Browsing by Subject "Spectral"

Border Images and Imaginaries: Spectral Aesthetics and Visual Media of Americanity at the U.S.-Mexico Border, proposes an emerging aesthetic of spectrality in visual media about the U.S.-Mexico border that challenges the power of militarized and racialized visibility. The visual media projects I work with, including cinema, electronic performance art, site specific video installation, and photography generate an aesthetic of spectrality as they try to conjure and express the socially invisible through sensual elements like affect, sound, kinaesthetics, and full embodiment. This aesthetic elicits the perceptions of our other senses beyond only the visual and makes visible the social flesh of the movements and socialities of migration rather than racialized, migrant bodies. The border, I claim, is an important site for understanding the continued deployment of visibility in the neoliberal legacies of what Quijano and Wallerstein call, Americanity, a term denoting the development of the modern capitalist system in the Americas which relied upon the imbricate logics of colonialism, racism, and the deification the modern. Images of spectrality are intermediaries between what Diana Taylor calls, archive and repertoire, being both documents and sites of embodied engagement that produce both certain and uncertain knowledges of race and migration at the border. The visual media projects in my dissertation cultivate spectral aesthetics to theorize an alternative visibility and the changing production of public memory. By making visible the social flesh of heterogeneous encounters with media, spectral aesthetics reforms collective memory making it a process of democratic editorialization that privileges experience as the site of a multivocal history. This project reclaims the image as a terrain for the multitude's inquiry and imagination about the US-Mexico border, and puts the imaginaries generated by these images in dialog with activist projects happening in relation to immigration.

This dissertation explores contemporary walking narratives – fictions formally organized around and guided by journeys “on foot” – in order to consider the deeply rooted literary-historical, aesthetic, and ethical relationships among practices of walking, writing, remembering, and haunting. By contemplating the ways in which contemporary authors reimagine the walking narrative as a historically embedded and physically embodied site of experimental spatial and social practices and radical modes of historical consciousness, this project encourages us to question the most obvious ways in which we encounter and represent the world. Walking narratives challenge readers and literary critics alike to reflect on the intimacies and intricacies of paths, place, pace, and point of view, thereby destabilizing familiar conceptualizations of perceptual, geographic, autobiographic, and historical knowledge. At the same time, these insistently and unsettlingly hybrid fictions also reconfigure the boundaries of genre, encouraging a theoretical and aesthetic reorientation of literary-critical knowledge guided by metaphors of spatial and temporal trespass, moments of historical surprise, recognition, and wonder, and networks of bodies in motion.

Integrating the critical-interpretive methods of literary studies with the practice-centered theories of cultural geography and performance studies, this project examines how texts by W. G. Sebald, Iain Sinclair, and Teju Cole engage with interdisciplinary questions of movement, personal identity, the philosophy of history, and place. Situating these works within the emerging critical field of “walking studies,” I argue that contemporary walking narratives become textual testing grounds for practices of wandering and wondering that (re)animate the histories embedded in landscape and allow the ghosts of the past to speak in ethically urgent ways. Through the linked practices of walking and writing, contemporary walking narratives open new routes through which to encounter literature and landscape and inspire spectral conversations designed to illuminate new points of entry into past and present, world and word.

In this work, the simulation (forward problem) and reconstruction (inverse problem) in Thermoacoustic Tomography (TAT) are studied using a pseudospectral time-domain (PSTD) method with 4th-order time integration.

The objective of the TAT simulation is to solve for the thermoacoustic pressure field in an inhomogeneous medium. Using the PSTD method, the spatial derivatives of pressure field and particle velocity can be obtained using fast fourier transform (FFT). Since the Fourier transforms used to represent the spatial derivatives of smooth functions are exact, only 2 points per wavelength are needed in the spatial discretization. The time integration is achieved by a 4th-order method to effectively reduce the computational time. The results of the algorithm are validated by analytical solutions. Perfectly Matched Layers (PMLs) are applied to absorb the outgoing waves and avoid ``wraparound'' effect. The maximum attenuation coefficient of the PMLs has an optimum value to minimize the reflections due to discretization and wraparound effect for 2D and 3D problems. Different PML profiles are also compared, quadratic profile is chosen because it can minimize the overall reflection. Spatial smoothing is needed for PSTD to avoid Gibbs' phenomenon in the modeling of a point source, and the effect of the smoothing function is studied.

In the TAT reconstruction problem, the PSTD method is used to reconstruct the thermoacoustic sources by solving the thermoacoustic wave equations in a reversed temporal order within the framework of time reversal imaging. The back-propagated pressure waves then refocus at the spatial locations of the original sources. Most other TAT reconstruction algorithms are based on the assumption that the tissue medium is acoustically homogeneous. In practice, however, even the mild tissue inhomogeneity will cause large phase errors and cause spatial misplacement and distortion of the sources. The proposed PSTD method utilizes a two-step process to solve this problem. In the first step, a homogeneous time reversal reconstruction is performed. Since an inhomogeneity itself is usually a source because of spatially dependent electrical conductivity (thus microwave absorption), the spatial location and the shape of the inhomogeneity can be estimated. In the second step, the updated acoustic property map is loaded followed by an inhomogeneous reconstruction. Numerical results show that this method greatly improves the reconstruction results. Images with improved quality are reconstructed from experimental data.

A 3D PSTD algorithm is developed and validated. Numerical results show that the PSTD algorithm with the 4th-order time integration is capable of simulating large 3D acoustic problems accurately and efficiently. A 3D breast phantom model is used to study the inhomogeneous reconstruction in 3D. Improved results over the homogeneous method are observed.

A preliminary study of the Thermoacoustic Tomography (TAT) using continuous-wave (CW) modulated microwaves is summarized. The theoretical background, system configuration, experiment setup, and measurement results are presented.

X-ray micro-CT is widely used for small animal imaging in preclinical studies of cardiopulmonary disease, but further development is needed to improve spatial resolution, temporal resolution, and material contrast. This study presents a set of tools that achieve these improvements. These tools include the mathematical formulation and computational implementation of algorithms for calibration, image reconstruction, and image analysis with our custom micro-CT system. These tools are tested in simulations and in experiments with live animals. With these tools, it is possible to visualize the distribution of a contrast agent throughout the body of a mouse as it changes over time, and produce 5-dimensional images (3 spatial dimensions + time + energy) of the cardiac cycle.