Volumetric in vivo imaging of intracochlear microstructures in mice by high-speed spectral domain optical coherence tomography

Optical Coherence Tomography-Based Atlas of the Human Cochlear Hook Region

Advancements in intracochlear diagnostics, as well as prosthetic and regenerative inner ear therapies, rely on a good understanding of cochlear microanatomy. The human cochlea is very small and deeply embedded within the densest skull bone, making nondestructive visualization of its internal microstructures extremely challenging. Current imaging techniques used in clinical practice, such as MRI and CT, fall short in their resolution to visualize important intracochlear landmarks, and histological analysis of the cochlea cannot be performed on living patients without compromising their hearing. Recently, optical coherence tomography (OCT) has been shown to be a promising tool for nondestructive micrometer resolution imaging of the mammalian inner ear. Various studies performed on human cadaveric tissue and living animals demonstrated the ability of OCT to visualize important cochlear microstructures (scalae, organ of Corti, spiral ligament, and osseous spiral lamina) at micrometer resolution. However, the interpretation of human intracochlear OCT images is non-trivial for researchers and clinicians who are not yet familiar with this novel technology. In this study, we present an atlas of intracochlear OCT images, which were acquired in a series of 7 fresh and 10 fresh-frozen human cadaveric cochleae through the round window membrane and describe the qualitative characteristics of visualized intracochlear structures. Likewise, we describe several intracochlear abnormalities, which could be detected with OCT and are relevant for clinical practice.

Outer hair cell driven reticular lamina mechanical distortion in living cochleae

Cochlear distortions afford researchers and clinicians a glimpse into the conditions and properties of inner ear signal processing mechanisms. Until recently, our examination of these distortions has been limited to measuring the vibration of the basilar membrane or recording acoustic distortion output in the ear canal. Despite its importance, the generation mechanism of cochlear distortion remains a substantial task to understand. The ability to measure the vibration of the reticular lamina in rodent models is a recent experimental advance. Surprising mechanical properties have been revealed. These properties merit both discussion in context with our current understanding of distortion, and appraisal of the significance of new interpretations of cochlear mechanics. This review focusses on some of the recent data from our research groups and discusses the implications of these data on our understanding of vocalization processing in the periphery, and their influence upon future experimental directions. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

In Vivo Cochlear imaging provides a tool to study endolymphatic hydrops

Exposure to noise trauma, such as that from improvised explosive devices, can lead to sensorineural hearing loss and a reduced quality of life. In order to elucidate the mechanisms underlying noise-induced hearing loss, we have adapted optical coherence tomography (OCT) for real-time cochlear visualization in live mice after blast exposure. We demonstrated that endolymphatic hydrops develops following blast injury, and that this phenomenon may be associated with glutamate excitotoxicity and cochlear synaptopathy. Additionally, osmotic stabilization of endolymphatic hydrops partially rescues cochlear synapses after blast trauma. OCT is thus a valuable research tool for investigating the mechanisms underlying acoustic trauma and dynamic changes in endolymph volume. It may also help with the diagnosis and treatment of human hearing loss and/or vertigo in the near future.

Quantitative spectroscopic comparison of the optical properties of mouse cochlea microstructures using optical coherence tomography at 1.06 µm and 1.3 µm wavelengths

Currently, the cochlear implantation procedure mainly relies on using a hand lens or surgical microscope, where the success rate and surgery time strongly depend on the surgeon's experience. Therefore, a real-time image guidance tool may facilitate the implantation procedure. In this study, we performed a systematic and quantitative analysis on the optical characterization of ex vivo mouse cochlear samples using two swept-source optical coherence tomography (OCT) systems operating at the 1.06-µm and 1.3-µm wavelengths. The analysis results demonstrated that the 1.06-µm OCT imaging system performed better than the 1.3-µm OCT imaging system in terms of the image contrast between the cochlear conduits and the neighboring cochlear bony wall structure. However, the 1.3-µm OCT imaging system allowed for greater imaging depth of the cochlear samples because of decreased tissue scattering. In addition, we have investigated the feasibility of identifying the electrode of the cochlear implant within the ex vivo cochlear sample with the 1.06-µm OCT imaging. The study results demonstrated the potential of developing an image guidance tool for the cochlea implantation procedure as well as other otorhinolaryngology applications.

Mammalian Cochlear Hair Cell Imaging Using Optical Coherence Tomography (OCT): A Preliminary Study

OBJECTIVES

This study aimed to investigate the feasibility of using optical coherence tomography (OCT) to provide information about cochlear microanatomy at a cellular level, specifically of cochlear hair cells in mammals.

MATERIALS AND METHODS

A total of 10 Sprague-Dawley rats were divided into 2 experimental groups for comparing the arrangement of normal and damaged hair cells. Postnatal day 3 Sprague-Dawley rats were used to test the swept-source OCT system, and the images recorded were compared with fluorescence microscope images.

RESULTS

Intracochlear structures (the inner hair cells, outer hair cells, and auditory nerve fibers) were clearly visualized at the individual cellular level.

CONCLUSION

These images reflect the ability of OCT to provide images of the inner hair cells, outer hair cells, and auditory nerve fibers (ex vivo). OCT is a promising technology, and these findings could be used to encourage research in the area of cochlear microstructure imaging in the future.

Live imaging and functional changes of the inner ear in an animal model of Meniere's disease

The symptoms of Meniere's disease (MD) are generally considered to be related to endolymphatic hydrops (EH). There are many recent reports supporting the possibility that vasopressin (VP) is closely linked to the formation of EH in Meniere's disease. Based on this, we developed a clinically relevant animal model of Meniere's disease in which a VP type 2 receptor agonist was administered after electrocauterization of the endolymphatic sac. We report live imaging of the internal structure, and functional changes of the inner ear after electrocauterization of the endolymphatic sac and administration of a VP type 2 receptor agonist. In this model, the development of EH was visualized in vivo using optical coherence tomography, there was no rupture of Reissner's membrane, and low-tone hearing loss and vertiginous attacks were observed. This study suggested that acute attacks are caused by the abrupt development of EH. This is the first report of live imaging of the development of EH induced by the administration of a VP type 2 receptor agonist.

High-resolution Imaging of the Human Cochlea through the Round Window by means of Optical Coherence Tomography

The human cochlea is deeply embedded in the temporal bone and surrounded by a thick otic capsule, rendering its internal structure inaccessible for direct visualization. Clinical imaging techniques fall short of their resolution for imaging of the intracochlear structures with sufficient detail. As a result, there is a lack of knowledge concerning best practice for intracochlear therapy placement, such as cochlear implantation. In the past decades, optical coherence tomography (OCT) has proven valuable for non-invasive, high-resolution, cross-sectional imaging of tissue microstructure in various fields of medicine, including ophthalmology, cardiology and dermatology. There is an upcoming interest for OCT imaging of the cochlea, which so far was mostly carried out in small animals. In this temporal bone study, we focused on high-resolution imaging of the human cochlea. The cochlea was approached through mastoidectomy and posterior tympanotomy, both standard surgical procedures. A commercially available spectral-domain OCT imaging system was used to obtain high-resolution images of the cochlear hook region through the intact round window membrane in four cadaveric human temporal bones. We discuss the qualitative and quantitative characteristics of intracochlear structures on OCT images and their importance for cochlear implant surgery.

Investigation of inner ear anatomy in mouse using X-ray phase contrast tomography

A thorough understanding of inner ear anatomy is important for investigators. However, investigation of the mouse inner ear is difficult due to the limitations of imaging techniques. X-ray phase contrast tomography increases contrast 100-1,000 times compared with conventional X-ray imaging. This study aimed to investigate inner ear anatomy in a fresh post-mortem mouse using X-ray phase contrast tomography and to provide a comprehensive atlas of microstructures with less tissue deformation. All experiments were performed in accordance with our institution's guidelines on the care and use of laboratory animals. A fresh mouse cadaver was scanned immediately after sacrifice using an inline phase contrast tomography system. Slice images were reconstructed using a filtered back-projection (FBP) algorithm. Standardized axial and coronal planes were adjusted with a multi-planar reconstruction method. Some three-dimensional (3D) objects were reconstructed by surface rendering. The characteristic features of microstructures, including otoconia masses of the saccular and utricular maculae, superior and inferior macula cribrosae, single canal, modiolus, and osseous spiral lamina, were described in detail. Spatial positions and relationships of the vestibular structures were exhibited in 3D views. This study investigated mouse inner ear anatomy and provided a standardized presentation of microstructures. In particular, otoconia masses were visualized in their natural status without contrast for the first time. The comprehensive anatomy atlas presented in this study provides an excellent reference for morphology studies of the inner ear.

In vivo detection of nanometer-scale structural changes of the human tympanic membrane in otitis media

Otitis media (OM) is a common ear infection and a leading cause of conductive hearing loss in the pediatric population. Current technologies such as otoscopy, pneumatic otoscopy, tympanometry, and acoustic reflectometry are used to diagnose OM, which can reasonably diagnose the infection with a sensitivity and specificity of 50-90% and 60-90%, respectively. However, these techniques provide limited information about the physical architecture of the tympanic membrane (TM), or what may lie behind it. Here, we report the detection of nanometer-scale structural changes of the TM using nano-sensitive optical coherence tomography (nsOCT). In total, an image dataset from 65 pediatric subjects from three different groups (normal, acute OM, and chronic OM) and with longitudinal image-based analysis of ear infections were included in this study. The nsOCT data were correlated with physician diagnosis and with OCT thickness measurements and were found to be in good agreement with these results. We report that nsOCT detects in vivo structural deformations of the TM earlier than OCT alone, and enhances the detection sensitivity of OCT measurements. This unique technique for early detection of nano-scale structural modifications in the TM has the potential to aid in our understanding of microbiological effects, and possibly for early diagnosis and more effective treatment of OM.

Endoscopic optical coherence tomography enables morphological and subnanometer vibratory imaging of the porcine cochlea through the round window

A highly phase stable hand-held (HH) endoscopic system has been developed for optical coherence tomography and vibrometry. Designed to transit the ear canal to the middle ear space and peer through the round window (RW), it is capable of imaging the vibratory function of the cochlear soft tissues with subnanometer scale sensitivity. A side-looking, 9 cm long rigid endoscope with a distal diameter of 1.2 mm, was able to fit within the RW niche and provide imaging access. The phase stability was achieved in part by fully integrating a Michelson interferometer into the HH device. Ex vivo imaging of a domestic pig demonstrated the system's ability for functional vibratory imaging of the cochlea via the RW.

ELHnet: a convolutional neural network for classifying cochlear endolymphatic hydrops imaged with optical coherence tomography

Detection of endolymphatic hydrops is important for diagnosing Meniere's disease, and can be performed non-invasively using optical coherence tomography (OCT) in animal models as well as potentially in the clinic. Here, we developed ELHnet, a convolutional neural network to classify endolymphatic hydrops in a mouse model using learned features from OCT images of mice cochleae. We trained ELHnet on 2159 training and validation images from 17 mice, using only the image pixels and observer-determined labels of endolymphatic hydrops as the inputs. We tested ELHnet on 37 images from 37 mice that were previously not used, and found that the neural network correctly classified 34 of the 37 mice. This demonstrates an improvement in performance from previous work on computer-aided classification of endolymphatic hydrops. To the best of our knowledge, this is the first deep CNN designed for endolymphatic hydrops classification.

Computer-aided detection and quantification of endolymphatic hydrops within the mouse cochlea in vivo using optical coherence tomography

Diseases that cause hearing loss and/or vertigo in humans such as Meniere’s disease are often studied using animal models. The volume of endolymph within the inner ear varies with these diseases. Here, we used a mouse model of increased endolymph volume, endolymphatic hydrops, to develop a computer-aided objective approach to measure endolymph volume from images collected <italic<in vivo</italic< using optical coherence tomography. The displacement of Reissner’s membrane from its normal position was measured in cochlear cross sections. We validated our computer-aided measurements with manual measurements and with trained observer labels. This approach allows for computer-aided detection of endolymphatic hydrops in mice, with test performance showing sensitivity of 91% and specificity of 87% using a running average of five measurements. These findings indicate that this approach is accurate and reliable for classifying endolymphatic hydrops and quantifying endolymph volume.

Micro-optical coherence tomography of the mammalian cochlea

The mammalian cochlea has historically resisted attempts at high-resolution, non-invasive imaging due to its small size, complex three-dimensional structure, and embedded location within the temporal bone. As a result, little is known about the relationship between an individual’s cochlear pathology and hearing function, and otologists must rely on physiological testing and imaging methods that offer limited resolution to obtain information about the inner ear prior to performing surgery. Micro-optical coherence tomography (μOCT) is a non-invasive, low-coherence interferometric imaging technique capable of resolving cellular-level anatomic structures. To determine whether μOCT is capable of resolving mammalian intracochlear anatomy, fixed guinea pig inner ears were imaged as whole temporal bones with cochlea in situ. Anatomical structures such as the tunnel of Corti, space of Nuel, modiolus, scalae, and cell groupings were visualized, in addition to individual cell types such as neuronal fibers, hair cells, and supporting cells. Visualization of these structures, via volumetrically-reconstructed image stacks and endoscopic perspective videos, represents an improvement over previous efforts using conventional OCT. These are the first μOCT images of mammalian cochlear anatomy, and they demonstrate μOCT’s potential utility as an imaging tool in otology research.

Decalcification using ethylenediaminetetraacetic acid for clear microstructure imaging of cochlea through optical coherence tomography

The aim of this study was to analyze the effectiveness of decalcification using ethylenediaminetetraacetic acid (EDTA) as an optical clearing method to enhance the depth visibility of internal soft tissues of cochlea. Ex vivo mouse and guinea pig cochlea samples were soaked in EDTA solutions for decalcification, and swept source optical coherence tomography (OCT) was used as imaging modality to monitor the decalcified samples consecutively. The monitored noninvasive cross-sectional images showed that the mouse and guinea pig cochlea samples had to be decalcified for subsequent 7 and 14 days, respectively, to obtain the optimal optical clearing results. Using this method, difficulties in imaging of internal cochlea microstructures of mice could be evaded. The obtained results verified that the depth visibility of the decalcified ex vivo samples was enhanced.

Investigation of middle ear anatomy and function with combined video otoscopy-phase sensitive OCT

We report the development of a novel otoscopy probe for assessing middle ear anatomy and function. Video imaging and phase-sensitive optical coherence tomography are combined within the same optical path. A sound stimuli channel is incorporated as well to study middle ear function. Thus, besides visualizing the morphology of the middle ear, the vibration amplitude and frequency of the eardrum and ossicles are retrieved as well. Preliminary testing on cadaveric human temporal bone models has demonstrated the capability of this instrument for retrieving middle ear anatomy with micron scale resolution, as well as the vibration of the tympanic membrane and ossicles with sub-nm resolution.

Optical Coherence Tomography to Measure Sound-Induced Motions Within the Mouse Organ of Corti In Vivo

The measurement of mechanical vibrations within the living cochlea is critical to understanding the first nonlinear steps in auditory processing, hair cell stimulation, and cochlear amplification. However, it has proven to be a challenging endeavor. This chapter describes how optical coherence tomography (OCT) can be used to measure vibrations within the tissues of the organ of Corti. These experimental measurements can be performed within the unopened cochlea of living mice routinely and reliably.

Quantitative polarized light microscopy of human cochlear sections

Dysfunction of the inner ear is the most common cause of sensorineural hearing loss, which is the most common sensory deficit worldwide. Conventional imaging modalities are unable to depict the microanatomy of the human inner ear, hence the need to explore novel imaging modalities. We provide the first characterization of the polarization dependent optical properties of human cochlear sections using quantitative polarized light microscopy (qPLM). Eight pediatric cadaveric cochlear sections, aged 0 (term) to 24 months, were selected from the US National Temporal Bone Registry, imaged with qPLM and analyzed using Image J. Retardance of the bony otic capsule and basilar membrane were substantially higher than that of the stria vascularis, spiral ganglion neurons, organ of Corti and spiral ligament across the half turns of the spiraling cochlea. qPLM provides quantitative information about the human inner ear, and awaits future exploration in vivo.

Real-time automated thickness measurement of the in vivo human tympanic membrane using optical coherence tomography

BACKGROUND

Otitis media (OM), an infection in the middle ear, is extremely common in the pediatric population. Current gold-standard methods for diagnosis include otoscopy for visualizing the surface features of the tympanic membrane (TM) and making qualitative assessments to determine middle ear content. OM typically presents as an acute infection, but can progress to chronic OM, and after numerous infections and antibiotic treatments over the course of many months, this disease is often treated by surgically inserting small tubes in the TM to relieve pressure, enable drainage, and provide aeration to the middle ear. Diagnosis and monitoring of OM is critical for successful management, but remains largely qualitative.

METHODS

We have developed an optical coherence tomography (OCT) system for high-resolution, depth-resolved, cross-sectional imaging of the TM and middle ear content, and for the quantitative assessment of in vivo TM thickness including the presence or absence of a middle ear biofilm. A novel algorithm was developed and demonstrated for automatic, real-time, and accurate measurement of TM thickness to aid in the diagnosis and monitoring of OM and other middle ear conditions. The segmentation algorithm applies a Hough transform to the OCT image data to determine the boundaries of the TM to calculate thickness.

RESULTS

The use of OCT and this segmentation algorithm is demonstrated first on layered phantoms and then during real-time acquisition of in vivo OCT from humans. For the layered phantoms, measured thicknesses varied by approximately 5 µm over time in the presence of large axial and rotational motion. In vivo data also demonstrated differences in thicknesses both spatially on a single TM, and across normal, acute, and chronic OM cases.

CONCLUSIONS

Real-time segmentation and thickness measurements of image data from both healthy subjects and those with acute and chronic OM demonstrate the use of OCT and this algorithm as a robust, quantitative, and accurate method for use during real-time in vivo human imaging.

Noninvasive depth-resolved optical measurements of the tympanic membrane and middle ear for differentiating otitis media

OBJECTIVE/HYPOTHESIS

In this study, optical coherence tomography (OCT) is used to noninvasively and quantitatively determine tympanic membrane (TM) thickness and the presence and thickness of any middle-ear biofilm located behind the TM. These new metrics offer the potential to differentiate normal, acute, and chronic otitis media (OM) infections in pediatric subjects.

STUDY DESIGN

Case series with comparison group.

METHODS

The TM thickness of 34 pediatric subjects was acquired using a custom-built, handheld OCT system following a traditional otoscopic ear exam.

RESULTS

Overall thickness (TM and any associated biofilm) was shown to be statistically different for normal, acute, and chronic infection groups (normal-acute and normal-chronic: P value < 0.001; acute-chronic: P value = 0.0016). Almost all observed scans from the chronic group had an accompanying biofilm structure. When the thickness of the TM and biofilm were considered separately in chronic OM, the chronic TM thickness correlated with the normal group (P value = 0.68) yet was still distinct from the acute OM group (P value < 0.001), indicating that the TM in chronic OM returns to relatively normal thickness levels.

CONCLUSION

Identifying these physical changes in vivo provides new metrics for noninvasively and quantitatively differentiating normal, acute, and chronic OM. This new diagnostic information has the potential to assist physicians to more effectively and efficiently screen, manage, and refer patients based on quantitative data.

LEVEL OF EVIDENCE

4.

Phase-sensitive optical coherence tomography using an Vernier-tuned distributed Bragg reflector swept laser in the mouse middle ear

Phase-sensitive optical coherence tomography (PhOCT) offers exquisite sensitivity to mechanical vibration in biological tissues. There is growing interest in using PhOCT for imaging the nanometer scale vibrations of the ear in animal models of hearing disorders. Swept-source-based systems offer fast acquisition speeds, suppression of common mode noise via balanced detection, and good signal roll-off. However, achieving high phase stability is difficult due to nonlinear laser sweeps and trigger jitter in a typical swept laser source. Here, we report on the initial application of a Vernier-tuned distributed Bragg reflector (VT-DBR) swept laser as the source for a fiber-based PhOCT system. The VT-DBR swept laser is electronically tuned and precisely controls sweeps without mechanical movement, resulting in highly linear sweeps with high wavelength stability and repeatability. We experimentally measured a phase sensitivity of 0.4 pm standard deviation, within a factor of less than 2 of the computed shot-noise limit. We further demonstrated the system by making ex vivo measurements of the vibrations of the mouse middle ear structures.

In vivo imaging of mouse cochlea by optical coherence tomography

HYPOTHESIS

Cochlear pathology can be evaluated in living animals using optical coherence tomography (OCT).

BACKGROUND

The current imaging methods available for the detailed analysis of cochlear pathology in a clinical setting provide only limited information. Thus, a cochlear imaging modality with high definition is needed for improving the diagnosis of cochlear pathology. OCT has been used in other fields for obtaining high-resolution subsurface images, and its use could potentially be extended to the analysis of cochlear pathogenesis.

METHODS

Slc26a4(-/-) mice, which generate endolymphatic hydrops, and their littermates were used in this study. Auditory function was monitored by the auditory brainstem responses (ABR). After the mice were placed under general anesthesia, OCT images of the cochlea were captured. The cochlea was subsequently dissected out and histologically evaluated. Three or 7 days later, the wild-type mice cochleae were visualized again.

RESULTS

In ABR assessments, Slc26a4(-/-) mice showed severe hearing loss, while no significant hearing loss was found in Slc26a4(+/-) or Slc26a4(+/+) mice. OCT demonstrated normal morphology in the cochlea of both Slc26a4(+/-) and Slc26a4(+/+) mice, including the location of Reissner's membrane. Meanwhile, in Slc26a4(-/-) mice, obvious dislocation of Reissner's membrane was observed, indicating severe endolymphatic hydrops. These findings in the OCT images were consistent with the histologic results for the cochlear morphology, as observed with hematoxylin and eosin staining. Three or 7 days later, wild-type cochleae were successfully visualized using OCT, and no otitis media or labyrinthitis was observed.

CONCLUSION

OCT can be applied in the detection of endolymphatic hydrops in living mice, indicating the potential of OCT for cochlear imaging analyses for clinical use in the near future.

In vivo imaging of middle-ear and inner-ear microstructures of a mouse guided by SD-OCT combined with a surgical microscope

We developed an augmented-reality system that combines optical coherence tomography (OCT) with a surgical microscope. By sharing the common optical path in the microscope and OCT, we could simultaneously acquire OCT and microscope views. The system was tested to identify the middle-ear and inner-ear microstructures of a mouse. Considering the probability of clinical application including otorhinolaryngology, diseases such as middle-ear effusion were visualized using in vivo mouse and OCT images simultaneously acquired through the eyepiece of the surgical microscope during surgical manipulation using the proposed system. This system is expected to realize a new practical area of OCT application.

Changes in cochlear blood flow in mice due to loud sound exposure measured with Doppler optical microangiography and laser Doppler flowmetry

In this work we determined the contributions of loud sound exposure (LSE) on cochlear blood flow (CoBF) in an in vivo anesthetized mouse model. A broadband noise system (20 kHz bandwidth) with an intensity of 119 dB SPL, was used for a period of one hour to produce a loud sound stimulus. Two techniques were used to study the changes in blood flow, a Doppler optical microangiography (DOMAG) system; which can measure the blood flow within individual cochlear vessels, and a laser Doppler flowmetry (LDF) system; which averages the blood flow within a volume (a hemisphere of ~1.5 mm radius) of tissue. Both systems determined that the blood flow within the cochlea is reduced due to the LSE stimulation.

Quantitative polarized light microscopy of unstained mammalian cochlear sections

Hearing loss is the most common sensory deficit in the world, and most frequently it originates in the inner ear. Yet, the inner ear has been difficult to access for diagnosis because of its small size, delicate nature, complex three-dimensional anatomy, and encasement in the densest bone in the body. Evolving optical methods are promising to afford cellular diagnosis of pathologic changes in the inner ear. To appropriately interpret results from these emerging technologies, it is important to characterize optical properties of cochlear tissues. Here, we focus on that characterization using quantitative polarized light microscopy (qPLM) applied to unstained cochlear sections of the mouse, a common animal model of human hearing loss. We find that the most birefringent cochlear materials are collagen fibrils and myelin. Retardance of the otic capsule, the spiral ligament, and the basilar membrane are substantially higher than that of other cochlear structures. Retardance of the spiral ligament and the basilar membrane decrease from the cochlear base to the apex, compared with the more uniform retardance of other structures. The intricate structural details revealed by qPLM of unstained cochlear sections ex vivo strongly motivate future application of polarization-sensitive optical coherence tomography to human cochlea in vivo.

In vivo vibrometry inside the apex of the mouse cochlea using spectral domain optical coherence tomography

Sound transduction within the auditory portion of the inner ear, the cochlea, is a complex nonlinear process. The study of cochlear mechanics in large rodents has provided important insights into cochlear function. However, technological and experimental limitations have restricted studies in mice due to their smaller cochlea. These challenges are important to overcome because of the wide variety of transgenic mouse strains with hearing loss mutations that are available for study. To accomplish this goal, we used spectral domain optical coherence tomography to visualize and measure sound-induced vibrations of intracochlear tissues. We present, to our knowledge, the first vibration measurements from the apex of an unopened mouse cochlea.

In vivo vibrometry inside the apex of the mouse cochlea using spectral domain optical coherence tomography

Sound transduction within the auditory portion of the inner ear, the cochlea, is a complex nonlinear process. The study of cochlear mechanics in large rodents has provided important insights into cochlear function. However, technological and experimental limitations have restricted studies in mice due to their smaller cochlea. These challenges are important to overcome because of the wide variety of transgenic mouse strains with hearing loss mutations that are available for study. To accomplish this goal, we used spectral domain optical coherence tomography to visualize and measure sound-induced vibrations of intracochlear tissues. We present, to our knowledge, the first vibration measurements from the apex of an unopened mouse cochlea.

Effects of hypoxia on cochlear blood flow in mice evaluated using Doppler optical microangiography

Reduced cochlear blood flow (CoBF) is a main contributor to hearing loss. Studying CoBF has remained a challenge due to the lack of available tools. Doppler optical microangiography (DOMAG), a method to quantify single-vessel absolute blood flow, and laser Doppler flowmetry (LDF), a method for measuring the relative blood flow within a large volume of tissue, were used for determining the changes in CoBF due to systemic hypoxia in mice. DOMAG determined the change in blood flow in the apical turn (AT) with single-vessel resolution, while LDF averaged the change in the blood flow within a large volume of the cochlea (hemisphere with ∼1 to 1.5 mm radius). Hypoxia was induced by decreasing the concentration of oxygen-inspired gas, so that the oxygen saturation was reduced from >95% to ∼80%. DOMAG determined that during hypoxia the blood flow in two areas of the AT near and far from the helicotrema were increased and decreased, respectively. The LDF detected a decrease in blood flow within a larger volume of the cochlea (several turns averaged together). Therefore, the use of DOMAG as a tool for studying cochlear blood flow due to its ability to determine absolute flow values with single-vessel resolution was proposed.

Quantitative X-ray tomography of the mouse cochlea

Imaging with hard X-rays allows visualizing cochlear structures while maintaining intrinsic qualities of the tissue, including structure and size. With coherent X-rays, soft tissues, including membranes, can be imaged as well as cells making use of the so-called in-line phase contrast. In the present experiments, partially coherent synchrotron radiation has been used for micro-tomography. Three-dimensional reconstructions of the mouse cochlea have been created using the EM3D software and the volume has been segmented in the Amira Software Suite. The structures that have been reconstructed include scala tympani, scala media, scala vestibuli, Reissner's membrane, basilar membrane, tectorial membrane, organ of Corti, spiral limbus, spiral ganglion and cochlear nerve. Cross-sectional areas of the scalae were measured. The results provide a realistic and quantitative reconstruction of the cochlea.

Quantitative imaging of cochlear soft tissues in wild-type and hearing-impaired transgenic mice by spectral domain optical coherence tomography

Human hearing loss often occurs as a result of damage or malformations to the functional soft tissues within the cochlea, but these changes are not appreciable with current medical imaging modalities. We sought to determine whether optical coherence tomography (OCT) could assess the soft tissue structures relevant to hearing using mouse models. We imaged excised cochleae with an altered tectorial membrane and during normal development. The soft tissue structures and expected anatomical variations were visible using OCT, and quantitative measurements confirmed the ability to detect critical changes relevant to hearing.

Phase-sensitive optical coherence tomography imaging of the tissue motion within the organ of Corti at a subnanometer scale: a preliminary study

Hearing loss can mean severe impairment to the quality of life. However, the biomechanical mechanisms of how the hearing organ, i.e., the organ of Corti (OC), responds to sound are still elusive, largely because there is currently no means available to image the 3-D motion characteristics of the OC. We present a novel use of the phase-sensitive spectral domain optical coherence tomography (PSOCT) to characterize the motion of cellular compartments within the OC at a subnanometer scale. The PSOCT system operates at 1310 nm with a spatial resolution of ∼16 μm and an imaging speed of 47,000 A-lines/s. The phase changes of the spectral interferograms induced by the localized tissue motion are used to quantify the vibration magnitude. Fourier transform analysis of the phase changes improves the system sensitivity to sense minute vibrations smaller than 1 nm. We demonstrate that the PSOCT system is feasible to image the meaningful vibration of cellular compartments within the OC with an unprecedented sensitivity down to ∼0.5 Å.