Hrebesh M. Subhash

Feasibility of spectral-domain phase- sensitive optical coherence tomography for middle ear vibrometry

We describe a novel application of spectral-domain phase-sensitive optical coherence tomography (SD PS-OCT) to detect the tiny motions of the middle ear structures, such as the tympanic membrane and ossicular chain, and their morphological features for differential diagnosis of CHL. This technique has the potential to provide meaningful vibration of ossicles with a vibration sensitivity of ≈ 0.5 nm at 1 kHz of acoustic stimulation. To the best of our knowledge, this is the first demonstration of depth-resolved vibration imaging of ossicles with a PS-OCT system at a nanometer scale.

Volumetric In Vivo Imaging of Microvascular Perfusion Within the Intact Cochlea in Mice Using Ultra-High Sensitive Optical Microangiography

Studying the inner ear microvascular dynamics is extremely important to understand the cochlear function and to further advance the diagnosis, prevention, and treatment of many otologic disorders. However, there is currently no effective imaging tool available that is able to access the blood flow within the intact cochlea. In this paper, we report the use of an ultrahigh sensitive optical micro-angiography (UHS-OMAG) imaging system to image 3-D microvascular perfusion within the intact cochlea in living mice. The UHS-OMAG image system used in this study is based on spectral domain optical coherence tomography, which uses a broadband light source centered at 1300 nm with an imaging rate of 47thinspace 000 A-scans/s, capable of acquiring high-resolution B scans at 300 frames/s. The technique is sensitive enough to image very slow blood flow velocities, such as those found in capillary networks. The 3-D imaging acquisition time for a whole cochlea is ~ 4.1 s. We demonstrate that volumetric reconstruction of microvascular flow obtained by UHS-OMAG provides a comprehensive perfusion map of several regions of the cochlea, including the otic capsule, the stria vascularis of the apical and middle turns and the radiating arterioles that emanate from the modiolus.

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

There is considerable interest in developing new methods for in vivo imaging of the complex anatomy of the mammalian cochlea for clinical as well as fundamental studies. In this study, we explored, the feasibility of spectral domain optical coherence tomography (SD-OCT) for 3-D in vivo imaging of the cochlea in mice. The SD-OCT system employed in this study used a broadband light source centered at 1300 nm, and the imaging speed of the system was 47,000 A-scans per second using the InGaAs camera. The system was capable of providing fully processed, high-resolution B-scan images [512 (axial) x 128 (lateral) pixels] at 280 frames per sec. The 3-D imaging acquisition time for a whole cochlea was approximately 0.45 sec. The traditional SD-OCT structural imaging algorithm was used to reconstruct 3-D cochlear morphology. We demonstrated that SD-OCT can be successfully used for in vivo imaging of important morphological features within the mouse cochlea, such as the otic capsule and structures within, including Reissners membrane, the basilar membrane, tectorial membrane, organ of Corti, and modiolus of the apical and middle turns.

Optical detection of indocyanine green encapsulated biocompatible poly (lactic-co-glycolic) acid nanoparticles with photothermal optical coherence tomography

We describe a functional imaging paradigm that uses photothermal optical coherence tomography (PT-OCT) to detect indocyanine green (ICG)-encapsulated biocompatible poly(lactic-co-glycolic) acid (PLGA) nanoparticles embedded in highly scattering tissue phantoms with high resolution and sensitivity. The ICG-loaded PLGA nanoparticles were fabricated using a modified emulsification solvent diffusion method. With a 20 kHz axial scan rate, PT-OCT based on spectral-domain interferometric configuration at 1310 nm was used to detect phase changes induced by a 808 nm photothermal excitation of ICG-encapsulated PLGA nanoparticles. An algorithm based on Fourier transform analysis of differential phase of the spectral interferogram was developed for detecting the depth resolved localized photothermal signal. Excellent contrast difference was observed with PT-OCT between phantoms containing different concentrations of ICG-encapsulated PLGA nanoparticles. This technique has the potential to provide simultaneous structural and molecular-targeted imaging with excellent signal-to-noise for various clinical applications.

Full-Field and Single-Shot Full-Field Optical Coherence Tomography: A Novel Technique for Biomedical Imaging Applications

Since its introduction, optical coherence tomography (OCT) technology has advanced dramatically in various field of both clinical and fundamental research. Full-field and Single-shot full-field OCT (FF-OCT and SS-FF-OCT) are alternative OCT concepts, which aims to improve the image acquisition speed and to simplify the optical setup of conventional point-scan OCT by realizing direct line field or full-field sample imaging onto an array or line detector such as CCD or CMOS camera. FF-OCT and SS-FF-OCT are based on bulk optics Linnik-type Michelson interferometer with relatively high numerical aperture (NA) microscopic objectives. This paper will give you an overview of the principle of various types of FF-OCT and SS-FF-OCT techniques and its associated system design concept and image reconstruction algorithms.

Full range complex spectral domain optical coherence tomography for volumetric imaging at 47 000 A-scans per second

In this paper, we demonstrate a high speed spectral domain optical coherence tomography (SDOCT) system capable of achieving full range complex imaging at 47 kHz line scan rate. By applying beam-offset method, a constant modulation frequency is introduced into each B-scan that enables reconstruction of the full range complex SDOCT images of in vivo tissue samples. To make use of the full capacity of detection camera used in the system, system control software is developed that streams the raw spectral fringe data directly into the computer memory. In order to assess performance of the high speed full range SDOCT system for imaging biological specimen, we present results imaged from the cuticle of fingernail of a human volunteer in vivo, and from the chicken embryos ex vivo. We also show the high sensitivity advantages of full range complex imaging as compared to the conventional SDOCT. To the best of our knowledge, 47,000 A-scan imaging rate is the highest imaging rate ever been reported for full range complex imaging. Notwithstanding, the method reported here has no limitations on the imaging speed, thus offers a useful tool to achieve volumetric imaging of living samples where the high sensitivity region around zero-delay line in the system can be utilized for imaging.

Depth-resolved dual-beamlet vibrometry based on Fourier domain low coherence interferometry

Abstract. We present an optical vibrometer based on delay-encoded, dual-beamlet phase-sensitive Fourier domain interferometric system to provide depth-resolved subnanometer scale vibration information from scattering biological specimens. System characterization, calibration, and preliminary vibrometry with biological specimens were performed. The proposed system has the potential to provide both amplitude and direction of vibration of tissue microstructures on a single two-dimensional plane.

Optogenetic Control of Mouse Outer Hair Cells.

Normal hearing in mammals depends on sound amplification by outer hair cells (OHCs) presumably by their somatic motility and force production. However, the role of OHC force production in cochlear amplification and frequency tuning are not yet fully understood. Currently, available OHC manipulation techniques for physiological or clinical studies are limited by their invasive nature, lack of precision, and poor temporal-spatial resolution. To overcome these limitations, we explored an optogenetic approach based on channelrhodopsin 2 (ChR-2), a direct light-activated nonselective cation channel originally discovered in Chlamydomonas reinhardtii. Three approaches were compared: 1) adeno-associated virus-mediated in utero transfer of the ChR-2 gene into the developing murine otocyst, 2) expression of ChR-2(H134R) in an auditory cell line (HEI-OC1), and 3) expression of ChR-2 in the OHCs of a mouse line carrying a ChR-2 conditional allele. Whole cell recording showed that blue light (470xa0nm) elicited the typical nonselective cation current of ChR-2 with reversal potential around zero in both mouse OHCs and HEI-OC1 cells and generated depolarization in both cell types. In addition, pulsed light stimulation (10xa0Hz) elicited a 1:1 repetitive depolarization and ChR-2 currents in mouse OHCs and HEI-OC1 cells, respectively. The time constant of depolarization in OHCs, 1.45xa0ms, is 10 times faster than HEI-OC1 cells, which allowed light stimulation up to rates of 10/s to elicit corresponding membrane potential changes. Our study demonstrates that ChR-2 can successfully be expressed in mouse OHCs and HEI-OC1 cells and that these present a typical light-sensitive current and depolarization. However, the amount of ChR-2 current induced in our inxa0vivo experiments was insufficient to result in measurable cochlear effects.

Ultra-high speed full range complex spectral domain optical coherence tomography for volumetric imaging at 140,000 A scans per second

We demonstrate an ultra-high speed full rang spectral domain optical coherence tomography system based on CMOS camera at 140,000 A-scans per second. By implementing beam-offset method, a constant modulation frequency is introduced into each B-scan that enables the reconstruction of the full range complex SDOCT images from in vivo biological specimens. To make use of the full acquisition capacity of detection camera used in the system, we developed system control software that streams the raw spectral fringe data directly into the computer memory. The feasibly of our high speed full range SDOCT system is demonstrated for imaging the dynamics of anterior segment of human eye in vivo.

Optical Coherence Tomography: Technical Aspects

Optical coherence tomography (OCT) is a high-resolution, noninvasive, 3D imaging technique with great potential in both clinical and fundamental research applications in many areas. Owing to its exceptionally high spatial resolution and velocity sensitivity, the functional extension of OCT techniques can simultaneously provide tissue structure, blood perfusion, birefringence, and other physiological information and it has great potential for basic biomedical research and clinical medicine. OCT has the far-reaching potential to be a quantitative imaging technique that could impact many, as yet unexplored, areas and should therefore be considered a vital measurement tool. In this chapter, we will first discuss the principle of operation and then the practical aspects of the OCT system; we will also provide detailed discussion on different OCT schemes and its functional extensions.