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UCL University College London Eprints (377,346 recursos)

UCL Eprints collects the work of UCL researchers and makes it freely available over the web, helping the worldwide scholarly community to discover UCL research. Institutional repositories like UCL Eprints complement the traditional academic publishing and scholarly communications processes. They raise the visibility of research and help to maximise its impact. UCL researchers are encouraged to deposit a copy of each journal article, conference paper, working paper, and any other research output, in the UCL Eprints at the earliest opportunity, ensuring that their research reaches as wide an audience as possible.

Mostrando recursos 61 - 80 de 381,051

  1. Monostatic and bistatic radar measurements of birds and micro-drone

    Ritchie, MA; Fioranelli, F; Torvik, B; Griffiths, H
    This paper analyses the experimental results from recent monostatic and bistatic radar measurements of multiple birds as well as a quadcopter micro-drone. The radar system deployed for these measurements was the UCL developed NetRAD system. The aim of this work is to evaluate the key differences observed by a radar system between different birds and a micro-drone. Measurements are presented from simultaneous monostatic co/cross polarized data as well as co-polar bistatic data. The results obtained show comparable signature within the time domain and a marked difference in the Doppler domain, from the various birds in comparison to the micro-drone. The...

  2. Spatiotemporal disaggregation of GB scenarios depicting increased wind capacity and electrified heat demand in dwellings

    Sharp, RE
    National Grid’s future energy scenarios present a range of futures where wind capacity and domestic heat pumps increasetovaryingdegrees. BothofthesechangeswillintroduceincreasedvariabilityintotheGBelectricitysystem. In order to understand the implications of this variability it is important to understand the impact that wind speeds and temperature will have on supply and demand. This study therefore presents a method that disaggregates and modelsthesescenariosatanhourlyresolutionina0.5ogridcoveringGBandoffshorewaters. Thegriddedmodelling approach facilitates the use of climate reanalysis data which provides spatiotemporally homogeneous and accurate hindcastedweatherdataoverthe25yearperiodofthescenarios. Methodsforredistributingnongriddedspatialdata and disaggregating non spatial data to the grid are discussed. Theresultsdemonstrate thathigh offshore capacityfactors haveasignificantimpactonthe projected generation ofdifferentwindcapacityscenarios. Analysisofextremeeventsundereachscenario,facilitatedbythedisaggregated approach, demonstrate that there will be periods of...

  3. A Model Officer: An Agent-based Model of Policing

    Wise, S; Cheng, T
    Crime is a complex phenomenon, emerging from the interactions of offenders, victims, and their environment, and in particular from the presence or absence of capable guardians. Researchers have historically struggled to understand how police officers create guardianship. This presents a challenge because, in order to understand how to advise the police, researchers must have an understanding of how the current system works. The work presents an agent-based model that simulates the movement of police vehicles, using a record of real calls for service and real levels of police staffing in spatially explicit environments to emulate the demands on the police...

  4. Beyond the baseline: Establishing the value in mobile phone based poverty estimates

    Smith-Clarke, C; Capra, L
    Within the remit of 'Data for Development' there have been a number of promising recent works that investigate the use of mobile phone Call Detail Records (CDRs) to estimate the spatial distribution of poverty or socio-economic status. The methods being developed have the potential to offer immense value to organisations and agencies who currently struggle to identify the poorest parts of a country, due to the lack of reliable and up to date survey data in certain parts of the world. However, the results of this research have thus far only been presented in isolation rather than in comparison to...

  5. Electromagnetic Imaging with Atomic Magnetometers: A Novel Approach to Security and Surveillance

    Hussain, S; Marmugi, L; Deans, C; Renzoni, F
    We describe our research programme on the use of atomic magnetometers to detect conductive objects via electromagnetic induction. The extreme sensitivity of atomic magnetometers at low frequencies, up to seven orders of magnitude higher than a coil-based system, permits deep penetration through different media and barriers, and in various operative environments. This eliminates the limitations usually associated with electromagnetic detection.

  6. Exciting positronium with a solid-state UV laser: the Doppler-broadened Lyman-alpha transition

    Deller, A; Edwards, D; Mortensen, T; Isaac, CA; van der Werf, DP; Telle, HH; Charlton, M
    A tunable, pulsed laser was used to excite the Lyman-α transition (1S–2P) of positronium (Ps). The laser system has a large bandwidth of $\Delta \nu =225$ GHz at $\lambda =243$ nm, providing significant coverage of the Doppler-broadened, single-photon transition. The infra-red fundamental of a Nd:YAG laser was converted to ultraviolet by a series of solid-state, nonlinear processes, centred about an unseeded optical parametric oscillator, from which the bulk of the ultimate bandwidth derives. The Ps atoms were created by bombarding mesoporous silica with positrons, and the Doppler-width of the 1S–2P transition of the resulting ensemble was measured to be $\Delta...

  7. III-V nanowires and nanowire optoelectronic devices

    Zhang, Y; Wu, J; Aagesen, M; Liu, H
    III–V nanowires (NWs) have been envisioned as nanoscale materials for next-generation technology with good functionality, superior performance, high integration ability and low cost, because of their special growth modes and unique 1D structure. In this review, we summarize the main challenges and important progress of the fabrication and applications of III–V NWs. We start with the III–V NW growth, that significantly influences the NW morphology and crystal quality. Attention is then given to the fabrication of some advanced III–V structures composed of axial and radial junctions. After that, we review the advantages, challenges, and major breakthroughs of using III–V NWs...

  8. Interventional multispectral photoacoustic imaging with a clinical linear array ultrasound probe for guiding nerve blocks

    Xia, W; West, SJ; Nikitichev, DI; Ourselin, S; Beard, PC; Desjardins, AE
    Accurate identification of tissue structures such as nerves and blood vessels is critically important for interventional procedures such as nerve blocks. Ultrasound imaging is widely used as a guidance modality to visualize anatomical structures in real-time. However, identification of nerves and small blood vessels can be very challenging, and accidental intra-neural or intra-vascular injections can result in significant complications. Multi-spectral photoacoustic imaging can provide high sensitivity and specificity for discriminating hemoglobin-and lipid-rich tissues. However, conventional surface-illumination-based photoacoustic systems suffer from limited sensitivity at large depths. In this study, for the first time, an interventional multispectral photoacoustic imaging (IMPA) system was...

  9. SmartLiver Image Guidance System for Laparoscopic Liver Resection

    Thompson, SA; Totz, J; Song, Y; Johnsen, S; Stoyanov, D; Ourselin, S; Gurusamy, K; Schneider, C; Davidson, B; Hawkes, D; Clarkson, M
    Minimally invasive surgery, both robotic and laparoscopic, has demonstrable benefits for the patient. However, inability to distinguish different tissues and target lesions reduces eligibity for laparoscopic surgery. Image guidance using previously acquired imaging has the potential to mitigate these effects increasing eligibity for minimally invasive surgery. We present the ''SmartLiver'' image guidance system for image guided laparoscopic and robotic liver resection. Existing image guidance systems for the liver [2,3] rely on separate probes to locate points on the liver surface relative to the theatre coordinate system. With sufficiently long digitizing probes these systems can be used for laparoscopic as well as open surgery. However, as well as requiring an additional tool in theatre...

  10. Comparative dynamics of monovalent and bivalent vaccination for immunologically unrelated pathogens

    Knipl, D; Moghadas, SM
    Multivalent vaccines are designed to immunize against two or more pathogens in a single dose vaccination. A challenge for wide spread use of these vaccines is their lower protection efficacy compared to monovalent vaccines that immunize individuals against a single pathogen. We sought, for the first time, to evaluate the outcomes of bivalent and monovalent vaccines in terms of the reduction in the number of infections over time. For this evaluation, we developed epidemiological models governing the transmission dynamics of two immunologically unrelated pathogens, where immunity conferred by vaccination or natural infection of one pathogen does not provide any cross-protection...

  11. Imaging fast neural activity in the brain with Electrical Impedance Tomography

    Packham, BC
    Electrical impedance tomography (EIT) is an emerging medical imaging technique that can be employed to reconstruct the internal conductivity of an object from measurements made on the boundary. One proposed application for EIT is in head imaging, including imaging of impedance changes that occur with neuronal depolarisation and the imaging of acute stroke. The work of this thesis was aimed at advancing the imaging of brain pathology and function, with particular focus on the imaging of fast neural activity. Chapter 1 is a review of other brain imaging techniques, the principles of bioimpedance and EIT, and of previous impedance recordings...

  12. Database-Based Estimation of Liver Deformation under Pneumoperitoneum for Surgical Image-Guidance and Simulation

    Johnsen, SF; Thompson, S; Clarkson, MJ; Modat, M; Song, Y; Totz, J; Gurusamy, K; Davidson, B; Taylor, ZA; Hawkes, DJ; Ourselin, S
    The insufflation of the abdomen in laparoscopic liver surgery leads to significant deformation of the liver. The estimation of the shape and position of the liver after insufflation has many important applications, such as providing surface-based registration algorithms used in image guidance with an initial guess and realistic patient-specific surgical simulation. Our proposed algorithm computes a deformation estimate for a patient subject from a database of known insufflation deformations, as a weighted average. The database is built from pre-operative and intra-operative 3D image segmentations. The estimation pipeline also comprises a biomechanical simulation to incorporate patient-specific boundary conditions (BCs) and eliminate...

  13. A dedicated vascular access clinic for children on haemodialysis: Two years' experience

    Shroff, R; Sterenborg, RB; Kuchta, A; Arnold, A; Thomas, N; Stronach, L; Padayachee, S; Calder, F
    BACKGROUND: Arteriovenous fistula (AVF) formation for long-term haemodialysis in children is a niche discipline with little data for guidance. We developed a dedicated Vascular Access Clinic that is run jointly by a transplant surgeon, paediatric nephrologist, dialysis nurse and a clinical vascular scientist specialised in vascular sonography for the assessment and surveillance of AVFs. We report the experience and 2-year outcomes of this clinic. METHODS: Twelve new AVFs were formed and 11 existing AVFs were followed up for 2 years. All children were assessed by clinical and ultrasound examination. RESULTS: During the study period 12 brachiocephalic, nine basilic vein transpositions and...

  14. The Benefit of Split Nonlinearity Compensation for Single-Channel Optical Fiber Communications

    Lavery, D; Ives, D; Liga, G; Alvarado, A; Savory, SJ; Bayvel, P
    In this letter, we analyze the benefit of digital compensation of fiber nonlinearity, where the digital signal processing is divided between the transmitter and the receiver. The application of the Gaussian noise model indicates that, where there are two or more spans, it is always beneficial to split the nonlinearity compensation. The theory is verified via numerical simulations, investigating the transmission of a single-channel 50-GBd polarization division multiplexed 4- and 256-ary quadrature amplitude modulation signals over 100-km standard single-mode fiber spans, using lumped amplification. It is shown, theoretically, that the signal-to-noise ratio gain for long distances and high bandwidth transmission...

  15. Adaptive-Rate Reconstruction of Time-Varying Signals With Application in Compressive Foreground Extraction

    Mota, JFC; Deligiannis, N; Sankaranarayanan, AC; Cevher, V; Rodrigues, MRD
    We propose and analyze an online algorithm for reconstructing a sequence of signals from a limited number of linear measurements. The signals are assumed sparse, with unknown support, and evolve over time according to a generic nonlinear dynamical model. Our algorithm, based on recent theoretical results for l1 - l1 minimization, is recursive and computes the number of measurements to be taken at each time on-the-fly. As an example, we apply the algorithm to online compressive video foreground extraction, a problem stated as follows: given a set of measurements of a sequence of images with a static background, simultaneously reconstruct...

  16. The worldwide trend to high participation higher education: dynamics of social stratification in inclusive systems

    Marginson, S
    Worldwide participation in higher education now includes one-third of the age cohort and is growing at an unprecedented rate. The tendency to rapid growth, leading towards high participation systems (HPS), has spread to most middle-income and some low-income countries. Though expansion of higher education requires threshold development of the state and the middle class, it is primarily powered not by economic growth but by the ambitions of families to advance or maintain social position. However, expansion is mostly not accompanied by more equal social access to elite institutions. The quality of mass higher education is often problematic. Societies vary in...

  17. Human Stem Cell-Derived Astrocytes: Specification and Relevance for Neurological Disorders

    Tyzack, G; Lakatos, A; Patani, R
    Astrocytes abound in the human central nervous system (CNS) and play a multitude of indispensable roles in neuronal homeostasis and regulation of synaptic plasticity. While traditionally considered to be merely ancillary supportive cells, their complex yet fundamental relevance to brain physiology and pathology have only become apparent in recent times. Beyond their myriad canonical functions, previously unrecognised region-specific functional heterogeneity of astrocytes is emerging as an important attribute and challenges the traditional perspective of CNS-wide astrocyte homogeneity. Animal models have undeniably provided crucial insights into astrocyte biology, yet interspecies differences may limit the translational yield of such studies. Indeed, experimental...

  18. Life in a dark biosphere: a review of circadian physiology in "arrhythmic" environments

    Beale, AD; Whitmore, D; Moran, D
    Most of the life with which humans interact is exposed to highly rhythmic and extremely predictable changes in illumination that occur with the daily events of sunrise and sunset. However, while the influence of the sun feels omnipotent to surface dwellers such as ourselves, life on earth is dominated, in terms of biomass, by organisms isolated from the direct effects of the sun. A limited understanding of what life is like away from the sun can be inferred from our knowledge of physiology and ecology in the light biosphere, but a full understanding can only be gained by studying animals...

  19. Characterization of brain-heart interactions in a rodent model of sepsis

    Pinto, BB; Ritter, C; Michels, M; Gambarotta, N; Ferrario, M; Dal-Pizzol, F; Singer, M
    Loss of heart rate variability (HRV) and autonomic dysfunction are associated with poor outcomes in critically ill patients. Neuronal networks comprising brainstem and hypothalamus are involved in the "flight-or-fight" response via control over the autonomic nervous system and circulation. We hypothesized that sepsis-induced inflammation in brain regions responsible for autonomic control is associated with sympathovagal imbalance and depressed contractility. Sepsis was induced by fecal slurry injection in fluid-resuscitated rats. Sham-operated animals served as controls. Echocardiography-derived peak velocity (PV) was used to separate septic animals into good (PV ≥0.93 m/s, low 72-h mortality) and bad (PV <0.93, high 72-h mortality) prognosis. Cytokine...

  20. Ultrasound guided placement of the distal catheter in paediatric ventriculoatrial shunts—an appraisal of efficacy and complications

    Clark, DJ; Chakraborty, A; Roebuck, DJ; Thompson, DNP
    Ventriculoatrial (VA) shunts are commonly used as a second-line treatment of hydrocephalus when the peritoneum is an unsuitable site for the distal catheter. Many centres now utilise ultrasound and interventional radiology techniques to aid placement of the distal catheter. The purpose of this study was to conduct a contemporary audit of VA shunting in children using interventional radiology techniques for placement of the distal catheter.

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