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A low-power reconfigurable analog-to-digital converter

Gulati, Kush
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 200 p.; 15127453 bytes; 15127213 bytes; application/pdf; application/pdf
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This thesis presents the concept, theory and design of a low power CMOS analog-to-digital converter that can digitize signals over a wide range of bandwidth and resolution with adaptive power consumption. The converter achieves the wide operating range by reconfiguring (1) its architecture between pipeline and delta-sigma modes (2) by varying its circuit parameters such as size of capacitors, length of pipeline, oversampling ratio, among others and (3) by varying the bias currents of the opamps in proportion with converter sampling frequency, accomplished through the use of a phase-locked loop. Target input signals for this ADC include high frequency and moderate resolution signals such as video and low I.F. in radio Receivers, low frequency and high resolution signals from seismic sensors and MEMs devices, and others that fall in between these extremes such as audio, voice and general purpose data-acquisition. This converter also incorporates several power reducing features such as thermal noise limited design, global converter chopping in the pipeline mode, opamp scaling, opamp sharing between consecutive stages in the pipeline mode, an opamp chopping technique in the delta-sigma mode, and other design techniques. The opamp chopping technique achieves faster closed-loop settling time and lower thermal noise than conventional design.; (cont.) At a converter power supply at 3.3V...

Information exchange between medical databases through automated identification of concept equivalence

Sun, Yao, 1962-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 148 p.; 13094524 bytes; 13094281 bytes; application/pdf; application/pdf
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The difficulty of exchanging information between heterogeneous medical databases remains one of the chief obstacles in achieving a unified patient medical record. Although methods have been developed to address differences in data formats, system software, and communication protocols, automated data exchange between disparate systems still remains an elusive goal. The Medical Information Acquisition and Transmission Enabler (MEDIATE) system identifies semantically equivalent concepts between databases to facilitate information exchange. MEDIATE employs a semantic network representation to model underlying native databases and to serve as an interface for database queries. This representation generates a semantic context for data concepts that can subsequently be exploited to perform automated concept matching between disparate databases. To test the feasibility of this system, medical laboratory databases from two different institutions were represented within MEDIATE and automated concept matching was performed. The experimental results show that concepts that existed in both laboratory databases were always correctly recognized as candidate matches.; (cont.) In addition, concepts which existed in only one database could often be matched with more "generalized" concepts in the other database that could still provide useful information. The architecture of MEDIATE offers advantages in system scalability and robustness. Since concept matching is performed automatically...

Pipelined oversampling analog-to-digital converters

Paul, Susanne A. (Susanne Anita)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 226 p.; 18943879 bytes; 18943636 bytes; application/pdf; application/pdf
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Oversampling and noise-shaping techniques, such as [delta sigma] modulation, are widely used in analog-to-digital conversion to achieve accuracy that exceeds that of integrated-circuit components. Such converters have an inherent tradeoff between accuracy and speed, whereby resolution in amplitude is achieved at the expense of resolution in time. Although much attention has been focused on improving the speed and power of [delta sigma] analog-to-digital converters, data rates remain limited to less than a few MHz and are not easily extended. A pipelined oversampling architecture is described that circumvents the speed-resolution tradeoff of conventional oversampling converters by performing spatial, rather than temporal, oversampling. It combines high-resolution capabilities of [delta sigma] techniques with the high speed of pipelined architectures so that both of these attributes are achievable. The architecture also differs from conventional oversampling in that it performs Nyquist-rate sampling. Power is improved as a result of a charge-domain implementation, reduced sensitivity to thermal noise, simplified decimation, and reduced circuit speed, which permits voltage scaling and use of low-power technologies. Circuit techniques for implementation of a pipelined oversampling converter are also presented. Although CCDs are not essential to the concept...

Composable system resources as an architecture for networked systems

Chatterjee, Sandeep, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 173 p.; 9102480 bytes; 9125984 bytes; application/pdf; application/pdf
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(cont.) In this thesis, I describe an architecture for network devices that is based on using pluggable system resource modules that can be composed together to create a close-to-optimal platform for a particular application mix and device. Frequently used applications execute efficiently, while infrequently used applications execute less efficiently. Metrics for calculating efficiencies and selected application domains and mixes are specified by individuals as opposed to one-size-fits- all metrics specified by manufacturers. I show that such a composable system architecture is effective in optimizing system performance with respect to user preferences and application requirements, while the modularity of the architecture introduces little overhead. I also explore opportunities that arise from segmenting devices into UI and computational resource components, and show that an automated design environment can be created that greatly simplifies custom device design, reducing time-to-market and lowering costs.; Network devices promise to provide a variety of user interfaces through which users can interact with network applications. The design of these devices stand in stark contrast to the design of personal computers in which new software content is accommodated by increased processor performance. Network device design...

A model for cerebral cortical neuron group electric activity and its implications for cerebral function

Karameh, Fadi Nabih
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 265 p.; 18782882 bytes; 18817790 bytes; application/pdf; application/pdf
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The electroencephalogram, or EEG, is a recording of the field potential generated by the electric activity of neuronal populations of the brain. Its utility has long been recognized as a monitor which reflects the vigilance states of the brain, such as arousal, drowsiness, and sleep stages. Moreover, it is used to detect pathological conditions such as seizures, to calibrate drug action during anesthesia, and to understand cognitive task signatures in healthy and abnormal subjects. Being an aggregate measure of neural activity, understanding the neural origins of EEG oscillations has been limited. With the advent of recording techniques, however, and as an influx of experimental evidence on cellular and network properties of the neocortex has become available, a closer look into the neuronal mechanisms for EEG generation is warranted. Accordingly, we introduce an effective neuronal skeleton circuit at a neuronal group level which could reproduce basic EEG-observable slow (< 15 Hz) oscillatory phenomenon. The circuit incorporates basic laminar organization principles of the cortex. Interaction between neuronal groups is defined on three scales, namely the columnar (0.3mm), columnar assembly (1-2mm) and areal (> 3mm). The effective circuit makes use of the dynamic properties of the layer 5 network to explain intra-cortically generated augmenting responses...

Determining articulator configuration in voiced stop consonants by matching time-domain patterns in pitch periods

Kondacs, Attila, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 104 p.; 3085753 bytes; 3113870 bytes; application/pdf; application/pdf
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In this thesis I will be concerned with linking the observed speech signal to the configuration of articulators. Due to the potentially rapid motion of the articulators, the speech signal can be highly non-stationary. The typical linear analysis techniques that assume quasi-stationarity may not have sufficient time-frequency resolution to determine the place of articulation. I argue that the traditional low and high-level primitives of speech processing, frequency and phonemes, are inadequate and should be replaced by a representation with three layers: 1. short pitch period resonances and other spatio-temporal patterns; 2. articulator configuration trajectories; 3. syllables. The patterns indicate articulator configuration trajectories (how the tongue, jaws, etc. are moving), which are interpreted as syllables and words. My patterns are an alternative to frequency. I use short time-domain features of the sound waveform, which can be extracted from each vowel pitch period pattern, to identify the positions of the articulators with high reliability. These features are important because by capitalizing on detailed measurements within a single pitch period, the rapid articulator movements can be tracked. No linear signal processing approach can achieve the combination of sensitivity to short term changes and measurement accuracy resulting from these nonlinear techniques. The measurements I use are neurophysiologically plausible: the auditory system could be using similar methods. I have demonstrated this approach by constructing a robust technique for categorizing the English voiced stops as the consonants B...

In vitro models for injurious compression of bovine and human articular cartilage; In vitro models for investigating effects of injurious compression of articular cartilage

Patwari, Parth, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 159 leaves; 12670803 bytes; 12692554 bytes; application/pdf; application/pdf
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Patients who have sustained a traumatic joint injury, such as a ligament rupture or cartilage fracture, are known to have an increased risk for the development of osteoarthritis (OA) in that joint. This has motivated the use of in vitro models of mechanical cartilage injury in order to identify processes that could lead to cartilage degradation. The overall aims of the work presented here has been to focus on further identification of the effects of in vitro injurious mechanical compression of cartilage on cell-mediated processes and to develop models for injurious compression of the cartilage that seek to incorporate interactions with other joint tissues, such as inflammatory mediators elaborated from the joint capsule tissue. Major results are that i) injurious compression of newborn bovine cartilage can result in cell death predominantly by an apoptotic mechanism; ii) incubation of mechanically injured bovine and human cartilage with exogenous cytokines produces a synergistic increase in proteoglycan loss from the cartilage; and iii) coincubation of cartilage with joint capsule tissue profoundly inhibits cartilage biosynthetic activity through an IL-1-independent pathway. We also characterize the activity and activation of ADAMTS-4 (aggrecanase-1) in the cartilage tissue...

Spatial software pipelining on distributed architectures for sparse matrix codes

Duvall, Michelle, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 103 p.; 5290118 bytes; 5302254 bytes; application/pdf; application/pdf
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Wire delays and communication time are forcing processors to become decentralized modules communicating through a fast, scalable interconnect. For scalability, every portion of the processor must be decentralized, including the memory system. Compilers that can take a sequential program as input and parallelize it (including the memory) across the new processors are necessary. Much research has gone towards the ensuing problem of optimal data layout in memory and instruction placement, but the problem is so large that some aspects have yet to be addressed. This thesis presents spatial software pipelining, a new mechanism for doing data layout and instruction placement for loops. Spatial software pipelining places instructions and memory to avoid communication cycles, decreases the dependencies of tiles on each other, allows the bodies of loops to be pipelined across tiles, allows branch conditions to be pipelined along with data, and reduces the execution time of loops across multiple iterations. This thesis additionally presents the algorithms used to effect spatial software pipelining. Results show that spatial software pipelining performs 2.14x better than traditional assignment and scheduling techniques for a sparse matrix benchmark...

Matching and compressing sequences of visual hulls

Goela, Naveen, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 63 p.; 2640606 bytes; 2646449 bytes; application/pdf; application/pdf
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In this thesis, we implement the polyhedral visual hull (PVH) algorithm in a modular software system to reconstruct 3D meshes from 2D images and camera poses. We also introduce the new idea of visual hull graphs. For data, using an eight camera synchronous system after multi-camera calibration, we collect video sequences to study the pose and motion of people. For efficiency in VH processing, we compress 2D input contours to reduce te number of triangles in the output mesh and demonstrate how subdivision surfaces smoothly approximate the irregular output mesh in 3D. After generating sequences of visual hulls from source video, to define a visual hull graph, we use a simple distance metric for pose by calculating Chamfer distances between 2D shape contours. At each frame of our graph, we store a view independent 3D pose and calculate the transition probability to any other frame based on similarity of pose. To test our approach, we synthesize new realistic motion by walking through cycles in the graph. Our results are new videos of arbitrary length and viewing direction based on a sample source video.; by Naveen Goela.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.; Includes bibliographical references (p. 61-63).

A specification and verification of intermittent global order broadcast

Matlon, Catherine A. (Catherine Ann), 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 73 leaves; 3254403 bytes; 3261819 bytes; application/pdf; application/pdf
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The goal of my thesis is to specify, model and verify intermittent global order broadcast. Broadcast means that every process in the system receives a copy of every message. Global order means that all processes deliver the messages in the same order. Intermittent global order means that global order holds (luring periods of stability and there are no guarantees during periods of instability. A group of processes is stable if each process can communicate with each other process with some minimal quality of service and no process can communicate with another process outside the group. Intermittent properties, such as intermittent global order, are useful for certain collaborative applications operating in dynamic environments. They help balance conflicting needs for the different applications. We want to be able to formally specify intermittent properties in order to precisely express the guarantees provided by these applications and to be able to verify the algorithms implementing these properties. Because the guarantees hold intermittently, simply stating a definition and building a state-machine specification for an intermittent property is non-trivial. The same is true about verifying that an algorithm satisfies an intermittent property. Existing specification and verification techniques may need to be adjusted.; by Catherine A. Matlon.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

Storm-wide precipitation retrievals

Loparo, Jessica A. (Jessica Ann), 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 95 p.; 4011333 bytes; 4022115 bytes; application/pdf; application/pdf
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The monitoring of precipitation is important for scientific purposes, such as the study of world weather patterns, the development of global precipitation maps, and the tracking of seasonal and diurnal variations in precipitation rates. Over time many observation methods have been used to estimate precipitation: rain gauges, ground based radar systems, and visible, infrared, and passive microwave sensors in orbiting satellites. This research project uses data from the Advanced Microwave Sounding Unit (AMSU-A and AMSU-B) which consists of passive microwave sensors that collect data in the opaque water vapor and oxygen microwave absorption bands. This data supports 15-km resolution global precipitation rate estimates. The goal of this research is to develop a computational method that will improve the accuracy of these precipitation estimates by including spatial information in the precipitation retrieval, which is currently pixel based. This spatial information, which consists of the precipitation rate at each pixel of the image, is used to divide the data into separate storms, where a storm is defined as a precipitation region that is separated from other regions by an area of low or zero precipitation. Once storms have been identified...

Efficiency of background suppression for arterial spin labeling

Garcia, Dairon, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 51 leaves; 2372035 bytes; 2376041 bytes; application/pdf; application/pdf
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Arterial spin labeling (ASL), a technique developed for the measurement of local tissue perfusion with MRI, is heavily dependent on distinguishing irrelevant static tissue signal from the labeled blood. Background suppression can greatly reduce motion and other sources of noise in Arterial Spin Labeling MRI. More sophisticated background suppression strategies with many inversion pulses may decrease the ASL signal causing reduced signal-to-noise ratios and quantitative accuracy. Numerical simulations and in-vivo measurements were used to measure the inefficiency of different adiabatic inversion pulses and to optimize pulse selection. The results emphasize the high potential efficiency of adiabatic inversion pulses but also the limited optimal range of pulse parameters.; by Dairon Garcia.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2005.; Includes bibliographical references (leaves 49-51).

Event structure and the encoding of arguments : the syntax of the Mandarin and English verb phrase

Lin, Jimmy J. (Jimmy Jr-Pin), 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 194 p.; 9880232 bytes; 9905049 bytes; application/pdf; application/pdf
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(cont.) to variations in the way functional elements interact with verbal roots. Overall, my work not only contributes to our understanding of how events are syntactically represented, but also explicates interactions at the syntax-semantics interface, clarifying the relationship between surface form, syntactic structure, and logical form. A theory of argument structure grounded in independently-motivated syntactic constraints, on the one hand, and the semantic structure of events, on the other hand, is able to account for a wide range of empirical facts with few stipulations.; This work presents a theory of linguistic representation that attempts to capture the syntactic structure of verbs and their arguments. My framework is based on the assumption that the proper representation of argument structure is event structure. Furthermore, I develop the hypothesis that event structure is syntactic structure, and argue that verb meanings are compositionally derived in the syntax from verbalizing heads, functional elements that license eventive interpretations, and verbal roots, abstract concepts drawn from encyclopedic knowledge. The overall goal of the enterprise is to develop a theory that is able to transparently relate the structure and meaning of verbal arguments. By hypothesis...

A heterojunction bipolar transistor with stepwise allog-graded base : analysis, design, fabrication, and characterization

Konistis, Konstantinos, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 p.; 7006227 bytes; 7022058 bytes; application/pdf; application/pdf
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(cont.) features but the device self-heating turned out to be crucial for the longevity of the base micro-airbridges. The short lifetime of the base micro-airbridges was prohibitive for the realization of high frequency measurements. This work serves as the foundation for the implementation of robust HBT transit-time oscillators with the incorporation of slight modifications in the fabrication process.; This thesis explores the potential benefits of a GaAs-based heterojunction bipolar transistor (HBT) with stepwise alloy-graded base. The step height is slightly greater than the longitidinal optical (LO) phonon energy h[omega]LO in order to facilitate LO-phonon-enhanced forward diffusion of minority carriers in the base. The intuitive theoretical approach of carrier transport in the base, as proposed by other workers for this type of alloy-grading, did not incorporate in detail the various mechanisms of transport. In this work, we solved the Botzmann transport equation (BTE) in one dimension across the base for arbritrary frequencies. Impurity and LO phonon scattering were considered as the dominant scattering mechanisms. The intrinsic and extrinsic elements were combined and a small-signal equivalent circuit was proposed for the evaluation of the high-frequency performance of the device. The unique feature of this HBT is that the base transport factor undergoes a moderate magnitude attenuation and phase delay. By choosing a suitable collector delay...

Minimally invasive diagnostic imaging using high resolution Optical Coherence Tomography

Herz, Paul Richard, 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 244 p.; 12832983 bytes; 12864998 bytes; application/pdf; application/pdf
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Advances in medical imaging have given researchers unprecedented capabilities to visualize, characterize and understand biological systems. Optical Coherence Tomography (OCT) is a high speed, high resolution imaging technique that utilizes low coherence interferometry to perform cross-sectional tomographic imaging of tissue in real time and in vivo. The design, development, and implementation of ultrahigh resolution OCT systems in both laboratory and clinical experiments has been pursued in this work. Biomedical imaging studies in the areas of arthroscopy, cardiology, and endoscopy have been investigated with ultrahigh resolution capability achieved through the use of broadband femtosecond oscillators such as Ti:Sapphire and Cr:Forsterite light sources. OCT image resolutions of 1-5um in tissue have been realized, an order of magnitude greater than conventional MRI or ultrasound resolutions. In addition, through the use of coherent heterodyne detection techniques, the capability to visualize pathological tissue architecture in vivo for both animal and human experimental trials has been demonstrated. Because OCT can perform such "optical biopsy" with resolutions approaching that of conventional excisional biopsy and histology, it has the potential to become a powerful diagnostic tool in the field of medical imaging. In combination with small fiber-optic catheters...

On-chip cross-talk analysis for multiple RF front ends of a wireless gigabit LAN system

Liang, Jie De Jacky, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 173 leaves; 7467790 bytes; 7489224 bytes; application/pdf; application/pdf
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In the Wireless-Gigabit-Local-Area-Network (WiGLAN) project, we proposes a system architecture that adopts multiple antennas [1, 2, 3, 4] to control the trade-off between data rate and transmission quality [5, 6] through Space-Time Coding (STC) [7, 8, 9] and Orthogonal Frequency Division Multiplexing (OFDM). However, along the multiple RF front-ends, there are multiple nodes that signal cross-talk can occur. Such signal cross-talk occurring on a silicon chip becomes more and more significant as the integration level and operating radio frequency rise, seriously degrading the system performance, the data rate and transmission quality. Most of the literature about on-chip crosstalk suppression have been focusing on adopting various process-technology techniques, such as using guard ring structures to separate the parallel RF front ends or inserting a ground plane to shield the cross-talk. In this study, we will take a different approach. We will investigate the effects of on-chip cross-talk upon the operations of the coding and modulation schemes adopted in the WiGLAN system and explore methods, other than those mentioned, to counteract them.; by Jie De Jacky Liang.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science...

Modeling analysis of core-shell Si/SiGe nanowires

Tang, Ming Y., 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 113 p.; 4851503 bytes; 4865184 bytes; application/pdf; application/pdf
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(cont.) a composition that results in a high mobility has a very promising thermoelectric performance. Lastly, the thermoelectric-related transport properties for a Si/SiGe core-shell nanowire are compared with the related properties for a Si nanowire and a SiGe nanowire. The Si/SiGe core-shell nanowire shows a better thermoelectric performance than its Si nanowire counterpart. On the other hand, by relaxing the harsh conditions imposed on the carrier mobility of the Si/SiGe core-shell nanowire structure in this thesis, the Si/SiGe core-shell nanowire structure is also expected to have a better thermoelectric performance than its SiGe nanowire counterpart.; In this thesis, I present a theoretical model for the Si core/SiGe shell core-shell nanowire system. A model for the single carrier pocket core-shell nanowire is first developed, along with the boundary conditions of a circular wire and sharp interfaces between the two media. A numerical scheme is then developed for the core-shell nanowire system, along with educated approximations for the numerical boundary conditions. The numerical model is designed such that low energy levels have higher accuracy than the high energy levels. The core-shell nanowire model is applied to a Si core/SiGe alloy shell structure...

Interconnect modeling and optimization in deep sub-micron technologies

Sotiriadis, Paul Peter P. (Paul Peter Peter-Paul), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 122 p.; 7621860 bytes; 7621668 bytes; application/pdf; application/pdf
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Interconnect will be a major bottleneck for deep sub-micron technologies in the years to come. This dissertation addresses the communication aspect from a power consumption and transmission speed perspective. A model for the energy consumption associated with data transmission through deep sub-micron technology buses is derived. The capacitive and inductive coupling between the bus lines as well as the distributed nature of the wires is taken into account. The model is used to estimate the power consumption of the bus as a function of the Transition Activity Matrix, a quantity generalizing the transition activity factors of the individual lines. An information theoretic framework has been developed to study the relation between speed (number of operations per time unit) and energy consumption per operation in the case of synchronous digital systems. The theory provides us with the fundamental minimum energy per input information bit that is required to process or communicate information at a certain rate. The minimum energy is a function of the information rate, and it is, in theory, asymptotically achievable using coding. This energy-information theory combined with the bus energy model result in the derivation of the fundamental performance limits of coding for low power in deep sub-micron buses. Although linear...

High frequency ultrasonic characterization of human skin In vivo

Raju, Balasundara I. (Balasundara Iyyavu), 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 161 p.; 7260250 bytes; 7260058 bytes; application/pdf; application/pdf
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High frequency (>20 MHz) ultrasound has numerous potential applications in dermatology because of its ability to penetrate several millimeters into the skin and provide information at a spatial resolution of tens of microns. However, conventional B-scan images of skin tissues often lack the capability to characterize and differentiate various skin tissues. In this work, quantitative ultrasonic methods using the attenuation coefficient, backscatter coefficient, and echo envelope statistics were studied for their potential to characterize human skin tissues in vivo. A high frequency ultrasound system was developed using polymer transducers, a pulser/receiver, high-speed digitizer, 3-axis scanning system, and a PC. Data collected using three different transducers with center frequencies of 28, 30 and 44 MHz were processed to determine the characteristics of normal human dermis and subcutaneous fat. Attenuation coefficients were obtained by computing spectral slopes vs. depth, with the transducers axially translated to minimize diffraction effects. Backscatter coefficients were obtained by compensating recorded backscatter spectra for system-dependent effects, and additionally for one transducer, using the reference phantom technique. Good agreement was seen between the results from the different transducers/methods. The attenuation coefficients were well described by a linear frequency dependence whose slope showed significant differences between the forearm and fingertip dermis...

Applications of optical parametric downconversion : I. Self-phase locking, [and] II. Generation of entangled photon pairs in periodically-poled lithium niobate

Mason, Elliott J. (Elliot James), 1972-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 114 p.; 4786445 bytes; 4786254 bytes; application/pdf; application/pdf
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Nonlinear optics plays a key role in many applications not only as a means of frequency synthesis and control, but also as a source of twin photons and other nonclassical states of light. Advances in nonlinear materials, including engineered nonlinear interactions through the use of quasi-phase matching, have increased the flexibility and performance of many nonlinear optical devices. One of the devices presented in this thesis is a 2:1 frequency divider based on self-phase locking in a type-II phase matched optical parametric oscillator. This device is investigated both theoretically and experimentally. Results are also presented on fabrication of periodically-poled nonlinear crystals for quasi-phase matching in lithium niobate and barium magnesium fluoride. Finally, a high-efficiency frequency-nondegenerate twin photon source is implemented in periodically-poled lithium niobate.; by Elliott J. Mason, III.; Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.; Includes bibliographical references (p. 108-114).