A study of stone fragmentation in shock wave lithotripsy by customizing the acoustic field and waveform shape

Shock wave lithotripsy is the preferred treatment modality for kidney stones in the United States. Despite clinical use for over twenty-five years, the mechanisms of stone fragmentation are still under debate. A piezoelectric array was employed to examine the effect of waveform shape and pressure distribution on stone fragmentation in lithotripsy. The array consisted of 170 elements placed on the inner surface of a 15 cm-radius spherical cap. Each element was driven independently using a 170 individual pulsers, each capable of generating 1.2 kV. The acoustic field was characterized using a fiber optic probe hydrophone with a bandwidth of 30 MHz and a spatial resolution of 100 μm. When all elements were driven simultaneously, the focal waveform was a shock wave with peak pressures p+ =65±3MPa and p−=−16±2MPa and the −6 dB focal region was 13 mm long and 2 mm wide. The delay for each element was the only control parameter for customizing the acoustic field and waveform shape, which was done with the aim of investigating the hypothesized mechanisms of stone fragmentation such as spallation, shear, squeezing, and cavitation. The acoustic field customization was achieved by employing the angular spectrum approach for modeling the forward wave propagation and regression of least square errors to determine the optimal set of delays. Results from the acoustic field customization routine and its implications on stone fragmentation will be discussed.

The Least of These: American Evangelical Parachurch Missions to the Poor, 1947-2005

This dissertation narrates the historical development of American evangelical missions to the poor from 1947-2005 and analyzes the discourse of its main parachurch proponents, especially World Vision, Compassion International, Food for the Hungry, Samaritan's urse, Sojourners, Evangelicals for Social Action, and the Christian Community Development Association. Although recent scholarship on evangelicalism has been prolific, much of the historical work has focused on earlier periods. Sociological and political scientific scholarship on the postwar period has been attracted mostly to controversies surrounding the Religious Right, leaving evangelicalism's resurgent concern for the poor relatively understudied. This dissertation addresses these lacunae. The study consists of three chronological parts, each marked by a distinctive model of mission to the poor. First, the 1950s were characterized by compassionate charity for individual emergencies, a model that cohered neatly with evangelicalism's individualism and emotionalism. This model should be regarded as the quintessential, bedrock evangelical theory of mission to the poor. It remained strong throughout the entire postwar period. Second, in the 1970s, a strong countercurrent emerged that advocated for penitent protest against structural injustice and underdevelopment. In contrast to the first model, it was distinguished by going against the grain of many aspects of evangelical culture, especially its reflexive patriotism and individualism. Third, in the 1990s, an important movement towards developing potential through hopeful holism gained prominence. Its advocates were confident that their integration of biblical principles with insights from contemporary economic development praxis would contribute to drastic, widespread reductions in poverty. This model signaled a new optimism in evangelicalism's engagement with the broader world. The increasing prominence of missions to the poor within American evangelicalism led to dramatic changes within the movement's worldview: by 2005, evangelicals were mostly unified in their expressed concern for the physical and social needs of the poor, a position that radically reversed their immediate postwar worldview of near-exclusive focus on the spiritual needs of individuals. Nevertheless, missions to the poor also paralleled, reinforced, and hastened the increasing fragmentation of evangelicalism's identity, as each missional model advocated for highly variant approaches to poverty amelioration that were undergirded by diverse sociological, political, and theological assumptions.

Scalability of Multicast Delivery for Non-sequential Streaming Access

To serve asynchronous requests using multicast, two categories of techniques, stream merging and periodic broadcasting have been proposed. For sequential streaming access where requests are uninterrupted from the beginning to the end of an object, these techniques are highly scalable: the required server bandwidth for stream merging grows logarithmically as request arrival rate, and the required server bandwidth for periodic broadcasting varies logarithmically as the inverse of start-up delay. However, sequential access is inappropriate to model partial requests and client interactivity observed in various streaming access workloads. This paper analytically and experimentally studies the scalability of multicast delivery under a non-sequential access model where requests start at random points in the object. We show that the required server bandwidth for any protocols providing immediate service grows at least as the square root of request arrival rate, and the required server bandwidth for any protocols providing delayed service grows linearly with the inverse of start-up delay. We also investigate the impact of limited client receiving bandwidth on scalability. We optimize practical protocols which provide immediate service to non-sequential requests. The protocols utilize limited client receiving bandwidth, and they are near-optimal in that the required server bandwidth is very close to its lower bound.