Dr. Srikanth Vasudevan Profile

Dr. Srikanth Vasudevan

Visiting Scientist at U.S. Food and Drug Administration

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Publications (5)

SPIE Journal Paper | 17 April 2023 Open Access

Development of polarization-sensitive optical coherence tomography imaging platform and metrics to quantify electrostimulation-induced peripheral nerve injury in vivo in a small animal model

Guillermo Monroy, Mohsen Erfanzadeh, Michael Tao, Damon DePaoli, Ilyas Saytashev, Ahhyun Stephanie Nam, Harmain Rafi, Kasey Kwong, Katherine Shea, Benjamin Vakoc, Srikanth Vasudevan, Daniel Hammer

NPh, Vol. 10, Issue 02, 025004, (April 2023) https://doi.org/10.1117/12.10.1117/1.NPh.10.2.025004

KEYWORDS: Nerve, Optical coherence tomography, Coherence imaging, Animals, Nervous system, Injuries, Biological imaging, Optical imaging, Image segmentation, Tissue optics

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SignificanceNeuromodulation devices are rapidly evolving for the treatment of neurological diseases and conditions. Injury from implantation or long-term use without obvious functional losses is often only detectable through terminal histology. New technologies are needed that assess the peripheral nervous system (PNS) under normal and diseased or injured conditions.AimWe aim to demonstrate an imaging and stimulation platform that can elucidate the biological mechanisms and impacts of neurostimulation in the PNS and apply it to the sciatic nerve to extract imaging metrics indicating electrical overstimulation.ApproachA sciatic nerve injury model in a 15-rat cohort was observed using a newly developed imaging and stimulation platform that can detect electrical overstimulation effects with polarization-sensitive optical coherence tomography. The sciatic nerve was electrically stimulated using a custom-developed nerve holder with embedded electrodes for 1 h, followed by a 1-h recovery period, delivered at above-threshold Shannon model k-values in experimental groups: sham control (SC, n = 5, 0.0 mA / 0 Hz), stimulation level 1 (SL1, n = 5, 3.4 mA / 50 Hz, and k = 2.57), and stimulation level 2 (SL2, n = 5, 6.8 mA / 100 Hz, and k = 3.17).ResultsThe stimulation and imaging system successfully captured study data across the cohort. When compared to a SC after a 1-week recovery, the fascicle closest to the stimulation lead showed an average change of +4 % / − 309 % (SL1/SL2) in phase retardation and −79 % / − 148 % in optical attenuation relative to SC. Analysis of immunohistochemistry (IHC) shows a +1 % / − 36 % difference in myelin pixel counts and −13 % / + 29 % difference in axon pixel counts, and an overall increase in cell nuclei pixel count of +20 % / + 35 % . These metrics were consistent with IHC and hematoxylin/eosin tissue section analysis.ConclusionsThe poststimulation changes observed in our study are manifestations of nerve injury and repair, specifically degeneration and angiogenesis. Optical imaging metrics quantify these processes and may help evaluate the safety and efficacy of neuromodulation devices.

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SPIE Journal Paper | 11 February 2023 Open Access

Improved in vivo optical coherence tomography imaging of animal peripheral nerves using a prism nerve holder

Ilyas Saytashev, Yong-Chul Yoon, Benjamin Vakoc, Srikanth Vasudevan, Daniel Hammer

JBO, Vol. 28, Issue 02, 026002, (February 2023) https://doi.org/10.1117/12.10.1117/1.JBO.28.2.026002

KEYWORDS: Nerve, Prisms, Optical coherence tomography, Animals, In vivo imaging, Coherence imaging, Biomedical optics, Optical imaging, Mirrors, Nervous system

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SignificanceModern optical volumetric imaging modalities, such as optical coherence tomography (OCT), provide enormous information about the structure, function, and physiology of living tissue. Although optical imaging achieves lateral resolution on the order of the wavelength of light used, and OCT achieves axial resolution on a similar micron scale, tissue optical properties, particularly high scattering and absorption, limit light penetration to only a few millimeters. In addition, in vivo imaging modalities are susceptible to significant motion artifacts due to cardiac and respiratory function. These effects limit access to artifact-free optical measurements during peripheral neurosurgery to only a portion of the exposed nerve without further modification to the procedure.AimWe aim to improve in vivo OCT imaging during peripheral neurosurgery in small and large animals by increasing the amount of visualized nerve volume as well as suppressing motion of the imaged area.ApproachWe designed a nerve holder with embedded mirror prisms for peripheral nerve volumetric imaging as well as a specific beam steering strategy to acquire prism and direct view volumes in one session with minimal motion artifacts.ResultsThe axially imaged volumes from mirror prisms increased the OCT signal intensity by >22 dB over a 1.25-mm imaging depth in tissue-mimicking phantoms. We then demonstrated the new imaging capabilities in visualizing peripheral nerves from direct and side views in living rats and minipigs using a polarization-sensitive OCT system. Prism views have shown nerve fascicles and vasculature from the bottom half of the imaged nerve which was not visible in direct view.ConclusionsWe demonstrated improved OCT imaging during neurosurgery in small and large animals by combining the use of a prism nerve holder with a specifically designed beam scanning protocol. Our strategy can be applied to existing OCT imaging systems with minimal hardware modification, increasing the nerve tissue volume visualized. Enhanced imaging depth techniques may lead to a greater adoption of structural and functional optical biomarkers in preclinical and clinical medicine.

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Proceedings Article | 28 April 2022 Presentation

Prism nerve holder for enhanced OCT volumetric imaging of peripheral nerves

Ilyas Saytashev, Anthony Parra, Yong-Chul Yoon, Benjamin Vakoc, Srikanth Vasudevan, Daniel Hammer

Proceedings Volume PC11946, PC119460D (2022) https://doi.org/10.1117/12.2610135

KEYWORDS: Nerve, Optical coherence tomography, Prisms, Tissue optics, In vivo imaging, Tissues, Polarization, Mirrors, Epilepsy, Electrodes

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Proceedings Article | 14 March 2018 Presentation

Optical coherence tomography angiography based biomarkers to assess the safety of peripheral nerve electrostimulation (Conference Presentation)

Srikanth Vasudevan, Ahhyun Nam, Benjamin Vakoc, Daniel Hammer

Proceedings Volume 10481, 1048115 (2018) https://doi.org/10.1117/12.2291646

KEYWORDS: Nerve, Optical coherence tomography, Safety, Electrodes, Injuries, Optical imaging, Relays, Nervous system, Arteries, Blood

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Proceedings Article | 9 March 2016 Paper

Real time imaging of peripheral nerve vasculature using optical coherence angiography

Srikanth Vasudevan, Doe Kumsa, Pavel Takmakov, Cristin Welle, Daniel Hammer

Proceedings Volume 9690, 96901D (2016) https://doi.org/10.1117/12.2213160

KEYWORDS: Nerve, Angiography, Safety, Electrodes, Coherence (optics), Nervous system, Real time imaging, Optical coherence tomography, Injuries, Blood vessels, Tissues, Reflectivity

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