1,090 research outputs found

    High Performance Optical Computed Tomography for Accurate Three-Dimensional Radiation Dosimetry

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    Optical computed tomography (CT) imaging of radiochromic gel dosimeters is a method for truly three-dimensional radiation dosimetry. Although optical CT dosimetry is not widely used currently due to previous concerns with speed and accuracy, the complexity of modern radiotherapy is increasing the need for a true 3D dosimeter. This thesis reports technical improvements that bring the performance of optical CT to a clinically useful level. New scanner designs and improved scanning and reconstruction techniques are described. First, we designed and implemented a new light source for a cone-beam optical CT system which reduced the scatter to primary contribution in CT projection images of gel dosimeters from approximately 25% to approximately 4%. This design, which has been commercially implemented, enables accurate and fast dosimetry. Second, we designed and constructed a new, single-ray, single-detector parallel-beam optical CT scanner. This system was able to very accurately image both absorbing and scattering objects in large volumes (15 cm diameter), agreeing within ∌1% with independent measurements. It has become a reference standard for evaluation of optical CT geometries and dosimeter formulations. Third, we implemented and characterized an iterative reconstruction algorithm for optical CT imaging of gel dosimeters. This improved image quality in optical CT by suppressing the effects of noise and artifacts by a factor of up to 5. Fourth, we applied a fiducial-based ray path measurement scheme, combined with an iterative reconstruction algorithm, to enable optical CT reconstruction in the case of refractive index mismatch between different media in the scanner’s imaged volume. This improved the practicality of optical CT, as time-consuming mixing of liquids can be avoided. Finally, we applied the new laser scanner to the difficult dosimetry task of small-field measurement. We were able to obtain beam profiles and depth dose curves for 4 fields (3x3 cm2 and below) using one 15 cm diameter dosimeter, within 2 hours. Our gel dosimetry depth-dose curves agreed within ∌1.5% with Monte Carlo simulations. In conclusion, the developments reported here have brought optical CT dosimetry to a clinically useful level. Our techniques will be used to assist future research in gel dosimetry and radiotherapy treatment techniques

    HYPERSPECTRAL LINE-SCANNING MICROSCOPY FOR HIGH-SPEED MULTICOLOR QUANTUM DOT TRACKING

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    One of the challenges in studying protein interactions in live cells lies in the capacity to obtain both spatial and temporal information that is sufficient to extend existing knowledge of the dynamics and interactions, especially when tracking proteins at high density. Here we introduce a high-speed laser line-scanning hyperspectral microscope that is designed to track quantum dot labeled proteins at 27 frames/sec over an area of 28 um2 using 128 spectral channels spanning the range from 500 to 750 nm. This instrument simultaneously excites 8 species of quantum dots and employs a custom prism spectrometer and high speed EMCCD to obtain spectral information that is then used to distinguish and track individual probes at high density. These emitters are localized to within 10s of nm in each frame and reconstructed trajectories yield information of the protein dynamics and interactions. This manuscript describes the design, implementation, characterization, and application of a high-speed laser line-scanning hyperspectral microscope (HSM). The intended primary application is that of investigating the dynamics of transmembrane antibody receptors using quantum dot labeled immunoglobulin E (QD-IgE). Several additional examples demonstrate other advantages and applications of this method, including 3D hyperspectral imaging of live cells and hyperspectral superresolution imaging

    Development and Implementation of Fully 3D Statistical Image Reconstruction Algorithms for Helical CT and Half-Ring PET Insert System

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    X-ray computed tomography: CT) and positron emission tomography: PET) have become widely used imaging modalities for screening, diagnosis, and image-guided treatment planning. Along with the increased clinical use are increased demands for high image quality with reduced ionizing radiation dose to the patient. Despite their significantly high computational cost, statistical iterative reconstruction algorithms are known to reconstruct high-quality images from noisy tomographic datasets. The overall goal of this work is to design statistical reconstruction software for clinical x-ray CT scanners, and for a novel PET system that utilizes high-resolution detectors within the field of view of a whole-body PET scanner. The complex choices involved in the development and implementation of image reconstruction algorithms are fundamentally linked to the ways in which the data is acquired, and they require detailed knowledge of the various sources of signal degradation. Both of the imaging modalities investigated in this work have their own set of challenges. However, by utilizing an underlying statistical model for the measured data, we are able to use a common framework for this class of tomographic problems. We first present the details of a new fully 3D regularized statistical reconstruction algorithm for multislice helical CT. To reduce the computation time, the algorithm was carefully parallelized by identifying and taking advantage of the specific symmetry found in helical CT. Some basic image quality measures were evaluated using measured phantom and clinical datasets, and they indicate that our algorithm achieves comparable or superior performance over the fast analytical methods considered in this work. Next, we present our fully 3D reconstruction efforts for a high-resolution half-ring PET insert. We found that this unusual geometry requires extensive redevelopment of existing reconstruction methods in PET. We redesigned the major components of the data modeling process and incorporated them into our reconstruction algorithms. The algorithms were tested using simulated Monte Carlo data and phantom data acquired by a PET insert prototype system. Overall, we have developed new, computationally efficient methods to perform fully 3D statistical reconstructions on clinically-sized datasets

    Attosecond dynamics of collective electron effects in nanostructures and molecules

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    In this work several time-resolved experimental studies on collective electron phenomena are presented. The first set of measurements aims to reveal the dynamics of surface plasmon polaritons (SPP), which emerge as coherent excitation of quasi-free conduction band electrons at a metal-dielectric-interface. The experiments employ a pump-probe scheme, where a few-cycle near-infrared (NIR) laser pulse, considered as pump beam, excites SPPs at a nanostructure grating. After propagation towards a nanoscale apex, where adiabatic focusing and localization is observed, the SPP electric field is probed by a second few-cycle laser field with variable delay, which constitutes a direct replica of the pump beam. The cross-correlation of SPP and near-infrared probe beam at the apex facilitates multi-photon ionization (MPI) from the metal sample and allows to obtain the convoluted temporal dynamics of both electric fields from the delay-dependent photoionization yield. To achieve temporal resolution on the attosecond (1 as = 10^{−18} s) scale, consecutive studies are performed to demonstrate the applicability of the attosecond streaking technique to nanoscale electric near-fields. These proof-of-principle measurements are conducted on tapered gold (Au) nanowires, which are illuminated by a few-cycle near-infrared laser pulse. The superposition of incident and scattered laser field gives rise to a characteristic near-field at the streaking target surface. A synchronized, extreme-ultraviolet (XUV) laser pulse of attosecond temporal duration generates photoelectrons from the target, whose final kinetic energy is modulated whilst propagating through the electric near-field. Acquisition of delaydependent photoelectron energy spectra unambiguously shows the signature of the nanoscale near field, which appears phase shifted relative to the incident laser field. Experimental findings are supported by theoretical simulations, which consider the specific target geometry. Based on trajectory calculations, generic prerequisites on the experimental setup and target geometry are formulated to facilitate sampling of nanoscale electric fields with attosecond temporal resolution. In the third set of experiments, the fundamental influence of collective electron effects on photoionization delays is investigated using the attosecond streaking technique on gaseous ethyl iodide molecules. Here, the photon energy of the extreme-ultraviolet radiation is chosen to overlap with the giant dipole resonance of the 4d shell in atomic iodine. The resulting photoionization delays are referenced by a simultaneous streaking measurement on atomic neon. The experiments, performed at different XUV photon energies, reveal a drastic increase of photoionization delays with decreasing XUV photon energy. To explain the observed temporal delays, simulations at different levels of theory are performed, including ab initio, quantum scattering and semi-classical calculations. Besides collective electron effects, the influence from molecular orbitals on the obtained photoionization delays is discussed.In der vorliegenden Arbeit werden mehrere zeitaufgelöste, experimentelle Studien ĂŒber die Dynamik kollektiver ElektronenphĂ€nomene vorgestellt. Im ersten Teil der Messungen wird versucht, die Dynamik von OberflĂ€chenplasmonen (SPP) offenzulegen, die als kohĂ€rente Anregung an GrenzflĂ€chen zwischen Metallen und Dielektrika in Erscheinung treten. Die Experimente nutzen ein Pump-Probe Schema, wobei der Pumppuls einen wenige Zyklen umfassenden Laserpuls im nah-infraroten Spektralbereich darstellt, der die SPPs an einem nanostrukturierten Gitter erzeugt. Ausgehend vom Gitter breiten sich die SPPs in Richtung einer Nanospitze aus, an der das elektrische Feld der Plasmonen adiabatisch fokussiert und eingegrenzt wird. Mit einem zweiten, zeitlich verzögerten Laserpuls, der im Folgenden als Probepuls bezeichnet ist und eine direkte Kopie des Pumppulses darstellt, wird das elektrische Feld der SPPs untersucht. Die Überlagerung des elektrischen SPP-Feldes mit dem Probepuls fĂŒhrt zur Multi-Photonen Ionisation (MPI) der metallischen Probe. Aus der Ionisationsrate, die von der zeitlichen Verzögerung zwischen Pump- und Probepuls abhĂ€ngt, ergibt sich die Dynamik von SPP und Probepuls als Faltung beider elektrischer Felder. Um eine zeitliche Auflösung auf der Attosekunden-Zeitskala (1 as = 10^{-18} s) zu ermöglichen, wurden weiterfĂŒhrende Untersuchungen durchgefĂŒhrt, die die Anwendbarkeit des Attosekunden Streakings bei nanoskaligen elektrischen Feldern demonstrieren. Die Pilotexperimente wurden an konisch verjĂŒngten Gold (Au) NanodrĂ€hten durchgefĂŒhrt. Dabei wurden die Proben von einem wenige Zyklen umfassenden Laserstrahl im nah-infraroten Spektralbereich beleuchtet, wodurch aus der Überlagerung von einfallendem und gestreutem Laserstrahl ein charakteristisches Nahfeld entsteht. Ein synchronisierter Attosekundenlaserstrahl im extremen ultravioletten (XUV) Spektralbereich fĂŒhrt zur Emission von Photoelektronen vom Nanodraht. Nach Propagation durch das elektrische Nahfeld wurde die kinetische Energie der Photoelektronen gemessen, die grundsĂ€tzlich von der zeitlichen Verzögerung zwischen XUV und NIR Laserstrahl abhĂ€ngt. Im aufgenommen Spektrogramm der Photoelektronen zeigt sich die eindeutige Signatur des Nahfeldes, das im Vergleich zum einfallenden NIR Laserstrahl phasenverschoben erscheint. Die experimentellen Ergebnisse werden durch theoretische Simulationen, die die spezifische Probengeometrie berĂŒcksichtigen, unterstĂŒtzt. Basierend auf Trajektorien Rechnungen, werden allgemeingĂŒltige Bedingungen an den experimentellen Aufbau sowie die Probengeometrie formuliert, unter denen die Untersuchung von nanoskaligen elektrischen Feldern möglich ist. Der dritte Teilbereich der Experimente untersucht den fundamentalen Einfluss kollektiver Elektroneneffekte auf die zeitliche Verzögerung der Photoionisation von gasförmigen EthyliodidmolekĂŒlen unter Anwendung der Attosekunden Streaking-Methode. Die Photonenenergie des XUV Pulses wurde dabei so gewĂ€hlt, dass die Ionisation im Bereich der Riesenresonanz der 4d-Elektronenschale des Iods erfolgt. Die Experimente, die bei verschiedenen XUV Photonenenergien durchgefĂŒhrt wurden, offenbaren einen deutlichen Anstieg der Verzögerung der Photoionisation in Richtung niedrigerer XUV Energien. Um die zeitliche Verzögerung zu erklĂ€ren, werden unterschiedliche theoretische AnsĂ€tze verfolgt, wobei semi-klassische, ab initio und Quanten-Streurechnungen durchgefĂŒhrt werden. Neben den kollektiven Elektroneneffekten werden auch mögliche EinflĂŒsse der MolekĂŒlorbitale diskutiert

    Correlative microscopic characterization of nanoscale assemblies at interfaces

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    Correlative microscopic characterization of nanoscale assemblies at interfaces

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    Lensless imaging with high-harmonic sources

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    Coherent diffractive imaging (CDI) is a family of computational imaging techniques that uses iterative reconstruction algorithms to decipher the information encoded in one or more interference patterns to reconstruct an image of an object located in another propagation plane. The lensless nature of these techniques makes them well-suited for imaging with coherent extreme ultraviolet (EUV) or x-ray illumination as refractive optics are limited at these wavelengths. In particular, this work investigates the use of CDI techniques in combination with high-harmonic generation. High-harmonic generation~(HHG) sources can generate EUV illumination beams with a high degree of spatial coherence in a compact tabletop setup. In this work we use Fourier-Transform spectroscopy~(FTS) to separate sets of nearly monochromatic diffraction patterns from a broadband HHG diffraction pattern. These monochromatic diffraction patterns can used to reconstruct spectrally resolved images through reconstruction methods that are similar to those applied in conventional CDI. In Chapter 4 we describe how we use a common path interferometer and a noncollinear chirped pulse amplifier system to generate phase locked 25 fs pulse pairs with a central wavelength of approximately 800 nm and a combined pulse energy of 10 mJ. These infrared driving laser pulses are focused at slightly separated locations in a noble gas jet to upconvert them into a pair of almost identical high-harmonic pulses. In FTS-based imaging experiments, we illuminate a sample with the HHG pulse pairs and record the far-field diffraction pattern as a function of pulse-to-pulse time delay. The spatial separation of our two harmonic beams results in spatial interference between two laterally sheared copies of the diffraction pattern. As a consequence of the geometry, the spectrally separated diffraction patterns obtained in these measurements are similar, but not identical to the standard CDI case. In this work, we demonstrated an algorithm, called diffractive shear interferometry (DSI), to reconstruct images from such diffraction patterns. Using this algorithm, the information present in these diffraction patterns is used to reconstruct complex images of the sample. The reverse problem is either constrained by combining an diffraction pattern with a finite object support prior in Chapter 5 or with other diffraction patterns with a different relative orientation between the shear and the object. One of the advantages of coherent diffractive imaging techniques is that it they reconstruct the full complex electric field at the sample. In reflection mode, such phase difference can be easily attributed to height differences of the reflecting surface. However, most research in diffractive imaging has focused on transmission mode imaging. At the EUV wavelengths generated by HHG sources normal incidence reflection coefficients are vanishingly small. However towards grazing incidence the reflection coefficients approach one. Such a geometry does come at a cost of added experimental and computational complexity. While far-field diffraction between colinear planes can be described by a straight forward Fourier transform of the electric field, for the propagation between non-collinear planes, an additional non-linear coordinate transformation is required. This coordinate transformation depends on the tilt angle of the fields and becomes very sensitive to the exact tilt-angle towards grazing incidence. While CDI itself requires accurate knowledge of the wave propagation, a technique known as ptychography offers more flexibility, as it is often possible to solve for more variables than just the object field. In Chapter 7 we use that property to demonstrate an auto-calibration algorithm that can iteratively calibrate the tilt-angle during a ptychographic reconstruction. Using this approach we were able to refine the tilt angle close to the correct value even when the initial estimates were off by more than 5 degrees, greatly improving flexibility in reflection-mode lensless imaging

    Feasibility study of fan-beam coherent scatter computed tomography

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    EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Adaptive, High-Resolution Ultrasound Phased Array Imaging for use in the Inspection of Laser Brazed Joints in the Automotive Sector

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    The inspection of welded and brazed joints has been performed in several industries using ultrasonic phased array. In the automotive sector, many of the current standards for brazed joint inspection do not apply due to the high variations in surface geometry and limited accessibility to the inspection region. As the automotive industry looks to integrate laser brazing into the production process, the need to determine the size and geometry of the joint, as well as the presence of any defects, is desirable to ensure product quality and reduce costs. Currently, the use of destructive techniques, such as cross-sectioning, is employed in the inspection process, with the ultimate desire being the shift to non-destructive methods. With this in mind, ultrasonic techniques have been investigated as a possible testing method. Ultrasound techniques have evolved over the decades, starting from a single element and eventually moving to phased array techniques. Recently, the investigation of the full matrix capture method has become popular in the field of ultrasound imaging. This technique, which separates the data acquisition process from the image formation process poses a viable solution to the inspection of laser brazed joints due to the ability to compensate for varying surfaces in post-processing.In this work, we make use of this technique, deriving the image formation process as an inverse problem for an arbitrary set of ultrasonic emitters and receivers. From this, the image formation process becomes equivalent to solving the inhomogeneous Helmholtz equation. By approximating the solutions to such an equation using the ray series expansion, an estimation of the solutions can be found in a time-efficient manner. When these solutions are found, the inverse process can be rewritten as a weighted, time-delayed summation of the acquired ultrasonic data. In current work, further approximations to this image formation process are often made; however, in the inspection of the laser braze process, these approximations are found to degrade image quality in a number of cases. In this work, we propose our second order corrections as a viable solution to increase the limit under which ultrasound imaging can currently occur. This is accomplished through the design of an ultrasonic array transducer and the manufacturing of a series of simulated defects, with the final assessment being performed on real joints.These techniques were found to improve imaging in a select set of samples when the radius of curvature dropped below 2 mm. In these cases, the use of the amplitude weighting was found to drastically improve system resolution, allowing for the determination of joint size, geometry and the presence of defects

    Dynamical control of one- and two-dimensional optical fibre scanning

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    This thesis investigates the dynamical control of one- and two-dimensional optical fibre scanning. One dimensional scanning is performed with a mechanically biaxial polarisation-preserving fibre mounted on a piezoelectric transducer with one of its principal mechanical axes aligned parallel to the excitation direction. The addition of an apertured reflector in front of the imaging lens allows a position sensing mechanism based on intermittent optical feedback to be integrated into the scanner. Over-scanning the lens generates timing pulses interlaced with back-scattered signals from the target. The timing information can be used for closed loop control of the phase and amplitude of vibration. Suitable control algorithms are developed and their convergence and stability is studied. This thesis also investigates the construction of fibres with enhanced mechanically asymmetry and their dynamical properties during two-dimensional imaging based on Lissajous scan patterns. Dip-coating is proposed as a method of forming two-cored waveguide cantilevers from two separate, parallel fibres that are encapsulated in a plastic coating. The frequency ratio between the two orthogonal bending mode resonances can be controlled with number of coatings. An exact image reconstruction algorithm based on Lissajous scanning is proposed. Latency, transient response and steady-state phase errors are all shown to cause dramatic deterioration of the reconstructed image. Solutions are provided by ensuring the correct starting time for data acquisition and introducing a drive phase correction to one of the axes. Two methods of resolution enhancement are demonstrated. The first is based on combining data sets obtained during separate scans carried out with deliberately applied phase offsets. The second operates by combining data sets from separate imaging operations carried out using the two different fibre cores. Finally, this thesis demonstrates potential applications in optogenetics by combining the two operations of imaging and writing, using different light sources that may also have different wavelengths.Open Acces
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