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Oxford, United Kingdom

Salvini S.,Oxford search Center | Wijnholds S.J.,Netherlands Institute for Radio Astronomy
2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014 | Year: 2014

Alternating Direction Implicit (ADI) methods provide a computationally efficient way to solve for antenna based gains in full polarization. In this paper, we analyze the convergence of such methods in simulations. We show that convergence of a basic implementation can be quite slow and we propose two forms of relaxation to improve convergence behavior. The algorithm can be shown to be statistically efficient in low-SNR scenarios, which makes this approach particularly suitable for calibration of large radio astronomical arrays like the Square Kilometre Array (SKA). This also led to the name Statistically Efficient and Fast Calibration (StEFCal). The algorithm was implemented in several stages of the Low Frequency Array (LOFAR) calibration pipeline. We report on calibration performance improvement achieved with StEFCal in this pipeline. © 2014 IEEE.

Timlin C.,Particle Therapy Cancer Research Institute | Houston M.,Particle Therapy Cancer Research Institute | Houston M.,Oxford search Center | Jones B.,Particle Therapy Cancer Research Institute | Jones B.,University of Oxford
British Journal of Radiology | Year: 2011

The aim of this study was to display malignant induction probability (MIP) maps alongside dose distribution maps for radiotherapy using X-ray and charged particles such as protons. Dose distributions for X-rays and protons are used in an interactive MATLAB® program (MathWorks, Natick, MA). The MIP is calculated using a published linear quadratic model, which incorporates fractionation effects, cell killing and cancer induction as a function of dose, as well as relative biological effect. Two virtual situations are modelled: (a) a tumour placed centrally in a cubic volume of normal tissue and (b) the same tumour placed closer to the skin surface. The MIP is calculated for a variety of treatment field options. The results show that, for protons, the MIP increases with field numbers. In such cases, proton MIP can be higher than that for X-rays. Protons produce the lowest MIPs for superficial targets because of the lack of exit dose. The addition of a dose bath to all normal tissues increases the MIP by up to an order of magnitude. This exploratory study shows that it is possible to achieve three-dimensional displays of carcinogenesis risk. The importance of treatment geometry, including the length and volume of tissue traversed by each beam, can all influence MIP. Reducing the volume of tissue irradiated is advantageous, as reducing the number of cells at risk reduces the total MIP. This finding lends further support to the use of treatment gantries as well as the use of simpler field arrangements for particle therapy provided normal tissue tolerances are respected. © 2011 The British Institute of Radiology.

Dasgupta A.,UNC Charlotte | Chen M.,Oxford search Center | Kosara R.,UNC Charlotte
Computer Graphics Forum | Year: 2012

Uncertainty is an intrinsic part of any visual representation in visualization, no matter how precise the input data. Existing research on uncertainty in visualization mainly focuses on depicting data-space uncertainty in a visual form. Uncertainty is thus often seen as a problem to deal with, in the data, and something to be avoided if possible. In this paper, we highlight the need for analyzing visual uncertainty in order to design more effective visual representations. We study various forms of uncertainty in the visual representation of parallel coordinates and propose a taxonomy for categorizing them. By building a taxonomy, we aim to identify different sources of uncertainty in the screen space and relate them to different effects of uncertainty upon the user. We examine the literature on parallel coordinates and apply our taxonomy to categorize various techniques for reducing uncertainty. In addition, we consider uncertainty from a different perspective by identifying cases where increasing certain forms of uncertainty may even be useful, with respect to task, data type and analysis scenario. This work suggests that uncertainty is a feature that can be both useful and problematic in visualization, and it is beneficial to augment an information visualization pipeline with a facility for visual uncertainty analysis. Computer Graphics Forum © 2012 The Eurographics Association and Blackwell Publishing Ltd.

Warren A.J.,Diamond Light Source | Armour W.,Diamond Light Source | Armour W.,Oxford search Center | Axford D.,Diamond Light Source | And 9 more authors.
Acta Crystallographica Section D: Biological Crystallography | Year: 2013

The focus in macromolecular crystallography is moving towards even more challenging target proteins that often crystallize on much smaller scales and are frequently mounted in opaque or highly refractive materials. It is therefore essential that X-ray beamline technology develops in parallel to accommodate such difficult samples. In this paper, the use of X-ray microradiography and microtomography is reported as a tool for crystal visualization, location and characterization on the macromolecular crystallography beamlines at the Diamond Light Source. The technique is particularly useful for microcrystals and for crystals mounted in opaque materials such as lipid cubic phase. X-ray diffraction raster scanning can be used in combination with radiography to allow informed decision-making at the beamline prior to diffraction data collection. It is demonstrated that the X-ray dose required for a full tomography measurement is similar to that for a diffraction grid-scan, but for sample location and shape estimation alone just a few radiographic projections may be required.

Harvey P.,Durham University | Kuprov I.,Oxford search Center | Parker D.,Durham University
European Journal of Inorganic Chemistry | Year: 2012

The physicochemical basis of probe design for 19F MRS and MRI applications is reviewed. Complexes that give a single major resonance in solution are described, in which the Ln 3+ ion is about 6 É from the 19F label. Sensitivity improvements of 15-fold are reported in both imaging and spectroscopy based on longitudinal relaxation enhancement. The pseudocontact shift allows an amplification of chemical shift non-equivalence in responsive 19F probes, e.g. for monitoring pH in the range between 5 and 8. Sensitivity in 19F magnetic resonance spectroscopy and imaging is enhanced by placing a paramagnetic lanthanide within 7 E of the spin label. Faster relaxation allows more rapid data acquisition for systems generating one main resonance, and the proximate lanthanide ion amplifies the chemical shift non-equivalence in responsive probes. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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