Browsing by Subject "imaging"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Combining TMS and EEG for Characterizing Motor Network Interactions and Improving Motor Recovery after Stroke
    (2016-12) Johnson, Nessa
    Imaging of electrophysiological activity within the brain is crucial to understanding function in both healthy and disease conditions. The overall goal of this dissertation is to use both non-invasive neuromodulation and non-invasive neuroimaging to characterize and manipulate underlying neurological network dynamics in both healthy and stroke affected subjects. The two main applications of work are for the evaluation of peripheral motor activity on motor network dynamics in healthy subjects, and as a brain-based treatment for motor recovery after stroke. Combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) imaging can be used to analyze cortical reactivity and connectivity of underlying brain networks. However, the effect of corticospinal and peripheral muscle activity on TMS-evoked potentials (TEPs), particularly in motor areas, is not well understood. One aim of the present dissertation is to evaluate the relationship between cortico-spinal activity, in the form of peripheral motor-evoked potentials (MEPs), and the TEPs from motor areas, along with the connectivity among activated brain areas. This research demonstrates that TMS-EEG, along with adaptive connectivity estimators, can be used to evaluate the cortical dynamics associated with sensorimotor integration and proprioceptive manipulation. Stroke is a devastating neurological disorder which can result in lasting impairment affecting quality-of-life. Combining contralesional repetitive TMS (rTMS) with EEG-based brain-computer interface (BCI) training can address motor impairment after stroke by down-regulating exaggerated inhibition from the contralesional hemisphere and encouraging ipsilesional activation. Another aim of this dissertation was to evaluate the efficacy of combined rTMS+BCI, compared to sham rTMS+BCI, and BCI alone, on motor recovery after stroke in subjects with lasting motor paresis. As evaluated in a series of stroke patients, such a brain-based neuromodulatory and imaging approach for rehabilitation could potentially lead to greater understanding of the influence of brain network dynamics in recovery and design of optimal treatment strategies for individual patients. Our findings demonstrate the feasibility and efficacy of not only combined rTMS+BCI but also BCI alone, as demonstrated by significant improvements over time in behavioral and electrophysiological measures. In summary, the present dissertation research developed and evaluated the combination of neuromodulation and neuroimaging for the non-invasive mapping of motor network activities in the diseased and normal brain. Evaluations were conducted in healthy controls to evaluate the influence of peripheral muscle activity on resulting neural network activity, as well as in stroke patients to provide a brain-based treatment for motor rehabilitation. The results obtained suggest the importance of non-invasive spatiotemporal neuroimaging, along with non-invasive neuromodulation, for providing insight into neuroscience questions and providing novel treatments for clinical problems in a brain-based manner.
  • Thumbnail Image
    Item
    Developing Stimulated Raman Spectroscopic Techniques For Imaging Below the Optical Diffraction Limit
    (2020-05) Graefe, Christian
    Stimulated Raman spectroscopy (SRS) is a technique that amplifies the normally weak Raman scattering process using an additional laser beam, resulting in increased signal amplitudes. For this reason, it has been developed as a biological imaging platform with the potential to be used as an alternative to fluorescence microscopy due to its chemical specificity. This eliminates the need for fluorescent tags, which can photobleach or disrupt the structure or dynamics of the system of interest. However, due to the optical diffraction limit SRS cannot compete with the spatial resolution that super-resolution fluorescence techniques are capable of. An SRS-based technique capable of breaking the diffraction limit would therefore allow for nanoscale research to occur on systems for which super-resolution fluorescence is not an option. To that end, we developed a method to improve spatial resolution in SRS using a toroidal beam to deplete SRS signal. As a result, signal is only generated in a reduced area at center of the beam. Initial experiments demonstrated up to 97% depletion of the signal and explored the properties of the depletion process. Additionally, we improved spatial resolution by approximately a factor of two using the toroidal beam to deplete signal while scanning the laser beams over the edge of a diamond plate. While the proof-of-concept experiments were successful, they were performed with a laser with high peak power and a relatively low repetition rate of 1 kHz. These high powers were not compatible with soft matter samples, causing significant photodamage. We therefore adapted super-resolution SRS on laser with a 2.04 MHz repetition rate to average faster and increase the peak power flexibility. Experiments on the 2.04 MHz laser corroborated many proof-of-concept results, including resolution improvement by about a factor of two. However, depletion iv was not achieved with the same efficiency and further improvements in resolution were not forthcoming. This is likely due to the inconsistent phase of the laser’s fundamental pulse profile, highlighting the importance of consistent and reproducible pulses when driving sensitive nonlinear optical processes. Additionally, we demonstrate the use of a new Raman tag using carboranes. By scanning a thin film of a carborane-terminated poly(N-isopropylacrylamide) (pNIPAAm), we show that their high density of B-H bonds and their unique vibrational frequency in the cell silent region make carboranes useful Raman imaging tags that expand multiplexing options. Carboranes’ role as reversible addition-fragmentation chain transfer (RAFT) polymerization agents make them especially good endogenous probes for polymers produced in this manner. Finally, we discuss planned experiments to further improve signal-to-noise ratio (SNR) and explore the mechanism of signal depletion. We also discuss applications of Raman imaging in lipid dynamics, using both diffraction-limited and sub-diffraction techniques. We propose possible methods to compare results from Raman and fluorescence microscopy to determine the impact of fluorescent tags on dynamics. In the research described herein, we develop and explore new Raman imaging methods and highlight the potential power of super-resolution SRS as a versatile chemical imaging tool.
  • Thumbnail Image
    Item
    Going Deeper into Laser Damage: Experiments and Methods for Characterizing Materials in High Power Laser Systems
    (2016-05) Taylor, Lucas
    Laser damage is a primary limiting factor to the design of high-power laser systems. This is true for short-pulse systems as well as long-pulse and continuous-wave (CW) systems. Unlike short-pulse laser damage, CW laser damage has been much less studied. This work comprises a background of laser damage and laser heating theory, a CW laser damage experiment and an imaging technique for monitoring laser heating. The damage experiment was performed on 100 nm thick hafnia coatings deposited on fused silica. Uniformly grown films were compared to hafnia-alumina nanolaminates. While the nanolaminates are known to perform better for 1 ns pulses, we found they had worse laser damage performance in the CW regime. We found the nanolaminates reduced crystallinity. The polycrystalline uniform films are thought to have increased absorption. We measured the thermal conductivity of the nanolaminates to be approximately 1/2 that of the uniform films. A theoretical model including the absorption and thermal conductivity of the nanolaminate and uniform film agreed with the experimental data for 1 ns pulses and CW tests. During laser damage experiments, anomalous damage morphologies were observed that we were unable to explain with theoretical techniques. We then developed an experimental method to observe high-speed laser damage events at the ms time-scale. We imaged laser heating and compared it to a theoretical model with good agreement. Our measurement method captured image data from a Mach- Zender interferometer that had do be processed ex-situ. We desired a system capable of providing real-time thermal data. We developed an image processing technique at least 66 times faster than the original method.
  • Thumbnail Image
    Item
    Photoacoustic Lifetime Imaging and its biomedical applications
    (2016-01) Shao, Qi
    Even though oxygen plays a crucial role in body function and cancer biology, methods of measuring oxygen level in tissue are all limited. The current gold standard relies on an invasive electrode for only single-point reading at a time. The photoacoustic lifetime imaging (PALI) approach overcomes these major limitations by applying photoacoustic probing to oxygen-sensitive optical transient absorption. The capability of assessing oxygen distribution is demonstrated by imaging tumor hypoxia in a small animal model, and monitoring changes of tissue oxygen induced by external modulations. Proposed applications of this imaging technique includes imaging-guided photodynamic therapy (PDT) and activatable probes for molecular imaging.