibration can prepare proper timing of stereocillia stimulation for the cochlear amplification.
Hearing in mammals, depend on an amplifying motion which hypothetically uses force from outer hair
cells (OHC) motility to enhance sound induced vibration of the organ of Corti of cochlea. In this
hypothesis the differential motion among key structures in this organ and the timing of the OHC force
generation is essential for cochlear amplification to occur. Using a time domain optical coherence
tomography system which allows us to make vibration measurements we were able to measure differential
motion of two functionally important surfaces, namely, basilar membrane and reticular lamina. The
reticular lamina vibrates at higher amplitude than the basilar membrane and has significant phase lead over
basilar membrane vibration. The differential motion, that is, different amplitude and phase of vibration,
become less as the energy of the sound stimulus is increased and the amplification processes in the organ of
Corti are quenched.
KEYWORDS: Optical coherence tomography, Interferometry, In vivo imaging, Heterodyning, Interferometers, System on a chip, Americium, Vibrometry, Bone, Ear
Studying the sound stimulated vibrations of various membranes that form the complex structure of the organ of Corti in
the cochlea of the inner ear is essential for understanding how the travelling sound wave of the basilar membrane couples
its energy to the organ structures. In this paper we report the feasibility of using phase-sensitive Fourier domain optical
coherence tomography (FD-OCT) to image the vibration of various micro-structures of the cochlea at the same time. An
excised cochlea of a guinea pig was stimulated using sounds at various frequencies and vibration image was obtained.
When measuring the apex area, vibration signal from different turns, which have different best response frequencies are
obtained in the same image. The method has the potential to measure the response from a much wider region of the
cochlea than any other currently used method. The noise floor for vibration image for the system at 200 Hz was ~0.3nm.
An optical coherence tomography (OCT) system was built to acquire in vivo, both images and vibration
measurements of the organ of Corti of the guinea pig. The organ of Corti was viewed through a ~500-μm
diameter hole in the bony wall of the scala tympani of the first cochlear turn. In imaging mode, the image
was acquired as reflectance R(x,z). In vibration mode, the basilar membrane (BM) or reticular lamina (RL)
was selected based on the image. Under software control, the system would move the scanning mirrors to
bring the sensing volume of the measurement to the desired tissue location. To address the gain stability
problem of the homodyne OCT system, arising from the system moving in and out of the quadrature point
and also to resolve the 180 degree ambiguity in the phase measurement using an interferometer, a vibration
calibration method is developed by adding a vibrating source to the reference arm to monitor the operating
point of the interferometric system. Amplitude gain and phase of various cochlear membranes was
measured for different sound pressure level (SPL) varying from 65dB SPL to 93 dB SPL.
KEYWORDS: Optical coherence tomography, Calibration, Interferometry, Reflectivity, In vivo imaging, Mirrors, Signal detection, Tissues, Vibrometry, Bone
An optical coherence tomography (OCT) system is built to acquire in vivo both images and vibration measurements of the organ of Corti of the guinea pig. The organ of Corti is viewed through a ~300-µm-diam hole in the bony wall of the cochlea at the scala tympani of the first cochlear turn. In imaging mode, the image is acquired as reflectance R(x,z). In vibration mode, the basilar membrane (BM) or reticular lamina (RL) are selected by the investigator interactively from the R(x,z) image. Under software control, the system moves the scanning mirrors to bring the sensing volume of the measurement to the desired membrane location. In vivo images of the organ of Corti are presented, indicating reflectance signals from the BM, RL, tectorial membrane, and Reissner's membrane. The tunnel of Corti and the inner sulcus are also visible in the images. Vibrations of ±2 and ±22 nm are recorded in the BM in response to low and high sound levels at 14 kHz above a noise floor of 0.2 nm.
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