Picometer scale vibrometry in the human middle ear using a surgical microscope based optical coherence tomography and vibrometry system

Dynamic X-ray Microtomography vs. Laser-Doppler Vibrometry: A Comparative Study

PURPOSE

There are challenges in understanding the biomechanics of the human middle ear, and established methods for studying this system show significant limitations. In this study, we evaluate a novel dynamic imaging technique based on synchrotron X-ray microtomography designed to assess the biomechanical properties of the human middle ear by comparing it to laser-Doppler vibrometry (LDV).

METHODS

We examined three fresh-frozen temporal bones (TB), two donated by white males and one by a Black female, using dynamic synchrotron-based X-ray microtomography for 256 and 512 Hz, stimulated at 110 dB and 120 dB sound pressure level (SPL). In addition, we performed measurements on these TBs using 1D LDV, a well-established method.

RESULTS

The normalized displacement values (µm/Pa) at the umbo and the posterior crus of the stapes are consistent or within 5-10 dB differences between all LDV and dynamic microtomography measurements and previously reported literature references. In general, the overall behavior is similar between the two measurement techniques.

CONCLUSION

In conclusion, our results demonstrate the suitability of dynamic synchrotron-based X-ray microtomography in studying the middle ear's biomechanics. However, this study shows that better standardization regarding acoustic stimulation and measurement points is needed to better compare the two measurement techniques.

Visualizing motions within the cochlea's organ of Corti and illuminating cochlear mechanics with optical coherence tomography

Beginning in 2006, optical coherence tomography (OCT) has been adapted for use as a vibrometer for hearing research. The application of OCT in this field, particularly for studying cochlear mechanics, represents a revolutionary advance over previous technologies. OCT provides detailed evidence of the motions of components within the organ of Corti, extending beyond the first-encountered surface of observation. By imaging through the bony capsule as well as through the round window membrane, OCT has measured vibration at multiple locations along the cochlear spiral, in vivo, under nearly natural conditions. In this document, we present examples of recent research findings to illustrate the applications of OCT in studying cochlear mechanics in both normal and impaired ears.

Generation of a zebrafish neurofibromatosis model via inducible knockout of nf2a/b

Neurofibromatosis type 2 (NF-2) is a dominantly inherited genetic disorder that results from variants in the tumor suppressor gene, neurofibromin 2 (NF2). Here, we report the generation of a conditional zebrafish model of neurofibromatosis established by inducible genetic knockout of nf2a/b, the zebrafish homologs of human NF2. Analysis of nf2a and nf2b expression revealed ubiquitous expression of nf2b in the early embryo, with overlapping expression in the neural crest and its derivatives and in the cranial mesenchyme. In contrast, nf2a displayed lower expression levels. Induction of nf2a/b knockout at early stages increased the proliferation of larval Schwann cells and meningeal fibroblasts. Subsequently, in adult zebrafish, nf2a/b knockout triggered the development of a spectrum of tumors, including vestibular Schwannomas, spinal Schwannomas, meningiomas and retinal hamartomas, mirroring the tumor manifestations observed in patients with NF-2. Collectively, these findings highlight the generation of a novel zebrafish model that mimics the complexities of the human NF-2 disorder. Consequently, this model holds significant potential for facilitating therapeutic screening and elucidating key driver genes implicated in NF-2 onset.

Dynamic X-ray Microtomography vs. Laser-Doppler Vibrometry: A Comparative Study

Purpose

There are challenges in understanding the biomechanics of the human middle ear, and established methods for studying this system show significant limitations. In this study, we evaluate a novel dynamic imaging technique based on synchrotron X-ray microtomography designed to assess the biomechanical properties of the human middle ear by comparing it to laser-Doppler vibrometry (LDV).

Methods

We examined three fresh-frozen temporal bones (TB) using dynamic synchrotron-based X-ray microtomography for 256 Hz and 512 Hz, stimulated at 110 dB and 120 dB SPL. In addition, we performed measurements on these TBs using 1D LDV, a well-established method.

Results

The normalized displacement values (μm/Pa) at the umbo and the posterior crus of the stapes are consistent or within 5-10 dB differences between all LDV and dynamic microtomography measurements and previously reported literature references. In general, the overall behavior is similar between the two measurement techniques.

Conclusion

In conclusion, our results demonstrate the suitability of dynamic synchrotron-based X-ray microtomography in studying the middle ear's biomechanics. However, this study shows that better standardization regarding acoustic stimulation and measurement points is needed to better compare the two measurement techniques.

Optical coherence tomography otoscope for imaging of tympanic membrane and middle ear pathology

Significance

Pathologies within the tympanic membrane (TM) and middle ear (ME) can lead to hearing loss. Imaging tools available in the hearing clinic for diagnosis and management are limited to visual inspection using the classic otoscope. The otoscopic view is limited to the surface of the TM, especially in diseased ears where the TM is opaque. An integrated optical coherence tomography (OCT) otoscope can provide images of the interior of the TM and ME space as well as an otoscope image. This enables the clinicians to correlate the standard otoscopic view with OCT and then use the new information to improve the diagnostic accuracy and management.

Aim

We aim to develop an OCT otoscope that can easily be used in the hearing clinic and demonstrate the system in the hearing clinic, identifying relevant image features of various pathologies not apparent in the standard otoscopic view.

Approach

We developed a portable OCT otoscope device featuring an improved field of view and form-factor that can be operated solely by the clinician using an integrated foot pedal to control image acquisition. The device was used to image patients at a hearing clinic.

Results

The field of view of the imaging system was improved to a 7.4 mm diameter, with lateral and axial resolutions of 38    μ m and 33.4    μ m , respectively. We developed algorithms to resample the images in Cartesian coordinates after collection in spherical polar coordinates and correct the image aberration. We imaged over 100 patients in the hearing clinic at USC Keck Hospital. Here, we identify some of the pathological features evident in the OCT images and highlight cases in which the OCT image provided clinically relevant information that was not available from traditional otoscopic imaging.

Conclusions

The developed OCT otoscope can readily fit into the hearing clinic workflow and provide new relevant information for diagnosing and managing TM and ME disease.

Generation of a zebrafish neurofibromatosis model via inducible knockout of nf2

Neurofibromatosis Type 2 (NF-2) is a dominantly inherited genetic disorder that results from mutations in the tumor suppressor gene, neurofibromin 2 (NF2) gene. Here, we report the generation of a conditional zebrafish model of neurofibromatosis established by an inducible genetic knockout of nf2a/b, the zebrafish homolog of human NF2. Analysis of nf2a and nf2b expression reveals ubiquitous expression of nf2b in the early embryo, with overlapping expression in the neural crest and its derivatives and in the cranial mesenchyme. In contrast, nf2a displays lower expression levels. Induction of nf2a/b knockout at early stages increases the proliferation of larval Schwann cells and meningeal fibroblasts. Subsequently, in adult zebrafish, nf2a/b knockout triggers the development of a spectrum of tumors, including vestibular schwannomas, spinal schwannomas, meningiomas, and retinal hamartomas, mirroring the tumor manifestations observed in patients with NF-2. Collectively, these findings highlight the generation of a novel zebrafish model that mimics the complexities of the human NF-2 disorder. Consequently, this model holds significant potential for facilitating therapeutic screening and elucidating key driver genes implicated in NF-2 onset.

The Dresden in vivo OCT dataset for automatic middle ear segmentation

Endoscopic optical coherence tomography (OCT) offers a non-invasive approach to perform the morphological and functional assessment of the middle ear in vivo. However, interpreting such OCT images is challenging and time-consuming due to the shadowing of preceding structures. Deep neural networks have emerged as a promising tool to enhance this process in multiple aspects, including segmentation, classification, and registration. Nevertheless, the scarcity of annotated datasets of OCT middle ear images poses a significant hurdle to the performance of neural networks. We introduce the Dresden in vivo OCT Dataset of the Middle Ear (DIOME) featuring 43 OCT volumes from both healthy and pathological middle ears of 29 subjects. DIOME provides semantic segmentations of five crucial anatomical structures (tympanic membrane, malleus, incus, stapes and promontory), and sparse landmarks delineating the salient features of the structures. The availability of these data facilitates the training and evaluation of algorithms regarding various analysis tasks with middle ear OCT images, e.g. diagnostics.

Electronic frequency shifting enables long, variable working distance optical coherence tomography

Increased imaging range is of growing interest in many applications of optical coherence tomography to reduce constraints on sample location, size, and topography. The design of optical coherence tomography systems with sufficient imaging range (e.g., 10s of centimeters) is a significant challenge due to the direct link between imaging range and acquisition bandwidth. We have developed a novel and flexible method to extend the imaging range in optical coherence tomography using electronic frequency shifting, enabling imaging in dynamic environments. In our approach, a laser with a quasi-linear sweep is used to limit the interferometric bandwidth, enabling decoupling of imaging range and acquisition bandwidth, while a tunable lens allows dynamic refocusing in the sample arm. Electronic frequency shifting then removes the need for high frequency digitization. This strategy is demonstrated to achieve high contrast morphological imaging over a > 21 cm working distance range, while maintaining high resolution and phase sensitivity. The system design is flexible to the application while requiring only a simple phase correction in post-processing. By implementing this approach in an auto-focusing paradigm, the proposed method demonstrates strong potential for the translation of optical coherence tomography into emerging applications requiring variable and centimeter-scale imaging ranges.

In vivo microstructural investigation of the human tympanic membrane by endoscopic polarization-sensitive optical coherence tomography

Significance

Endoscopic optical coherence tomography (OCT) is of growing interest for in vivo diagnostics of the tympanic membrane (TM) and the middle ear but generally lacks a tissue-specific contrast.

Aim

To assess the collagen fiber layer within the in vivo TM, an endoscopic imaging method utilizing the polarization changes induced by the birefringent connective tissue was developed.

Approach

An endoscopic swept-source OCT setup was redesigned and extended by a polarization-diverse balanced detection unit. Polarization-sensitive OCT (PS-OCT) data were visualized by a differential Stokes-based processing and the derived local retardation. The left and right ears of a healthy volunteer were examined.

Results

Distinct retardation signals in the annulus region of the TM and near the umbo revealed the layered structure of the TM. Due to the TM's conical shape and orientation in the ear canal, high incident angles onto the TM's surface, and low thicknesses compared to the axial resolution limit of the system, other regions of the TM were more difficult to evaluate.

Conclusions

The use of endoscopic PS-OCT is feasible to differentiate birefringent and nonbirefringent tissue of the human TM in vivo. Further investigations on healthy as well as pathologically altered TMs are required to validate the diagnostic potential of this technique.

Use of simulated data to explore the application of optical coherence tomography for classifying middle-ear pathologies

Optical coherence tomography (OCT) vibrometry is a non-invasive tool for functional imaging of the middle ear. It provides spatially resolved vibrational responses and also anatomical images of the same ear. Our objective here was to explore the potential of OCT vibration measurements at the incus, as well as at the umbo, to distinguish among middle-ear disorders. Our approach was to build finite-element models of normal and pathological ears, generate large amounts of synthetic data, and then classify the simulated data into normal and pathological groups using a decision tree based on features extracted from simulated vibration magnitudes. We could distinguish between normal ears and ears with incudomallear joint (IMJ) disarticulation or stapes fixation, with the sensitivity and specificity both being 1.0; distinguish between stapes fixation and IMJ disarticulation with a sensitivity of 0.900 and a specificity of 0.889; and distinguish ears with ISJ disarticulation from normal ears with a sensitivity of 0.784 and a specificity of 0.872. Less extreme pathologies were also simulated. The results suggest that the vibration measurements within the middle ear that can be provided by OCT (e.g., at the incus) may be very valuable for diagnosis.

Geometrically accurate real-time volumetric visualization of the middle ear using optical coherence tomography

We introduce a novel system for geometrically accurate, continuous, live, volumetric middle ear optical coherence tomography imaging over a 10.9mm×30∘×30∘ field of view (FOV) from a handheld imaging probe. The system employs a discretized spiral scanning (DC-SC) pattern to rapidly collect volumetric data and applies real-time scan conversion and lateral angular distortion correction to reduce geometric inaccuracies to below the system's lateral resolution over 92% of the FOV. We validate the geometric accuracy of the resulting images through comparison with co-registered micro-computed tomography (micro-CT) volumes of a phantom target and a cadaveric middle ear. The system's real-time volumetric imaging capabilities are assessed by imaging the ear of a healthy subject while performing dynamic pressurization of the middle ear in a Valsalva maneuver.

In vivo optical mapping of the tympanic membrane impulse response

The wide frequency range of the human hearing could be narrowed by various pathologies in the middle ear and in the tympanic membrane that lead to conductive hearing loss. Diagnosing such hearing problems is challenging, however, often relying on subjective hearing tests supported by functional tympanometry. Here we present a method for in vivo 2D mapping of the impulse response of the tympanic membrane, and demonstrate its potential on a healthy human volunteer. The imaging technique is based on interferometric spectrally encoded endoscopy, with a handheld probe designed to scan the human tympanic membrane within less than a second. The system obtains high-resolution 2D maps of key functional parameters including peak response, rise and decay times, oscillation bandwidth and resonance frequency. We also show that the system can identify abnormal regions in the membrane by detecting differences in the local mechanical parameters of the tissue. We believe that by offering a full 2D mapping of broad-bandwidth dynamics of the tympanic membrane, the presented imaging modality would be useful for effective diagnosis of conductive hearing loss in patients.

Transtympanic Visualization of Cochlear Implant Placement With Optical Coherence Tomography: A Pilot Study

OBJECTIVE

This study aimed to evaluate the ability of transtympanic middle ear optical coherence tomography (ME-OCT) to assess placement of cochlear implants (CIs) in situ.

PATIENT

A 72-year-old man with bilateral progressive heredodegenerative sensorineural hearing loss due to work-related noise exposure received a CI with a slim modiolar electrode for his right ear 3 months before his scheduled checkup.

INTERVENTION

A custom-built swept source ME-OCT system (λo = 1550 nm, ∆λ = 40 nm) designed for transtympanic middle ear imaging was used to capture a series of two- and three-dimensional images of the patient's CI in situ. Separately, transtympanic OCT two-dimensional video imaging and three-dimensional imaging were used to visualize insertion and removal of a CI with a slim modiolar electrode in a human cadaveric temporal bone through a posterior tympanotomy.

MAIN OUTCOME MEASURE

Images and video were analyzed qualitatively to determine the visibility of implant features under ME-OCT imaging and quantitatively to determine insertion depth of the CI.

RESULTS

After implantation, the CI electrode could be readily visualized in the round window niche under transtympanic ME-OCT in both the patient and the temporal bone. In both cases, characteristic design features of the slim modiolar electrode allowed us to quantify the insertion depth from our images.

CONCLUSIONS

ME-OCT could potentially be used in a clinic as a noninvasive, nonionizing means to confirm implant placement. This study shows that features of the CI electrode visible under ME-OCT can be used to quantify insertion depth in the postoperative ear.

Multimodal Handheld Probe for Characterizing Otitis Media - Integrating Raman Spectroscopy and Optical Coherence Tomography

Otitis media (OM) is a common disease of the middle ear, affecting 80% of children before the age of three. The otoscope, a simple illuminated magnifier, is the standard clinical diagnostic tool to observe the middle ear. However, it has limited contrast to detect signs of infection, such as clearly identifying and characterizing middle ear fluid or biofilms that accumulate within the middle ear. Likewise, invasive sampling of every subject is not clinically indicated nor practical. Thus, collecting accurate noninvasive diagnostic factors is vital for clinicians to deliver a precise diagnosis and effective treatment regimen. To address this need, a combined benchtop Raman spectroscopy (RS) and optical coherence tomography (OCT) system was developed. Together, RS-OCT can non-invasively interrogate the structural and biochemical signatures of the middle ear under normal and infected conditions.In this paper, in vivo RS scans from pediatric clinical human subjects presenting with OM were evaluated in parallel with RS-OCT data of physiologically relevant in vitro ear models. Component-level characterization of a healthy tympanic membrane and malleus bone, as well as OM-related middle ear fluid, identified the optimal position within the ear for RS-OCT data collection. To address the design challenges in developing a system specific to clinical use, a prototype non-contact multimodal handheld probe was built and successfully tested in vitro. Design criteria have been developed to successfully address imaging constraints imposed by physiological characteristics of the ear and optical safety limits. Here, we present the pathway for translation of RS-OCT for non-invasive detection of OM.

Automated classification of otitis media with OCT: augmenting pediatric image datasets with gold-standard animal model data

Otitis media (OM) is an extremely common disease that affects children worldwide. Optical coherence tomography (OCT) has emerged as a noninvasive diagnostic tool for OM, which can detect the presence and quantify the properties of middle ear fluid and biofilms. Here, the use of OCT data from the chinchilla, the gold-standard OM model for the human disease, is used to supplement a human image database to produce diagnostically relevant conclusions in a machine learning model. Statistical analysis shows the datatypes are compatible, with a blended-species model reaching ∼95% accuracy and F1 score, maintaining performance while additional human data is collected.

Convolutional dictionary learning for blind deconvolution of optical coherence tomography images

In this study, we demonstrate a sparsity-regularized, complex, blind deconvolution method for removing sidelobe artefacts and stochastic noise from optical coherence tomography (OCT) images. Our method estimates the complex scattering amplitude of tissue on a line-by-line basis by estimating and deconvolving the complex, one-dimensional axial point spread function (PSF) from measured OCT A-line data. We also present a strategy for employing a sparsity weighting mask to mitigate the loss of speckle brightness within tissue-containing regions caused by the sparse deconvolution. Qualitative and quantitative analyses show that this approach suppresses sidelobe artefacts and background noise better than traditional spectral reshaping techniques, with negligible loss of tissue structure. The technique is particularly useful for emerging OCT applications where OCT images contain strong specular reflections at air-tissue boundaries that create large sidelobe artefacts.

Endolymphatic Hydrops is a Marker of Synaptopathy Following Traumatic Noise Exposure

After acoustic trauma, there can be loss of synaptic connections between inner hair cells and auditory neurons in the cochlea, which may lead to hearing abnormalities including speech-in-noise difficulties, tinnitus, and hyperacusis. We have previously studied mice with blast-induced cochlear synaptopathy and found that they also developed a build-up of endolymph, termed endolymphatic hydrops. In this study, we used optical coherence tomography to measure endolymph volume in live CBA/CaJ mice exposed to various noise intensities. We quantified the number of synaptic ribbons and postsynaptic densities under the inner hair cells 1 week after noise exposure to determine if they correlated with acute changes in endolymph volume measured in the hours after the noise exposure. After 2 h of noise at an intensity of 95 dB SPL or below, both endolymph volume and synaptic counts remained normal. After exposure to 2 h of 100 dB SPL noise, mice developed endolymphatic hydrops and had reduced synaptic counts in the basal and middle regions of the cochlea. Furthermore, round-window application of hypertonic saline reduced the degree of endolymphatic hydrops that developed after 100 dB SPL noise exposure and partially prevented the reduction in synaptic counts in the cochlear base. Taken together, these results indicate that endolymphatic hydrops correlates with noise-induced cochlear synaptopathy, suggesting that these two pathologic findings have a common mechanistic basis.

In vivo functional imaging of the human middle ear with a hand-held optical coherence tomography device

We describe an optical coherence tomography and vibrometry system designed for portable hand-held usage in the otology clinic on awake patients. The system provides clinically relevant point-of-care morphological imaging with 14-44 µm resolution and functional vibratory measures with sub-nanometer sensitivity. We evaluated various new approaches for extracting functional information including a multi-tone stimulus, a continuous chirp stimulus, and alternating air and bone stimulus. We also explored the vibratory response over an area of the tympanic membrane (TM) and generated TM thickness maps. Our results suggest that the system can provide real-time in vivo imaging and vibrometry of the ear and could prove useful for investigating otologic pathology in the clinic setting.

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.

Doppler Optical Coherence Tomography for Otology Applications: From Phantom Simulation to In Vivo Experiment

In otology, visualization and vibratory analysis have been crucial to enhance the success of diagnosis and surgical operation. Optical coherence tomography (OCT) has been employed in otology to obtain morphological structure of tissues non-invasively, owing to the ability of measuring the entire region of tympanic membrane, which compensates the limitations of conventional methods. As a functional extension of OCT, Doppler OCT, which enables the measurement of the motion information with structural data of tissue, has been applied in otology. Over the years, Doppler OCT systems have been evolved in various forms to enhance the measuring sensitivity of phase difference. In this review, we provide representative algorithms of Doppler OCT and various applications in otology from preclinical analysis to clinical experiments and discuss future developments.

Rapid optical tomographic vibrometry using a swept multi-gigahertz comb

We propose a rapid tomographic vibrometer technique using an optical comb to measure internal vibrations, transient phenomena, and tomographic distributions in biological tissue and microelectromechanical system devices at high frequencies. This method allows phase-sensitive tomographic measurement in the depth direction at a multi-MHz scan rate using a frequency-modulated broadband electrooptic multi-GHz supercontinuum comb. The frequency spacing was swept instantaneously in time and axisymmetrically about the center wavelength via a dual-drive Mach-Zehnder modulator driven by a variable radio frequency signal. This unique sweeping method permits direct measurement of fringe-free interferometric amplitude and phase with arbitrarily changeable measurement range and scan rate. Therefore, a compressive measurement can be made in only the depth region where the vibration exists, reducing the number of measurement points. In a proof-of-principle experiment, the interferometric amplitude and phase were investigated for in-phase and quadrature phase-shifted interferograms obtained by a polarization demodulator. Tomographic transient displacement measurements were performed using a 0.12 mm thick glass film and piezo-electric transducer oscillating at 10-100 kHz with scan rates in the range 1-20 MHz. The depth resolution and precision of the vibrometer were estimated to be approximately 25 µm and 1.0 nm, respectively.

Handheld Briefcase Optical Coherence Tomography with Real-Time Machine Learning Classifier for Middle Ear Infections

A middle ear infection is a prevalent inflammatory disease most common in the pediatric population, and its financial burden remains substantial. Current diagnostic methods are highly subjective, relying on visual cues gathered by an otoscope. To address this shortcoming, optical coherence tomography (OCT) has been integrated into a handheld imaging probe. This system can non-invasively and quantitatively assess middle ear effusions and identify the presence of bacterial biofilms in the middle ear cavity during ear infections. Furthermore, the complete OCT system is housed in a standard briefcase to maximize its portability as a diagnostic device. Nonetheless, interpreting OCT images of the middle ear more often requires expertise in OCT as well as middle ear infections, making it difficult for an untrained user to operate the system as an accurate stand-alone diagnostic tool in clinical settings. Here, we present a briefcase OCT system implemented with a real-time machine learning platform for middle ear infections. A random forest-based classifier can categorize images based on the presence of middle ear effusions and biofilms. This study demonstrates that our briefcase OCT system coupled with machine learning can provide user-invariant classification results of middle ear conditions, which may greatly improve the utility of this technology for the diagnosis and management of middle ear infections.

Rapid imaging of tympanic membrane vibrations in humans

Functional imaging of the human ear is an extremely challenging task because of its minute anatomic structures and nanometer-scale motion in response to sound. Here, we demonstrate noninvasive in vivo functional imaging of the human tympanic membrane under various acoustic excitations, and identify unique vibration patterns that vary between human subjects. By combining spectrally encoded imaging with phase-sensitive spectral-domain interferometry, our system attains high-resolution functional imaging of the two-dimensional membrane surface, within a fraction of a second, through a handheld imaging probe. The detailed physiological data acquired by the system would allow measuring a wide range of clinically relevant parameters for patient diagnosis, and provide a powerful new tool for studying middle and inner ear physiology.

Optical coherence tomography: current and future clinical applications in otology

PURPOSE OF REVIEW

This article reviews literature on the use of optical coherence tomography (OCT) in otology and provides the reader with a timely update on its current clinical and research applications. The discussion focuses on the principles of OCT, the use of the technology for the diagnosis of middle ear disease and for the delineation of in-vivo cochlear microarchitecture and function.

RECENT FINDINGS

Recent advances in OCT include the measurement of structural and vibratory properties of the tympanic membrane, ossicles and inner ear in healthy and diseased states. Accurate, noninvasive diagnosis of middle ear disease, such as otosclerosis and acute otitis media using OCT, has been validated in clinical studies, whereas inner ear OCT imaging remains at the preclinical stage. The development of recent microscopic, otoscopic and endoscopic systems to address clinical and research problems is reviewed.

SUMMARY

OCT is a real-time, noninvasive, nonionizing, point-of-care imaging modality capable of imaging ear structures in vivo. Although current clinical systems are mainly focused on middle ear imaging, OCT has also been shown to have the ability to identify inner ear disease, an exciting possibility that will become increasingly relevant with the advent of targeted inner ear therapies.

Methylene blue-filled biodegradable polymer particles as a contrast agent for optical coherence tomography

Optical coherence tomography (OCT) images largely lack molecular information or molecular contrast. We address that issue here, reporting on the development of biodegradable micro and nano-spheres loaded with methylene blue (MB) as molecular contrast agents for OCT. MB is a constituent of FDA approved therapies and widely used as a dye in off-label clinical applications. The sequestration of MB within the polymer reduced toxicity and improved signal strength by drastically reducing the production of singlet oxygen and leuco-MB. The former leads to tissue damage and the latter to reduced image contrast. The spheres are also strongly scattering which improves molecular contrast signal localization and enhances signal strength. We demonstrate that these contrast agents may be imaged using both pump-probe OCT and photothermal OCT, using a 830 nm frequency domain OCT system and a 1.3 µm swept source OCT system. We also show that these contrast agents may be functionalized and targeted to specific receptors, e.g. the VCAM receptor known to be overexpressed in inflammation.

Characterisation of the static offset in the travelling wave in the cochlear basal turn

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

In Situ Characterization of Micro-Vibration in Natural Latex Membrane Resembling Tympanic Membrane Functionally Using Optical Doppler Tomography

Non-invasive characterization of micro-vibrations in the tympanic membrane (TM) excited by external sound waves is considered as a promising and essential diagnosis in modern otolaryngology. To verify the possibility of measuring and discriminating the vibrating pattern of TM, here we describe a micro-vibration measurement method of latex membrane resembling the TM. The measurements are obtained with an externally generated audio stimuli of 2.0, 2.2, 2.8, 3.1 and 3.2 kHz, and their respective vibrations based tomographic, volumetric and quantitative evaluations were acquired using optical Doppler tomography (ODT). The micro oscillations and structural changes which occurred due to diverse frequencies are measured with sufficient accuracy using a highly sensitive ODT system implied phase subtraction method. The obtained results demonstrated the capability of measuring and analyzing the complex varying micro-vibration of the membrane according to implied sound frequency.

Noise and sensitivity in optical coherence tomography based vibrometry

There is growing interest in using the exquisite phase sensitivity of optical coherence tomography (OCT) to measure the vibratory response in organ systems such as the middle and inner ear. Using frequency domain analysis, it is possible to achieve picometer sensitivity to vibration over a wide frequency band. Here we explore the limits of the frequency domain vibratory sensitivity due to additive noise and consider the implication of phase noise statistics on the estimation of vibratory amplitude and phase. Noise statistics are derived in both the Rayleigh (s/n = 0) and Normal distribution (s/n > 3) limits. These theoretical findings are explored using simulation and verified with experiments using a swept-laser system and a piezo electric element. A metric for sensitivity is proposed based on the 98% confidence interval for the Rayleigh distribution.