Detection of Neural Action Potentials Using Optical Coherence Tomography: Intensity and Phase Measurements with and without Dyes

We review the use of optical coherence tomography (OCT) for detection of neural activity, and present a new approach for depth-localization of neural action potentials (APs) using voltage-sensitive dyes as contrast agents in OCT. A stained squid giant axon is imaged by spectral-domain OCT. Changes in the intensity and phase of back-scattered light coming from regions around the membrane are measured during AP propagation. The depth-resolved change in back-scattered intensity coincides with the arrival of AP at the measurement area, and is synchronous with the changes in transmitted light intensity and reflection-mode cross-polarized light intensity measured independently. The system also provides depth-resolved phase changes as an additional indication of activity. With further investigation our results could open a new era in functional imaging technology to localize neural activity at different depths in situ

ART(S) OF BECOMING: PERFORMATIVE ENCOUNTERS IN CONTEMPORARY POLITICAL ART

This thesis analyses Deleuze & Guattari’s notion of becoming through certain performative encounters in contemporary political art, and re-conceptualizes them as “art(s) of becoming”. Art(s) of becoming are actualizations of a non-representational –minoritarian– mode of becoming and creation as well as the political actions of fleeing quanta. The theoretical aim of the study is, on the one hand, to explain how Platonic Idealism is overturned by Deleuze’s reading of Nietzsche and Leibniz, and on the other hand, how Cartesian dualism of mind and body is surpassed by following a Spinozistic theory of affects. In this respect, the dissertation has both theoretical and practical dimensions. Since art(s) of becoming are bodies without organs which constitute their own lines of flight through a process of minoration, the concepts of body, affect, becoming, and intensity are central to this study. For the same reason, this is an attempt to show the intersections of philosophical, political and aesthetic domains in Deleuze’s theory of sensation which is part of his general practice of philosophy, that is, a quest for establishing an ontology of immanence as opposed to identitarian metaphysics

Spectral-domain optical coherence reflectometric sensor for highly sensitive molecular detection

We describe what we believe to be a novel use of spectral-domain optical coherence reflectometry (SD-OCR) for highly sensitive molecular detection in real time. The SD-OCR sensor allows identification of a sensor surface of interest in an OCR depth scan and monitoring the phase alteration due to molecular interaction at that surface with subnanometer optical thickness sensitivity. We present subfemtomole detection sensitivity for etching of Si

Phase-sensitive imaging of the outer retina using optical coherence tomography and adaptive optics

The cone photoreceptor’s outer segment (OS) experiences changes in optical path length, both in response to visible stimuli and as a matter of its daily course of renewal and shedding. These changes are of interest, to quantify function in healthy cells and assess dysfunction in diseased ones. While optical coherence tomography (OCT), combined with adaptive optics (AO), has permitted unprecedented three-dimensional resolution in the living retina, it has not generally been able to measure these OS dynamics, whose scale is smaller than OCT’s axial resolution of a few microns. A possible solution is to take advantage of the phase information encoded in the OCT signal. Phase-sensitive implementations of spectral-domain optical coherence tomography (SD-OCT) have been demonstrated, capable of resolving sample axial displacements much smaller than the imaging wavelength, but these have been limited to ex vivo samples. In this paper we present a novel technique for retrieving phase information from OCT volumes of the outer retina. The key component of our technique is quantification of phase differences within the retina. We provide a quantitative analysis of such phase information and show that–when combined with appropriate methods for filtering and unwrapping–it can improve the sensitivity to OS length change by more than an order of magnitude, down to 45 nm, slightly thicker than a single OS disc. We further show that phase sensitivity drops off with retinal eccentricity, and that the best location for phase imaging is close to the fovea. We apply the technique to the measurement of sub-resolution changes in the OS over matters of hours. Using custom software for registration and tracking, these microscopic changes are monitored in hundreds of cones over time. In two subjects, the OS was found to have average elongation rates of 150 nm/hr, values which agree with our previous findings

Single-shot two-dimensional full-range optical coherence tomography achieved by dispersion control

We present a full-range Fourier-domain optical coherence tomography (OCT) system that is capable of acquiring two-dimensional images of living tissue in a single shot. By using line illumination of the sample in combination with a two-dimensional imaging spectrometer, 1040 depth scans are performed simultaneously on a sub-millisecond timescale. Furthermore, we demonstrate an easy and flexible real-time single-shot technique for full-range (complex-conjugate cancelled) OCT imaging that is compatible with both two-dimensional as well as ultrahighresolution OCT. By implementing a dispersion imbalance between reference and sample arms of the interferometer, we eliminate the complex-conjugate signal through numerical dispersion compensation, effectively increasing the useful depth range by a factor of two. The system allows us to record 6.7 × 3.2 mm images at 5 μm depth resolution in 0.2 ms. Data postprocessing requires only 4 s. We demonstrate the capability of our system by imaging the anterior chamber of a mouse eye in vitro, as well as human skin in vivo. © 2009 Optical Society of America

Contributions and complexities from the use of in-vivo animal models to improve understanding of human neuroimaging signals.

Many of the major advances in our understanding of how functional brain imaging signals relate to neuronal activity over the previous two decades have arisen from physiological research studies involving experimental animal models. This approach has been successful partly because it provides opportunities to measure both the hemodynamic changes that underpin many human functional brain imaging techniques and the neuronal activity about which we wish to make inferences. Although research into the coupling of neuronal and hemodynamic responses using animal models has provided a general validation of the correspondence of neuroimaging signals to specific types of neuronal activity, it is also highlighting the key complexities and uncertainties in estimating neural signals from hemodynamic markers. This review will detail how research in animal models is contributing to our rapidly evolving understanding of what human neuroimaging techniques tell us about neuronal activity. It will highlight emerging issues in the interpretation of neuroimaging data that arise from in-vivo research studies, for example spatial and temporal constraints to neuroimaging signal interpretation, or the effects of disease and modulatory neurotransmitters upon neurovascular coupling. We will also give critical consideration to the limitations and possible complexities of translating data acquired in the typical animals models used in this area to the arena of human fMRI. These include the commonplace use of anaesthesia in animal research studies and the fact that many neuropsychological questions that are being actively explored in humans have limited homologues within current animal models for neuroimaging research. Finally we will highlighting approaches, both in experimental animals models (e.g. imaging in conscious, behaving animals) and human studies (e.g. combined fMRI-EEG), that mitigate against these challenges

Biomedical applications of a fiber based low-coherence interferometer for quantitative differential phase measurements

textThis dissertation presents design, implementation, and biomedical applications of a novel polarization-maintaining-fiber based phase sensitive optical low-coherence reflectometer (PS-OLCR). Using dual channels, PS-OLCR detects Angstrom/nanometer scale optical path length changes by calculating phase difference between depth resolved interference fringes. Ability to detect such small changes in optical path length at specific depths is useful in a wide variety of biomedical imaging and sensing applications. Application areas investigated in this dissertation include (i) measurement of analyte concentrations, (ii) imaging surface topography, (iii) cartilage surface response to electrical stimulation, (iv) arterial tissue response to photothermal stimulation, (v) birefringence measurement, and (vi) optical detection of neural activity. For each application, results of PS-OLCR differential phase measurements are presented.Electrical and Computer Engineerin

OPHTHALMIC SURGERY AND LASERS

A novel suture technique for regular stellate corneal lacerations, called a star-shaped suture is described. The suturing begins from inside any of the wound tips in a clockwise direction. Suture placement proceeds in a counterclockwise direction by the adjacent wound limb, while the suture passes are all in clock-wise direction. Finally, a star-shaped suture is achieved with the knot self-buried in the corneal stroma at the initial entry site. This is a continuous suture, of which tractional forces direct to the center of the wound providing good apical apposition for stellate lacerations. This suture technique can be an alternative for the reconstruction of stellate corneal lacerations