Optical Coherence Tomography (OCT) is non-invasive, real time optical imaging method based on low coherence interferometry with high resolution. It is capable of imaging microstructure by measuring the light backscattered from the sample. Under the AGI grant, I have built a Swept Source OCT (SS-OCT) / phase resolved OCT to detect the nanometer(nm) scale changes in the cell membrane that occur during membrane depolarization and Ion influx. This type of small changes can be seen using phase. Intensity is robust but is less sensitive to the small-scale changes whereas phase is highly sensitive. The noise level for phase difference quantification depends on the lateral motion, triggering of the wavelength mismatch and signal to noise ratio (SNR). To detect the nm scale changes phase noise should be minimum. The phase noise is inversely proportional to SNR. The SNR of the system should be maximum. The SS-OCT has swept source with sweep rate 100 kHz and it is not phase stable. It needs post processing algorithm to match the triggering wavelength to stabilize the phase. I wrote the code in GPU to reduce the computation time for post processing algorithm. Due to a few setbacks related to moving the system to a collaborating lab, the biological portion of my thesis work, trying to find the neural activity in the walking leg nerve of the Lobster using phase, was done with a spectral domain OCT (SD-OCT). To see any action potential change in the nerve I realigned the SD-OCT system with center wavelength to increase the SNR of the system and the sample arm with the nerve chamber (3D printed) to stimulate the nerve at one end and take the OCT image at the other end to see the action potential.

Optical coherence tomography (OCT) is an optical imaging method based on low-coherence interferometry. OCT uses light waves to take cross-section images of your internal microstructure such as retina by measuring light backscattered from the sample. It can provide reliable and high resolution images of biological tissues at different levels. Real-time OCT can be a challenge due to the heavy computational load required to process acquired data streams. Reconstructing functional images with multi-functional OCT imaging requires additional processing, further increasing processing time. So we introduced graphics processing unit (GPU) processing and some code opti- mization strategies of MATLAB to accelerate the image processing. Also, we implemented a web application to run MATLAB program remotely for the convenience of the researchers. It can allow multiple users to remote control MATLAB programs and do some basic oper- ations through both mobile phone and computer. In Chapter1, I will explain the OCT data processing workflow and describe the motivation of OCT data processing acceleration. vi Chapter2 will mainly focus on the methods that I used to improve the performance of the MATLAB code and show the comparison of the total processing time. In Chapter3, I will introduce the features of the web application as well as the structure of the whole application for both front-end and back-end. Chapter4 is the conclusion part of the entire post-processing acceleration project, some future work will also be listed in chapter4.

Our lab aims to apply OCT to the study of the nervous system. In order to detect neural activity using this technology, we measure optical changes that occur in the neural tissue as a result of transient volume changes. Though these changes are critical to our studies, they are poorly understood and so, through analysis of the current literature on the subject, I narrowed down the possible mechanisms for this phenomenon. As such,

our system needs to be able to resolve on the order of microns. We can increase the resolution by increasing the numerical aperture, but this leads to a characteristic trade-off with the depth of field. In order to maximize our resolution and depth of field, I implemented phase sensitive synthetic aperture technology into our OCT system which allows for an increase in the intensity and phase across a DOF beyond the Rayleigh range through modification of the wavefront curvature.

No abstract was necessary

Current methods for detection of nervous system activity have been limited to electrophysiology, which requires the electrode to contact the nervous system or optical-based techniques that require incorporation of reporter agents. Both methods are golden standards but have limitations. Optical coherence tomography (OCT) is an imaging technique based on light that measures intrinsic structural changes associated with an action potential. A benefit of using OCT is that it does not require any exogenous agents and is minimally invasive. Phase-resolved OCT detects 1-30 nm swelling of an axon during neural activity. In my studies described here, phase-resolved OCT was used to detect the swelling Drosophila neurons when presented with ecdysis triggering hormone as well as a 1-3 nm swelling of a cockroach axon caused by action. However, since phase imaging is limited to one point, the dissertation ends with development of the line field swept source OCT system that can provide images of a b-line or cross-sectional image without using a scanning device.

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