Tomography is a powerful technique to non-destructively determine the interior structure of an object.Usually, a series of projection images (e.g.\ X-ray images) is acquired from a range of... Show moreTomography is a powerful technique to non-destructively determine the interior structure of an object.Usually, a series of projection images (e.g.\ X-ray images) is acquired from a range of different positions.from these projection images, a reconstruction of the object's interior is computed. Many advanced applications require fast acquisition, effectively limiting the number of projection images and imposing a level of noise on these images. These limitations result in artifacts (deficiencies) in the reconstructed images. Recently, deep neural networks have emerged as a powerful technique to remove these limited-data artifacts from reconstructed images, often outperformingconventional state-of-the-art techniques. To perform this task, the networks are typically trained on a dataset of paired low-quality and high-quality images of similar objects. This is a major obstacle to their use in many practical applications. In this thesis, we explore techniques to employ deep learning in advanced experiments where measuring additional objects is not possible. Show less
This dissertation is an experimental study of laser-generated, atmospheric pressure, transient toroidal helium plasmas.The formation mechanism of these toroidal plasmas is identified and an... Show moreThis dissertation is an experimental study of laser-generated, atmospheric pressure, transient toroidal helium plasmas.The formation mechanism of these toroidal plasmas is identified and an estimate of their main plasma parameters is obtained. Furthermore, preliminary experiments are presented, aimed at heating these plasmas by absorption of microwave radiation, in order to counteract their transient nature.Through a tomographic reconstruction, cross-sectional images of the toroidal plasma are obtained, visualising the fluid flow responsible for the generation of the toroidal structure. The origin of the flow is traced back to the structure of the plasma kernel. The shocks generated by this kernel interact akin to a Mach reflection and generate a low pressure region whose replenishment transforms the plasma into a toroid. Schlieren imaging, complemented with a novel scanning-probe technique, and thermodynamic modelling, as well as deliberately breaking the flow symmetry, confirm the formation mechanism. A high-power, sub-microsecond rise time, pulsed magnetron source has been designed for the microwave heating experiments. Its detailed design and the effect of the microwave pulse on the plasma are discussed.This work is part of a larger study on self-organising knotted magnetic structures in plasma, which may find their application in nuclear fusion and astrophysical research. Show less
With tomography it is possible to reconstruct the interior of an object without destroying. It is an important technique for many applications in, e.g., science, industry, and medicine. The runtime... Show moreWith tomography it is possible to reconstruct the interior of an object without destroying. It is an important technique for many applications in, e.g., science, industry, and medicine. The runtime of conventional reconstruction algorithms is typically much longer than the time it takes to perform the tomographic experiment, and this prohibits the real-time reconstruction and visualization of the imaged object. The research in this dissertation introduces various techniques such as new parallelization schemes, data partitioning methods, and a quasi-3D reconstruction framework, that significantly reduce the time it takes to run conventional tomographic reconstruction algorithms without affecting image quality. The resulting methods and software implementations put reconstruction times in the same ballpark as the time it takes to do a tomographic scan, so that we can speak of real-time tomographic reconstruction. Show less
The research in this thesis is focused on tomographic reconstruction based on two imaging modalities in electron microscopy. The first modality is high angle annular dark field scanning... Show moreThe research in this thesis is focused on tomographic reconstruction based on two imaging modalities in electron microscopy. The first modality is high angle annular dark field scanning transmission microscopy (HAADF-STEM), and the second modality is energy-dispersive X-ray spectroscopy (EDS). In this Ph.D. thesis, we propose several approaches to pave the way for HAADF- STEM + EDS tomography: (1) the HAADF-EDS bimodal tomographic reconstruction technique, which is based on jointly modeling the consistency of the two imaging modalities; (2) TNV-regularized joined reconstruction which allows to incorporate the prior knowledge that common edges exist in the reconstructions from HAADF and EDS data respectively; (3) a set of algorithmic recipes to tailor various reconstruction algorithms for given experimental conditions and sample properties; (4) an algorithm for automatically correcting the nonlinear damping effects in HAADF-STEM tomographic data. Show less
Computed Tomography (CT) is an imaging technique that is used to calculate the interior of an object using X-rays under multiple projection angles. A well-known application is medical imaging with... Show moreComputed Tomography (CT) is an imaging technique that is used to calculate the interior of an object using X-rays under multiple projection angles. A well-known application is medical imaging with a CT-scanner. The reconstruction methods can roughly be divided into two categories: analytical reconstruction methods and algebraic reconstruction methods (ARMs). An example of an algorithm from the first category is Filtered Backprojection (FBP). This method has a high computational efficiency and it performs well in cases with many equiangularly distributed projection angles and high signal-to-noise ratio. ARMs require in general more computation time. They are more robust with respect to noise and can handle few projection angles or a limited angular range better. In this dissertation, the new algorithm Algebraic filter – Filtered Backprojection (AF-FBP) is introduced, which uses an ARM to create filters that can be used in FBP. The reconstruction quality of AF-FBP approximates that of the corresponding (locally) linear ARM, while the reconstructions are obtained with the computational efficiency of FBP. In cases with a small number of different scanning geometries, using AF-FBP enables the reconstruction of images of relatively high quality for few projection angles, limited angular range, or low signal-to-noise ratio. Show less
In X-ray tomography, a three-dimensional image of the interior of an object is computed from multiple X-ray images, acquired over a range of angles. Two types of methods are commonly used to... Show moreIn X-ray tomography, a three-dimensional image of the interior of an object is computed from multiple X-ray images, acquired over a range of angles. Two types of methods are commonly used to compute such an image: analytical methods and iterative methods. Analytical methods are computationally efficient, but in many applications, they produce reconstructions that are not accurate enough for further analysis. More accurate reconstructions can be obtained by using (regularized) iterative methods, but these can have computational costs that are too high to be used in practice. In this thesis, new reconstruction methods are developed that combine the analytical and algebraic approaches, resulting in methods that are as computationally efficient as analytical methods, but with a reconstruction accuracy of iterative methods. Analytical methods allow for changing their filter without increasing the needed computation time. We use this freedom in filter choice to develop new filter-based reconstruction methods, which are based on the analytical FBP method with specific filters. The filters can be defined and computed in different ways, and can depend on the acquisition geometry, the scanned object, and/or a separate pre-computing step. Several filter-based methods are introduced in this thesis and reconstruction results are compared with other popular methods. Show less
We first investigate the microscopic working principle of the nanowire superconducting single-photon detectors (SSPDs), and we find that the edge of the nanowire is much more sensitive than the... Show moreWe first investigate the microscopic working principle of the nanowire superconducting single-photon detectors (SSPDs), and we find that the edge of the nanowire is much more sensitive than the central part. The experimental results agree quantitatively with the theory based on a photon-assisted vertex- entry model. We then explore its potential to be used as a probe for near-field detection. Our calculations show that the SSPDs have a much higher signal collection efficiency than the conventional near-field optical microscopy, and that their spacial resolution can reach 20 nm. With photon number resolving ability, a SSPD fabricated on a micro-pillar can be used to investigate the quantum information of the optical near-field at subwavelength scales. Show less
Part 1: We studied the interaction of light with various devices made from nanoslits in metal: a quarter-wave retarder, a spin-to-orbital angular momentum converter, and a vortex analyzer. Part 2:... Show morePart 1: We studied the interaction of light with various devices made from nanoslits in metal: a quarter-wave retarder, a spin-to-orbital angular momentum converter, and a vortex analyzer. Part 2: We demonstrate anomalous dispersion of surface plasmons on an aluminum surface. Show less
In this thesis several aspects on the diagnosing and management of patients with acute abdominal pain are investigated. 1; The efficacy and safety of standard outpatient re-evaluation for patients... Show moreIn this thesis several aspects on the diagnosing and management of patients with acute abdominal pain are investigated. 1; The efficacy and safety of standard outpatient re-evaluation for patients not admitted to the hospital after emergency department evaluation for acute abdominal pain. 2; The use of a diagnostic strategy for patients clinically suspected to have appendicitis incorporating non-invasive modalities such as clinical evaluation, routine ultrasound and clinical re-evaluation, and a minimal use of complementary CT or laparoscopy. 3; The possibility to clinically differentiate between children with acute appendicitis and those with acute mesenteric lymphadenitis. 4; The value of secondary signs during the ultrasonographic evaluation of children with suspected appendicitis. 5; The impact of radiological imaging on diagnostic accuracy and clinical decision making for patients with acute colonic diverticulitis. 6; A systematic review on laparoscopic peritoneal lavage for patients with peritonitis due to perforated colonic diverticulitis. Show less
