Point-of-care depth resolved 3D imaging of the tympanic membrane and middle ear with OCT, combined with quantitative image analysis, could improve the diagnosis and management of patients in the clinical setting. We imaged the TMs and MEs of 55 patients in a neurotology clinic, using a custom-built hand-held OCT (HHOCT) device. Patients with a diagnosis of TM retraction pockets, perforations, cholesteatomas, and postoperative states were included in this study. Healthy volunteers were also imaged to provide a baseline for quantitative metrics. Images were post processed to perform segmentation of the TM and create thickness maps of the TM, derive mean TM thickness values, and conduct ear symmetry analysis. The normal mean TM thickness was found to be significantly different from every other condition explored. Ear symmetry of healthy subjects was found to be 80% between left and right ears. Quantitative metrics derived from OCT images can be used to characterize TM pathologies and potentially aid in diagnosis and management.
SignificancePathologies 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.AimWe 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.ApproachWe 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.ResultsThe field of view of the imaging system was improved to a 7.4 mm diameter, with lateral and axial resolutions of 38 μm and 33.4 μm, 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.ConclusionsThe developed OCT otoscope can readily fit into the hearing clinic workflow and provide new relevant information for diagnosing and managing TM and ME disease.
We have been investigating Optical Coherence Tomography (OCT) as a tool to measure the tympanic membrane and middle ear morphology and vibrational response. The hand-held OCT ostoscope system, based on a 1.3 µm swept laser, is integrated into an endoscopy cart. It has an ~ 8 mm diameter field of view, 38 µm lateral resolution, 35 µm axial resolution, A-line rate of 200 kHz, and subnanometer sensitivity to vibration within the tympanic membrane and middle ear. The system has been used in the clinic at USC Keck Medical Center to image over 100 patients and healthy volunteers. Total imaging time is ~2 minutes, which allows it to easily fit into the clinic workflow, while providing high-resolution images and vibrometric assessment of the tympanic membrane and middle ear. The functional and morphological features visible within these image sets that allow us to readily differentiate among pathologies, will be discussed.
Early diagnosis of ear disorders is difficult in part because patients do not seek out an otologist until they have significant hearing loss. Early detection could happen in the primary care provider’s office, however the sensitivity of an otoscopic examination by a primary care provider during an annual physical is very low. On the other hand, Optical Coherence Tomography (OCT) imaging of the tympanic membrane and middle ear can provide detailed volumetric images of the structure and function. These detailed images can form the basis for an approach for finding early signs of ear disease. Our hypothesis is that asymmetry between the ears could be used for early diagnosis. In order to test this, we need to understand the naturally occurring asymmetry in healthy volunteers. We have collected volumetric OCT images from 8 healthy subjects using a hand-held otoscopic OCT system. As part of a registration algorithm, we crop and down sample the data before finding the transformation matrix that registers the volumes. This matrix is then used to register the original volumes. Then the quantitative analysis of the symmetry between the left and right ears was applied through the similarity coefficient and overall, the left and right ears similarity of 8 healthy subjects has a mean of 0.7892, and a standard deviation of 0.0186. From a scientific perspective, this is the first quantitative measure of how symmetric the right and left ears are in humans. From a diagnostic perspective, this approach could provide a simple method to find early signs of ear disease.
Optical coherence tomography (OCT) has been shown to provide detailed images of the morphology and vibratory response in the living cochlea. As a part of the cochlea, the organ of Corti (OC) has a complex tissue structure including three rows of outer hair cells which act to amplify sound, supporting cells and one row of inner hair cells which transduce sound-induced vibrations into electrical signals. Unfortunately, OCT images of the OC have relatively low contrast, in spite of the fact that the microstructures have very different function and morphology. That fact has led us to explore alternative approaches to extracting contrast from these OCT images. In this paper, we propose a contrast-enhanced method based on spatial frequency to identify structures within the cochlea, including the OC. In total, 15 mice have been imaged with our customed OCT system and analyzed. A two-dimension spatial frequency analysis was performed over subregions of the images, using a sliding window. Then the power spectral density was fit to a 2-D Gaussian. Finally, we extracted several Gaussian fitting coefficients and constructed a coefficients map to enhance the visualization of the cochlea and identify structures within the OC. This method improves our ability to identify specific microstructures within the cochlea and ultimately map the functional vibratory response to these microstructures. Application of this approach can elucidate the micromechanical function of the cochlea.
Magnetic Resonance Imaging and x-ray Computed Tomography have limitations when applied to diseases of the human inner ear due to insufficient resolution. Key morphological features of the inner ear are below the resolving power of both modalities; thus, they are unable to measure functional aspects of the microstructures in the cochlea. Furthermore, general access to the cochlea is a challenge due to its location in the inner ear and its bony encapsulation. These limitations cause clinicians to rely on clinical history when diagnosing and managing hearing loss in patients, which is not ideal. This paper explores the application of Optical Coherence Tomography (OCT) as a diagnostic tool for inner ear diseases. OCT’s high spatial and temporal resolution allows for detailed imaging of inner ear structures and their function. To address the challenge of accessing the cochlea in humans, a hand-held endoscopic OCT device has been developed that can image through the round window membrane. The technology has been tested in cadaver temporal bone, enabling functional and morphological imaging of the cochlea when navigated to the round window. Alongside the device, we are developing an algorithm to perform subsequent stitching of volumes to overcome limitations with a small field of view. Applying this algorithm on cadaver tissue serves as a preliminary step before advancing to live human cochlear imaging. By utilizing our hand-held OCT endoscope, clinicians will have the ability to record changes in morphological and functional information, thereby improving the approach to diagnosing and treating patients with inner ear diseases.
Cross-sectional 4-D imaging of vocal fold morphology and function is desirable for accurate diagnosis of many vocal fold pathologies, which occur throughout the epithelial layer and alter the mucosal wave. Clinical videoendoscopy provides qualitative diagnostic information but remains limited to surface visualization of layered vocal fold structure and two-dimensional mucosal vibration. While OCT has been investigated to address the shortcomings of standard 2-D endoscopy, challenges remain in reconstruction of the 4-D mucosal wave over the entire vocal fold structure. To address these challenges, we have developed a fast-scanning OCT laryngoscope to enable asynchronous Nyquist sampling of the human voice fundamental frequency range (and its harmonics, up to 1 kHz). We present a new algorithm for reconstruction of the 4-D vocal fold dynamics during phonation using OCT volume data of the entire anterior-posterior vocal fold structure. Reconstruction of the vibration of a vocal fold phantom confirmed feasibility of the algorithm and preliminary reconstruction of the in vivo vocal fold glottal cycle is presented. This work represents the first cross-sectional Nyquist sampling of the in vivo human mucosal wave using an OCT system with hardware capable of encompassing the human fundamental frequency range (i.e., 90-260 Hz). The developed OCT laryngoscope and algorithm will enable volumetric representations of vocal fold dynamics in the clinic and development of quantitative metrics for diagnostic and interventional guidance.
OCT has been exploited extensively in studies of cochlear mechanics due to its ability to non-invasively measure vibrations of various cochlear structures. A key limitation has been the ability to measure only in one dimension, along the optical axis. However, recent evidence suggests the organ of Corti has complex, three-dimensional vibratory micromechanics. Therefore, a 3D-OCT vibrometry system has been developed to measure the vector of motion within the cochlea and hopefully shed light on the underlying mechanics that lead to cochlear amplification and the exquisite sensitivity and frequency selectivity of mammalian hearing. The system uses three independent sample arms (channels) with a single reference arm to acquire vibrations, exploiting the long coherence length of the laser to depth encode the three channels. The system was first validated using a piezoelectric actuator. This yielded an RMS error of ≤0.3° in both polar angles with expected sensitivity to vibrational amplitude. Preliminary measurements in the cochlea of a live mouse demonstrate direction-dependent differences in vibratory responses.
Increased cellular metabolic activity, a hallmark of malignant epithelial cells, can be quantified by imaging the oral tissue autofluorescence originated from the metabolic cofactors NADH and FAD. We report a novel multispectral autofluorescence lifetime imaging (maFLIM) handheld probe capable of simultaneous autofluorescence excitation at 375 nm (for NADH) and 445 nm (for FAD), and simultaneous multispectral time-resolved fluorescence measurement at four emission spectral channels. The performance of the dual-wavelength excitation maFLIM handheld probe was assessed by imaging fluorescent dye standards with well characterized fluorescence lifetimes, and the oral mucosa of human subjects in oral health care settings.
Significance: Detailed biochemical and morphological imaging of the plaque burdened coronary arteries holds the promise of improved understanding of atherosclerosis plaque development, ultimately leading to better diagnostics and therapies.
Aim: Development of a dual-modality intravascular catheter supporting swept-source optical coherence tomography (OCT) and frequency-domain fluorescence lifetime imaging (FD-FLIM) of endogenous fluorophores with UV excitation.
Approach: We instituted a refined approach to endoscope development that combines simulation in a commercial ray tracing program, fabrication, and a measurement method for optimizing ball-lens performance. With this approach, we designed and developed a dual-modality catheter endoscope based on a double-clad fiber supporting OCT through the core and fluorescence collection through the first cladding. We varied the relative percent of UV excitation launched into the core and first cladding to explore the potential resolution improvement for FD-FLIM. The developed catheter endoscope was optically characterized, including measurement of spatial resolution and fluorescent lifetimes of standard fluorophores. Finally, the system was demonstrated on fresh ex vivo human coronary arteries.
Results: The developed endoscope was shown to have optical performance similar to predictions derived from the simulation approach. The FLIM resolution can be improved by over a factor of 4 by primarily illuminating through the core rather than the first cladding. However, time-dependent solarization losses need to be considered when choosing the relative percentage. We ultimately chose to illuminate with 7% of the power transmitting through the core. The resulting catheter endoscope had 40-μm lateral resolution for OCT and <100 μm lateral resolution for FD-FLIM. Images of ex vivo coronary arteries are consistent with expectations based on histopathology.
Conclusions: The results demonstrate that our approach for endoscope simulation produces reliable predictions of endoscope performance. Simulation results guided our development of a multimodal OCT/FD-FLIM catheter imaging system for investigating atherosclerosis in coronary arteries.
A principal tool for the visual inspection of the middle ear in the hearing clinic is the surgical stereo-microscope. We have developed a compact accessory for the surgical microscope that enables volumetric optical coherence tomography (OCT) imaging of the middle ear as well as functional vibratory imaging with subnanometer sensitivity. The sensitivity to vibration is achieved by careful engineering of the microscope attachment and frequency-domain processing. The microscope attachment integrates the entire OCT interferometer onto a custom aluminum base that mounts directly to the accessory area at the foot of most surgical microscopes. This approach is effective at removing high-frequency phase-noise, thus enabling near shot-noise limited sensitivity above ~2 kHz even though the OCT system is suspended above the patient by a boom arm. We analyze the vibratory response in the frequency domain, hence our ability to measure vibrations in the tympanic membrane and ossicles is near the shot-noise limit. As a demonstration of this system we have recorded in vivo volumetric images of a healthy human patient as well as the vibratory response at the tympanic membrane down to the hearing threshold. We also show that it is possible to use time averaging to drive the noise floor down below 2 pm which allowed us to make the first measure of distortion product otoacoustic emissions (DPOAE) using OCT. Finally, the system can easily be taken on and off of the surgical microscope and when in use does not impinge on the normal view through the surgical microscope.
Atherosclerosis, a condition in which plaque accumulates on the inner wall of arteries, is often recognized as a precursor to cardiovascular diseases (CVDs), the most common causes of death in the US. Optical Coherence Tomography (OCT) is an intravascular optical diagnosis tool, which can be used to obtain high resolution morphological images of atherosclerotic plaque. However, atherosclerotic plaque components, such as macrophages, can be misclassified due to their signal similarities to fibrin accumulations, cholesterol crystals and microcalcifications. To overcome these challenges, we develop a biocompatible contrast agent to enhance molecular imaging of a Pump-Probe OCT (PPOCT) system. Methylene blue (MB) was encapsulated into poly lactic-co-glycolic acid (PLGA) particles by an emulsion/solvent evaporation technique. Fabrication parameters were controlled to synthesize particles with desired properties such as: size, encapsulation efficiency, degradation rate, and particle surface functionalization. The encapsulation of MB protects it from the enzymatic reduction to leuco-methylene blue (92.8 % protection), and reduces the singlet oxygen generation by the excited MB molecules by 78.3%. Likewise, the PLGA shells improve the OCT signal by enhancing the scattering of light. The surface of particles was modified with ligands that can target molecular biomarkers involved in atherosclerotic plaque formation such as vascular cell adhesion molecules (VCAM-1) and apoptotic macrophages. This modification is expected to enhance tissue selectivity, provides detailed information on the local biochemistry and yields visualization of pathological processes. PLGA-based contrast agents were tested in human postmortem artery sections to study particles permeability as a function of particle size and its molecular selectivity.
Atherosclerosis is a progressive asymptomatic disease that has the highest rate of death and morbidity in the United States. High macrophage infiltration and thin cap fibroatheromas are known to be the precursor lesions of plaque rupture. Lipid-laden macrophages called foam cells are formed by the uptake of lipids within the plaque. These foam cells eventually die forming a necrotic core. Ruptured plaques are characterized by a necrotic core with an overlying thin-ruptured cap highly infiltrated by macrophages. Imaging modalities capable of identifying macrophage clusters in atherosclerotic plaques could be used for plaque vulnerability assessment. In this study, Multispectral Fluorescence Lifetime Imaging (FLIM) is used to retrieve information of biochemical markers present in atherosclerotic tissue. Here, we present a computational methodology that makes use of FLIM-based biochemical plaque features in order to identify macrophage/foam cells in atherosclerotic plaques. In the proposed methodology, the FLIM lifetime map obtained from a spectral channel of 494 ± 20.5 nm provides information about the accumulation of macrophages, which produce long lifetimes (>6 ns). This methodology was validated against histopathological assessment (CD68 staining specific for macrophages) in terms of statistical correlation, a 10-fold cross validation (sensitivity = 88.45%; specificity= 91.21%), and receiver operating characteristic (ROC AUC = 0.91) analyses.
We report the design and validation of a novel ball lens-based imaging catheter based on dual-clad fiber for frequency-domain fluorescence lifetime imaging microscopy (FLIM) of atherosclerosis. The illumination and collection performance of the catheter endoscope was modeled and optimized with the ray-tracing program Zemax. A 1.55-m-long dual-clad fiber was spliced with a short length of coreless fiber, and then heated and polished to fabricate the angled ball lens. The fiber endoscope was enclosed in a torque cable and had a diameter of 2Fr. The catheter was affixed to a custom built lensless rotary joint which had high coupling efficiency (>90%) over a broad spectral range, accommodating both the UV (375 nm) excitation and the broad fluorescence emission (385 nm - 600 nm). The computer controlled rotary joint and translation stage for pullback imaging can routinely achieve rotation rates of 6000 rpm. The endoscope has two configurations depending on different illumination methods. Lateral resolution was improved more than twice by illuminating the core instead of the inner cladding, while SNR decreased due to higher attenuation of the core. Experiments conducted using a resolution target demonstrate a lateral resolution 80 μm at 1 mm lens-to-sample distance. Experiments conducted using a fluorescein phantom and a segment of ex vivo human coronary artery demonstrate the system performance for fluorescence lifetime imaging with pullback velocities of >10mm/s. This study demonstrates the novel design of a ball lens-based FLIM catheter system to record fluorescence in a continuous helical scanning method across broad-spectral emission bands.
We have shown in an ex vivo human coronary artery study that the biochemical information derived from FLIM interpreted in the context of the morphological information from OCT enables a detailed classification of human coronary plaques associated with atherosclerosis. The identification of lipid-rich plaques prone to erosion or rupture and associated with sudden coronary events can impact current clinical practice as well as future development of targeted therapies for “vulnerable” plaques. In order to realize clinical translation of intravascular OCT/FLIM we have had to develop several key technologies. A multimodal catheter endoscope capable of delivering near UV excitation for FLIM and shortwave IR for OCT has been fabricated using a ball lens design with a double clad fiber. The OCT illumination and the FLIM excitation propogate down the inner core while the large outer multimode core captures the fluorescence emission. To enable intravascular pullback imaging with this endoscope we have developed an ultra-wideband fiber optic rotary joint using the same double clad fiber. The rotary joint is based on a lensless design where two cleaved fibers, one fixed and one rotating, are brought into close proximity but not touching. Using water as the lubricant enabled operation over the near UV-shortwave IR range. Transmission over this bandwidth has been measured to be near 100% at rotational frequencies up to 147 Hz. The entire system has been assembled and placed on a mobile cart suitable for cath lab based imaging. System development, performance, and early ex vivo imaging results will be discussed.
Molecular contrast imaging can target specific molecules or receptors to provide detailed information on the local biochemistry and yield enhanced visualization of pathological and physiological processes. When paired with Optical Coherence Tomography (OCT) it can simultaneously supply the morphological context for the molecular information. We recently demonstrated in vivo molecular contrast imaging of methylene blue (MB) using a 663 nm diode laser as a pump in a Pump-Probe OCT (PPOCT) system. The simple addition of a dichroic mirror in the sample arm enabled PPOCT imaging with a typical 830-nm band spectral-domain OCT system. Here we report on the development of a microencapsulated MB contrast agent. The poly lactic-co-glycolic acid (PLGA) microspheres loaded with MB offer several advantages over bare MB. The microsphere encapsulation improves the PPOCT signal both by enhancing the scattering and preventing the reduction of MB to leucomethylene blue. The surface of the microsphere can readily be functionalized to enable active targeting of the contrast agent without modifying the excited state dynamics of MB that enable PPOCT imaging. Both MB and PLGA are used clinically. PLGA is FDA approved and used in drug delivery and tissue engineering applications. 2.5 μm diameter microspheres were synthesized with an inner core containing 0.01% (w/v) aqueous MB. As an initial demonstration the MB microspheres were imaged in a 100 μm diameter capillary tube submerged in a 1% intralipid emulsion.
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