Ultra-fast Imaging of Two-Phase Flow in Structured Monolith Reactors; Techniques and Data Analysis

This thesis will address the use of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques to probe the “monolith reactor”, which consists of a structured catalyst over which reactions may occur. This reactor has emerged as a potential alternative to more traditional chemical engineering systems such as trickle bed and slurry reactors. However, being a relatively new design, its associated flow phenomena and design procedures are not rigorously understood, which is retarding its acceptance in industry. Traditional observations are unable to provide the necessary information for design since the systems are opaque and dynamic. Therefore, NMR is proposed as an ideal tool to probe these systems in detail. The theory of NMR is summarised and the development of novel NMR techniques is presented. Novel techniques are validated in simple systems, and tested in more complex systems to ascertain their quantitative nature, and to find their limitations. These techniques are improvements over existing techniques in that they either decrease the acquisition time (allowing the observation of dynamically-changing systems) or allow us to probe systems in different ways to extract useful information. The goal of this research is to better understand the flow phenomena present in such systems, and to use this information to design better, more efficient, more controllable industrial reactors. The analysis of the NMR data acquired is discussed in detail, and several novel image-processing techniques have been developed to aid in the quantification of features within the images, and also to measure quantities such as holdup and velocity. These novel techniques are validated, and then applied to the systems of interest. Various configurations of monolith reactor, ranging from low flow rate systems to more challenging (and more industrially relevant) turbulent systems, are probed using these methods, and the contrasting flow phenomena and performance of these systems are discussed, with a view to optimisation of the choice of design parameters

Denoising OCT Images Using Steered Mixture of Experts with Multi-Model Inference

In Optical Coherence Tomography (OCT), speckle noise significantly hampers image quality, affecting diagnostic accuracy. Current methods, including traditional filtering and deep learning techniques, have limitations in noise reduction and detail preservation. Addressing these challenges, this study introduces a novel denoising algorithm, Block-Matching Steered-Mixture of Experts with Multi-Model Inference and Autoencoder (BM-SMoE-AE). This method combines block-matched implementation of the SMoE algorithm with an enhanced autoencoder architecture, offering efficient speckle noise reduction while retaining critical image details. Our method stands out by providing improved edge definition and reduced processing time. Comparative analysis with existing denoising techniques demonstrates the superior performance of BM-SMoE-AE in maintaining image integrity and enhancing OCT image usability for medical diagnostics.Comment: This submission contains 10 pages and 4 figures. It was presented at the 2024 SPIE Photonics West, held in San Francisco. The paper details advancements in photonics applications related to healthcare and includes supplementary material with additional datasets for revie

Investigation of Neonatal Pulmonary Structure and Function via Proton and Hyperpolarized Gas Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a modality that utilizes the phenomenon of nuclear magnetic resonance (NMR) to yield tomographic images of the body. Proton (1H) MRI has historically been successful in soft tissues but has suffered in the lung due to a variety of technical challenges, such as the low proton-density, rapid T2* relaxation time of the lung parenchymal tissue, and inherent physiological motion in the chest. Recent developments in radial ultrashort echo time (UTE) MRI have in part overcome these issues. In addition, there has been much progress in techniques for hyperpolarization of noble gases (3He and 129Xe) out of thermal equilibrium via spin exchange optical pumping, which can greatly enhance the gas NMR signal such that it is detectable within the airspaces of the lung on MRI. The lung is a unique organ due to its complex structural and functional dynamics, and its early development through the neonatal (newborn) period is not yet well understood in normal or abnormal conditions. Pulmonary morbidities are relatively common in infants and are present in a majority of patients admitted to the neonatal intensive care unit, often stemming from preterm birth and/or congenital defects. Current clinical lung imaging in these patients is typically limited to chest x-ray radiography, which does not provide tomographic information and so has lowered sensitivity. More rarely, x-ray computed tomography (CT) is used but exposes infants to ionizing radiation and typically requires sedation, both of which pose increased risks to pediatric patients. Thus the opportunity is ripe for application of novel pulmonary MRI techniques to the infant population. However, MR imaging of very small pulmonary structure and microstructure requires fundamental changes in the imaging theory of both 1H UTE MRI and hyperpolarized gas diffusion MRI. Furthermore, such young patients are often non-compliant, yielding a need for new and innovative techniques for monitoring respiratory and bulk motion. This dissertation describes methodology development and provides experimental results in both 1H UTE MRI and hyperpolarized 3He and 129Xe gas diffusion MRI, with investigation into the structure and function of infant lungs at both the macrostructural and microstructural level. In particular, anisotropically restricted gas diffusion within infant alveolar microstructure is investigated as a measurement of airspace size and geometry. Additionally, the phenomenon of respiratory and bulk motion-tracking via modulation of the k-space center\u27s magnitude and phase is explored and applied via UTE MRI in various neonatal pulmonary conditions to extract imaging-based metrics of diagnostic value. Further, the proton-density regime of pulmonary UTE MRI is validated in translational applications. These techniques are applied in infants with various pulmonary conditions, including patients diagnosed with bronchopulmonary dysplasia, congenital diaphragmatic hernia, esophageal atresia/tracheoesophageal fistula, tracheomalacia, and no suspected lung disease. In addition, explanted lung specimens from both infants with and without lung disease are examined. Development and implementation of these techniques involves a strong understanding of the physics-based theory of NMR, hyperpolarization, and MR imaging, in addition to foundations in hardware, software, and image analysis techniques. This thesis first outlines the theory and background of NMR, MRI, and pulmonary physiology and development (Part I), then proceeds into the theory, equipment, and imaging experiments for hyperpolarized gas diffusion MRI in infant lung airspaces (Part II), and finally details the theory, data processing methods, and applications of pulmonary UTE MRI in infant patients (Part III). The potential for clinical translation of the neonatal pulmonary MRI methods presented in this dissertation is very high, with the foundations of these techniques firmly rooted in the laws of physics

Design and performance analysis of a picosecond-pulsed laser raman spectrometer for fluorescence rejection in raman spectroscopy

Many attempts have been made to reduce fluorescence backgrounds in Raman spectra. A critical appraisal of fluoresence rejection techniques reveals that while many techniques are available which improve the Raman/fluorescence ratio (R/F), very few actually increase the spectral signal/noise (R/N), which is the most important parameter. Temporal-resolution of Raman and fluorescence photons was investigated in this laboratory, using a picosecond-laser system and gated photon detection. Two detection methods were evaluated. The first, an intensified diode array detector (DAD), could be gated "on" for periods of ca. 5 ns, at rates of up to 5kHz. This gave a 5-fold increase in R/F, but a slight reduction in R/N, for a fluorescor with τ(_f) ̴̱ 1O.5 ns. The R/N degradation arose as a result of the low laser output intensity at kHz pulse rates, rather than inefficiency in fluorescence rejection. The second method used a continuously-operated photomultiplie tube (PMT), and time-correlated photon counting with ca. 1 ns timing-resolution. This yielded R/F and R/N improvements of ca. 15 and 3 respectively (τ(_f) ̴̱ 12 ns).Although efficient fluorescence rejection was obtained with each system, the corresponding R/N enhancements were not practically significant. However, the development of theoretical models describing the performance of each system has identified modifications which should give valuable improvements. These include the use of a laser with MW peak powers at kHz pulse rates (DAD system), and use of a microchannel-plate PMT with 50 ps timing resolution. When these (and other) modifications are made, significant R/N enhancements (ca. 7 and 13 (DAD and PMT systems respectively)) are expected, thus enabling the study of the majority of "real world" samples. In addition, the limiting theoretical and practical performance of time-resolved rejection is considered, and several hitherto unreported aspects of the behaviour of the laser and detection systems are discussed. Other techniques were also evaluated, in particular utilising the differing Raman and fluorescence response to variations in laser intensity. While the non-linear fluorescence responseto intensity variations of cw lasers has been previously exploited, simple calculations indicate that the use of high-powered pulsed sources could allow discrimination at ca. 100- fold lower average powers. However, a satisfactory test of the calculations requires the construction of apparatus not presently available in this .laboratory

Quantification in time-domain diffuse optical tomography using mellin-laplace transforms

Simulations and phantom measurements are used to evaluate the ability of time-domain diffuse optical tomography using Mellin-Laplace transforms to quantify the absorption perturbation of centimetric objects immersed at depth 1-2 cm in turbid media. We find that the estimated absorption coefficient varies almost linearly with the absorption change in the range of 0-0.15 cm-1 but is underestimated by a factor that depends on the inclusion depth (~2, 3 and 6 for depths of 1.0, 1.5 and 2.0 cm respectively). For larger absorption changes, the variation is sublinear with ~20% decrease for δμa = 0.37 cm-1. By contrast, constraining the absorption change to the actual volume of the inclusion may considerably improve the accuracy and linearity of the reconstructed absorption

Few-femtosecond deep-UV Pulses for transient-absorption experiments

In this thesis I describe the development, implementation and characterisation of a source of wavelength-tunable few-femtosecond laser pulses in the deep ultraviolet spectral region for use in time-resolved experiments. I also propose and model an extension of this source capable of simultaneously generating a single-cycle driving pulse for extreme nonlinear optics as well as a few-femtosecond ultraviolet pulse. Building on advances in the field of femtochemistry, ultrafast science is moving towards ever shorter timescales and more complex systems. One of the key building blocks for the next generation of experiments studying ultrafast dynamics in molecules will be the availability of few-femtosecond pulses to directly address electronic resonances whose corresponding photon energy lies in the vacuum and deep ultraviolet spectral regions. By harnessing the capabilities of soliton self-compression in novel micro-structured waveguides, we have generated pulses in the deep ultraviolet with energies of hundreds of nanojoules. The delivery of these pulses to an experiment as well as the measurement of their temporal profile pose significant challenges due to the dispersive properties of optical materials in the ultraviolet. We have developed an in-vacuum device for ultrafast pulse characterisation, and by directly coupling the waveguide to vacuum we were able to measure distortion-free pulses with durations below 10 fs at a range of different central wavelengths. Numerical modelling of a scaled-up version of the apparatus shows that the self-compressed driving pulse in the ultraviolet pulse generation process can maintain its shape when delivered directly to vacuum. The single-cycle pulse duration makes it an ideal driver for extreme nonlinear optics and the generation of isolated attosecond pulses in the soft X-ray spectral region.Open Acces