VictorJuiz/thesis huggingface dataset:
Lag correction in amorphous silicon flat-panel x-ray computed tomography [electronic resource]

Cone-beam computed tomography (CBCT), which uses much of the same hardware as traditional 2-D x-ray fluoroscopy, is now being used during patient treatment. The ability to perform CBCT scans has been driven by the development of amorphous silicon (a-Si) digital flat-panel (FP) x-ray detector technology. These detectors were originally designed for radiography and fluoroscopy, but because of their compactness, flexibility, low spatial distortion, and relative low-cost, are also practical for CBCT. One factor limiting the CBCT image quality for a-Si x-ray detectors is detector lag, stemming from charge trapping in the a-Si layer. Lag is defined as residual signal present in the detector, and can lead to a range of image artifacts in CBCT reconstructions, such as severe image shading for elliptical or off-center objects that could potentially obscure anatomy of interest. Current lag correction methods for a-Si FP detectors assume that the detector is linear and time invariant (LTI) and determine a temporal impulse response for the system. The detector output is deconvolved with the measured impulse response to compute the lag-corrected data. However a conventional FP is neither linear nor time invariant. Different techniques for measuring the FP impulse response are examined, as well as their effect on the final CBCT image reconstruction. A range of results are achieved by the different techniques, highlighting the non-linearity and time variance of the system. A novel non-LTI algorithm is then presented that better describes the x-ray detector dynamics, gives exposure independent results, and provides significant error reduction in CBCT for large objects where lag artifacts are most severe. A second method to reduce lag based on a hardware change to the a-Si detector is also described in this dissertation. The photodiode at each detector pixel is briefly operated in a forward bias mode to maintain saturation of charge trap states. Detector measurements and CBCT scans with and without the detector forward biasing are made to show image improvement and the trade-offs of using the hardware method.

Keywords: Electrical Engineering
Advisor: Fahrig Rebecca primary advisor. Thesis advisor
Pauly John (John M.) advisor. Thesis advisor
Pelc Norbert J. advisor. Thesis advisor
Land is not the setting [electronic resource] : the lightning field and environments, 1960-1980

"Land is not the setting: the lightning field and environments, 1960-1980" examines land art and ecological thought in the 1960s and 1970s through a sustained and unconventional investigation of Walter De Maria's single work The Lightning Field of 1977. A large, site-specific sculpture consisting of four hundred sharpened steel poles arranged in a regular grid, The Lightning Field is a remarkably variable object and site which present a range of experiences from reflective metal palpably shifting in form and orientation with the angle of the sun to a dramatic stage for electrical fireworks. The focus of this study extends the manifold nature of The Lightning Field to examine how this single work enfolds not only various modes of experience, but, more importantly, the most significant strands of De Maria's artistic practice beginning around 1960 alongside competing conceptualizations of the environment throughout the 1960s and 1970s. During this period, the idea of an environment was rapidly on the move. Artists and critics employed the term to suggest gallery installations, ecosystems, multimedia projections, and natural sanctuaries. In short, the concepts "environment" and "ecology" experienced two decades of rich investigation in social and intellectual registers, and the emergence of land art provides one unique access to their larger turns within the period.

Keywords: Art and Art History
Advisor: Lee Pamela M. primary advisor. Thesis advisor
Gough Maria advisor. Thesis advisor
Levi Pavle advisor. Thesis advisor
Wolf Bryan Jay advisor. Thesis advisor
Language, ideology and identity in rural eastern Kentucky [electronic resource]

Linguists still know relatively little about the speech of the rural US South, in part because rural speech is thought to be more conservative than urban speech with regard to language change. In order to fill in a gap in our dialect map and explore the innovative capacity of rural speakers, this dissertation examines language use in its social context in Wilson County (a pseudonym), a small rural community in Eastern Kentucky. Data come from interviews with thirty women, stratified by age and education level, who have spent most of their lives in the community. I employ a combination of ethnographic and quantitative methods to analyze interviewees' use of three local dialect features, monophthongization of /ay/ before voiceless consonants (n=270), raising and fronting of /^/ (n=309), and leveling of standard-were to was (n=450). Results show that speakers use pre-voiceless /ay/-monophthongization and /^/- fronting and raising nearly categorically, but use was-leveling infrequently. Metapragmatic commentary indicates that the overall low level of morphosyntactic variation is driven by standard language ideology (Lippi-Green 1997) and negative ideologies that characterize rural and Mountain Southerners as old-fashioned, unsophisticated and ignorant. Such commentary also indicates that, inversely, the high use of local phonetic features is motivated by those same negative ideologies about rural and Mountain Southerners: Wilson Countians appear to have developed a strong sense of local identity and pride in oppositional reaction to cultural and linguistic marginalization. I conclude that speakers combine strongly locally-accented phonology with relatively prescriptive grammar in an effort to appear local and authentic, yet at the same time competent and modern. As predicted, younger and more-educated speakers use less of the local phonetic features than older and college-educated speakers do. More-educated speakers also use less was-leveling than less-educated speakers do. Mainstream language norms appear to be entering the community through those speakers who feel the greatest pressure to appear competent and modern. The high rate of pre-voiceless /ay/-monophthongization (a relatively new Southern feature), as well as the complete reorganization of linguistic constraints on wasleveling since Northern British settled the region, indicate that rural speech can be highly innovative.

Keywords: Linguistics
Advisor: Eckert Penelope primary advisor. Thesis advisor
Rickford John R.
Inoue Miyako
Sumner Meghan advisor. Thesis advisor
Large eddy simulation of high pressure turbine cascade [electronic resource]

In gas turbine engine design, better efficiency is achieved through higher turbine inlet temperature. As a result, the nozzle guide vanes and rotor blades of a High Pressure Turbine (HPT) are subject to an increasingly hostile thermal environment. High levels of turbulence and hot streaks from the upstream combustor impinge on the HPT blade rows. Heat transfer prediction of the thermally critical regions of a HPT is a vital element in the design of gas turbine engines. The effect of inlet turbulence on heat transfer and boundary layer development on a HPT vane cascade has been studied numerically using a high-order overset Large Eddy Simulation procedure. Numerical visualizations of instantaneous boundary layer flow show presence of long, meandering, streamwise oriented streaky structures on both the suction and pressure sides of the blade. Stretching of vortices from the inlet turbulence around the leading edge and subsequent transport of them into the blade passage produces these boundary layer streaks. Significant enhancement in laminar heat transfer, especially around the leading edge and on the pressure side is observed due to these structures. Backward jets are created in the fluctuation velocity field in the boundary layer, which interact with eddies in the turbine passage leading to bypass transition of the suction surface boundary layer. Simulations with organized spanwise disturbance show sensitivity of leading edge region heat transfer to the intensity and length scale of the inlet disturbance.

Keywords: Mechanical Engineering
Advisor: Lele Sanjiva K. (Sanjiva Keshava)
Eaton John K. advisor. Thesis advisor
Moin Parviz advisor. Thesis advisor
Laser-tissue interactions in retinal photo-thermal therapy [electronic resource] : mechanisms and applications

Since its introduction nearly 40 years ago, laser photocoagulation has been the standard of care for treatment of numerous retinal pathologies. Recently, new approaches have been introduced for more selective targeting of various retinal layers, including patterned scanning photocoagulation, selective retinal therapy, and sub-lethal treatment. Despite its broad use in clinical practice, there remains a need for the quantitative description of laser-tissue interactions involved in retinal phototherapy. A unified description of various treatment regimes and associated mechanisms of tissue damage would allow for optimization of laser parameters to improve selectivity and safety of retinal photocoagulation, and for avoidance of undesirable collateral damage. The presented work describes an investigation into the dynamics of the retinal response to hyperthermia and vaporization. A finite-element computational model of photocoagulation and rupture was constructed based on experimental measurements of laser interactions with tissue, and verified in vivo in the millisecond time domain. Two approaches towards improvement of tissue heating uniformity were studied: spatial and temporal modulation of the treatment beam. After optimization using the computational model, beam shaping and pulse modulation systems were constructed. Experimental studies in vivo confirmed improvements in safety of the retinal treatment, potentially allowing for reductions in treatment time, thermal damage extent, and perceived pain. In addition, tissue response to sub-lethal thermal stress in the retina was explored using expression of heat shock protein in an animal model. Computational modeling of the corresponding treatment regime demonstrated that a similar response is likely to occur in clinical application of sub-lethal exposures. Photo-mechanical interactions in the retina were investigated in model systems and in vivo with microsecond-range exposures. The dominant mechanisms of tissue damage were identified and the corresponding limits of the safe therapeutic window were computed over a broad range of pulse durations - from microseconds to seconds. An understanding of the thermal and mechanical interactions involved in laser heating of the retina allows for the realization of safer and more selective treatment regimes. All three mechanisms investigated in the current study -- photocoagulation, photomechanical interactions and sub-lethal hyperthermia -- play a role in clinical treatment. The developed quantitative models of these interactions have immediate applicability to clinical practice, providing guidance towards optimization of retinal phototherapy, evaluation of retinal safety, and development of new clinical applications.

Keywords: Applied Physics
Advisor: Doniach S. primary advisor. Thesis advisor
Palanker Daniel primary advisor. Thesis advisor
Blumenkranz Mark S. advisor. Thesis advisor
Learning and control with inaccurate models [electronic resource]

A key challenge in applying model-based Reinforcement Learning and optimal control methods to complex dynamical systems, such as those arising in many robotics tasks, is the difficulty of obtaining an accurate model of the system. These algorithms perform very well when they are given or can learn an accurate dynamics model, but often times it is very challenging to build an accurate model by any means: effects such as hidden or incomplete state, dynamic or unknown system elements, and other effects, can render the modeling task very difficult. This work presents methods for dealing with such situations, by proposing algorithms that can achieve good performance on control tasks even using only \emph{inaccurate} models of the system. In particular, we present three algorithmic contributions in this work that exploit inaccurate system models in different ways: we present an approximate policy gradient method, based on an approximation we call the Signed Derivative, that can perform well provided only that the sign of certain model derivative terms are known; we present a method for using a distribution over possible inaccurate models to identify a linear subspace of control policies that perform well in all models, then learn a member of this subspace on the real system; finally, we propose an algorithm for integrating previously observed trajectories with inaccurate models in a probabilistic manner, achieving better performance than is possible with either element alone. In addition to these algorithmic contributions, a central focus of this thesis is the application of these methods to challenging robotic domains, extending the state of the art. The methods have enabled a quadruped robot to cross a wide variety of challenging terrain, using a combination of slow static walking, dynamic trotting gaits, and dynamic jumping maneuvers. We also apply these methods to a full-sized autonomous car, where they enable it to execute a "powerslide'' into a narrow parking spot, one of the most challenging maneuvers demonstrated on an autonomous car. Both these domains represent highly challenging robotics tasks where the dynamical system is difficult to model, and our methods demonstrate that we can attain excellent performance on these tasks even without an accurate model of the system.

Advisor: Ng Andrew Y.
Koller Daphne advisor. Thesis advisor
Thrun Sebastian
Learning from the positivity effect [electronic resource] : informed and motivated behavior change

The story of old age is not one of pure loss. Indeed, old age represents a time of certain cognitive decline in some domains, but also a time of growth and stability in others. Wisdom, maturity, expertise, and emotional regulation show improvements across adulthood. Goals also shift over the life span, from those aimed at gaining information and preparing for the future to emotionally meaningful pursuits later in life. These differences lead to goal-consistent cognitive biases in the young and the old. Younger people attend to negative information more than positive information. Older adults attend to positive information more than negative information. This developmental shift, called the positivity effect, is presumably adaptive in a life-span context. Yet either set of chronically activated goals can be maladaptive in certain contexts. Because the positivity effect is malleable -- influenced by the way task relevant goals are structured -- it may be possible to frame information in ways that reduce biases in young and old. The present program of research aims to show how biases can be reduced when participants are given information about relevant research findings and have the necessary psychological tools to adjust their behavior accordingly. No previous work has investigated the impact of disclosing information about the positivity effect directly to participants. Such an informed debiasing attempt could prove fruitful for future attempts to educate the public about their own attentional biases. Study 1 demonstrates the positivity effect in a picture recall task; Study 2 adds images to an existing image set in order to more accurately study debiasing of the positivity effect in future studies; Study 3 demonstrates that people are generally unaware of the positivity effect; Study 4 demonstrates that simple information disclosure about the positivity effect is insufficient to change participants' behavior on a picture presentation/recall task and a health care decision task; Study 5 demonstrates that participants' motivation to change is key to behavior change.

Keywords: Psychology
Advisor: Carstensen Laura L. primary advisor. Thesis advisor
Dweck Carol S.
Tsai Jeanne Ling advisor. Thesis advisor
Learning position-tolerant object representations [electronic resource]

Human observers have a remarkable ability to recognize and discriminate objects despite a high degree of variation in their visual appearance in different contexts. The mechanisms underlying this tolerance to object transformation are largely unknown and it is unclear if learning can influence sensitivity to object transformations. In these studies, I examine the role of learning on our ability to visually discriminate objects across multiple retinal positions. The findings from this research program indicate that prior experience will influence an observer's ability to discriminate objects and that the quality of prior experience is of utmost importance to the development of position-tolerant object representations.

Keywords: Psychology
Advisor: Grill-Spector Kalanit primary advisor. Thesis advisor
Wagner Anthony David advisor. Thesis advisor
Wandell Brian A. advisor. Thesis advisor
Learning via prediction [electronic resource] : mapping continuous stimuli to discrete symbols

How do we use and represent words? How do we learn to break up the dimensions of continuous variation of the world into discrete categories? In this thesis, I explore how recasting this problem in terms of simple prediction provides insight into the computational nature of word learning. Through a series of computational simulations and human experiments that manipulate the structure of information in time, I find that when using features of the world to predict words, the representations learned are likely to be more useful for discriminating the appropriate response. However, these representations are also likely to be distorted, favoring diagnostic information, and thus sacrificing general utility across contexts. At the same time, trying to use words to predict the relevant features of the world will result in less distorted representations, but will not enhance discrimination. Demonstrating these effects while controlling for various potentially confounding variables strengthens the case that prediction is central to word learning.

Keywords: Psychology.
Advisor: Ramscar Michael primary advisor. Thesis advisor
Thomas Ewart A. C. primary advisor. Thesis advisor
McClelland James L. advisor. Thesis advisor
Lensless holography methods for soft X-ray resonant coherent imaging [electronic resource]

The ability to interpret and inverse x-ray diffraction patterns from crystals has largely shaped our understanding of the structure of matter. However, structure determination of noncrystalline objects from their diffraction patterns is a much more difficult task. The dramatic increase in available coherent x-ray photon flux over the past decade has made possible a technique known as lensless coherent diffractive imaging (CDI), that addresses exactly this problem. The central question around CDI is the so-called phase problem: upon detection of the diffraction intensity, the phase information of the diffracted wave is inevitably lost. Generally, the phase problem is approached using iterative phase retrieval algorithms. Holographic methods, through interference with reference diffractions, encode the phase information directly inside the measured x-ray holograms, and are therefore able to avoid the stagnation and uniqueness problems commonly encountered by the iterative algorithms. This dissertation discusses two novel holographic methods for coherent lensless imaging using resonant soft x-rays. The first part focuses on generalizing the multiple-wavelength anomalous diffraction technique, a highly successful method for solving the crystal structures of biomacromolecules, into a multiple-wavelength holography technique for nanoscale resonant x-ray imaging. Using this method I show element specific reconstructions of nanoparticles and magnetization distribution in magnetic thin films with sub 50 nm resolution. The second part discusses progress in X-ray Fourier holography, an ultrafast lensless imaging platform that can be used with the upcoming x-ray free electron lasers. In particular, I will present experiments using two novel types of extended reference structures that bring the resolution beyond the precision of reference fabrication, previously regarded as the resolution limit for x-ray Fourier transform holography. Finally, future applications of holographic methods, especially experimental considerations for time-resolved studies of nanostructures using X-FELs, will be discussed.

Keywords: Applied Physics
Advisor: Moler Kathryn A. primary advisor. Thesis advisor
Stöhr Joachim primary advisor. Thesis advisor
Lindenberg Aaron Michael advisor. Thesis advisor
Life history and livelihood strategies in the Colombian Chocó [electronic resource]

Colombia has been characterized by extreme levels of violence throughout the latter part of the twentieth century, and the country continues to be one of the most dangerous in the world in terms of violent deaths. There were at least 527,718 deaths due to violence in Colombia during 1985-2005, of which nearly 90 percent occurred among men. As a result, Colombia is one of few countries in the world where women outnumber men in both young and old age. Human behavioral ecology and life history theory offer a number of implications for phenomena such as marriage, migration, and gender relations in violent settings where men are in short supply. This work combines demographic data from government registries and household surveys with ethnographic research among Afro-Colombian and indigenous communities in the Chocó region. Multiple decrement life table analysis reveals that intentional injury deaths were responsible for as much as 14 years of lost life expectancy among men in affected regions. Analysis suggests that by weakening women's bargaining position in gender interactions, female-biased sex ratios may be partly responsible for increasingly high frequencies of consensual unions, female rural-urban migration, and intimate partner violence. Clear interactions between sex ratios, time preferences, and life history decision-making in Colombia underscore the importance of life history perspectives for public policy in regions affected by violence.

Keywords: Anthropology.
Advisor: Jones James Holland primary advisor. Thesis advisor
Bird Rebecca (Rebecca Bliege) advisor. Thesis advisor
Durham William H. advisor. Thesis advisor