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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Point source compensation ??? a backpropagation method for underwater acoustic imaging

Yee, Clifford Wing Wei, Physics, Faculty of Science, UNSW January 2003 (has links)
The backpropagation method of image reconstruction has been known for some time with the advantage of fast processing due to the use of Fast Fourier Transform. But its applicability to underwater imaging has been limited. At present the shift-and-add method is the more widely used method in underwater imaging. This is due to the fact that backpropagation has been derived for plane wave insonification, with the scattered waves detected in transmission-mode, or synthetic aperture set-up. One of the methods being used for underwater imaging is to use a point source for the insonification of the target and the scattered waves detected in reflection-mode by a receiver array. An advantage of this scanning method is only one transmission of the source is required to capture an image, instead of multiple transmissions. Therefore motion artifacts are kept to minimum. To be able to exploit the processing speed of the backpropagation method, it must be adapted for point source insonification. The coverage of this configuration in the literature has been scant, methods for spherical sources have been proposed for transmission mode and arbitrary surfaces in geophysical applications. These methods are complex and difficult to use. A novel point source compensation method is proposed in this thesis so that the backpropagation image formation method can be used for the point source insonification set-up. The method of investigation undertaken to derive this new backpropagation method was through theoretical analysis, numerical simulation and experimental verification. The effect of various compensation factors on the image quality was studied in simulation. In the experimental verification, practical issues relating to the application of the new method was addressed. The final proof of concept of our method was undertaken with our experimental verification. The quality of images formed with the point source compensation methods has also been compared with that with the shiftand- add method. Experimental and simulation results show that the point source compensated backpropagation algorithm can produce images of comparable quality with those formed with shift-and-add method for the set-up of wideband point-source insonification with detection in reflection-mode, with the advantage of faster image formation.
22

Automatic Imaging for Face Biometrics and Eye Localization

Wang, Tao, Lin, Weifeng January 2009 (has links)
<p>A proposal for a person authentication system, which localizes facial landmarks and extracts biometrical features for face authentication, is presented in this thesis. An efficient algorithm for eye localization and biometrical feature extraction and person identification is developed by using Gabor filters. In the eye localization part, we build artificial average eye models for eye location. In the person identification part, we construct databases of biometrical features around the eye area of clients and, for authentication, Schwartz inequality and the sum square error (SSE) are used.</p><p>This project is implemented in the ‘Matlab’ programming language, on a personal computer system, and experimental results on the proposed system are presented.</p>
23

Functional Deficits in Motor Terminals and their Mitochondria in Mouse Models of Amyotrophic Lateral Sclerosis

Nguyen, Khanh Tu 06 December 2009 (has links)
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which the upper and lower motor neurons die. Most studies aimed at elucidating the cause of this disease have focused on the motor neuron cell body. However, recent work has suggested that the disease may begin in motor nerve terminals. The experiments described in Chapters 2-4 of this dissertation studied functional defects in motor nerve terminals of mice expressing mutant human superoxide dismutase 1 (SOD1-G93A, SOD1-G85R), models of familial ALS. In Chapter 2, the proximal hind limbs of SOD1-G93A mice were subjected to varying durations of a tourniquet-induced ischemia/reperfusion injury to determine whether these motor terminals were more vulnerable to this stress than wild-type terminals. Confocal imaging of yellow fluorescent protein (YFP expressed in neurons) and alpha-bungarotoxin (labels acetylcholine receptors on muscle) was used to determine endplate occupancy. In the distal hind limb of SOD1-G93A/YFP terminals innervating fast type muscles (extensor digitorum longus (EDL) and plantaris) were more vulnerable to ischemia/reperfusion injury than those occupying the slow type muscle (soleus). Increased vulnerability to endplate denervation was evident in presymptomatic mice as early as 31 days old. Experiments in Chapters 3 tested whether mitochondrial handling of Ca2+ loads is altered at presymptomatic stages. These experiements used rhodamine-123 to measure depolarization of the mitochondrial membrane potential (Ψm) evoked by trains of action potentials delivered to the motor nerve in levator auris longus motor terminals. These Ψm depolarizations depended on Ca2+ entry into motor terminals and were relatively small (~1-2 mV) in wild-type terminals. Consistent with the hypothesis that reduced ability to accelerate the electron transport chain (ETC) activity results in larger stimulation-induced Ψm depolarizations, presymptomatic SOD1-G93A (maintains dismutase activity) and SOD1-G85R (lacks dismutase activity) terminals displayed ~5 times greater depolarizations than wild-type terminals. Expression of normal human SOD1 or knockout of SOD1 did not significantly alter Ψm depolarizations. In the presence of a low concentration of rotenone (inhibits complex 1 of the ETC) wild-type terminals also displayed larger Ψm depolarizations. Experiments in Chapter 4 studied stimulation-induced Ψm depolarizations in terminals of older, symptomatic SOD1-G93A and SOD1-G85R mice. These depolarizations decayed more slowly than those in wild-type terminals and incremented with successive trains. Asynchronous depolarizations that were not time linked to the stimulus train were also noted. These behaviors were attenuated when opening of the mitochondrial permeability transition pore (mPTP) was inhibited with cyclosporin A or by replacing bath Ca2+ with Sr2+. Incrementing Ψm depolarizations could be elicited in wild-type terminals when subjected to an oxidative stress (diamide-induced depletion of glutathione). These findings indicate that motor terminals in mutant SOD1 mice display functional deficits even at presymptomatic ages, and that deficits associated with mitochondrial handling of stimulation-induced Ca2+ loads increase with age and may contribute to motor terminal degeneration in mutant SOD1 mice.
24

Ion Imaging Studies on NO2 Photodissociation

Xiao, Chun-yi 16 August 2007 (has links)
none
25

Automatic Imaging for Face Biometrics and Eye Localization

Wang, Tao, Lin, Weifeng January 2009 (has links)
A proposal for a person authentication system, which localizes facial landmarks and extracts biometrical features for face authentication, is presented in this thesis. An efficient algorithm for eye localization and biometrical feature extraction and person identification is developed by using Gabor filters. In the eye localization part, we build artificial average eye models for eye location. In the person identification part, we construct databases of biometrical features around the eye area of clients and, for authentication, Schwartz inequality and the sum square error (SSE) are used. This project is implemented in the ‘Matlab’ programming language, on a personal computer system, and experimental results on the proposed system are presented.
26

Acoustic imaging with blazed arrays and time-frequency beamforming

Thompson, Roger Lee, January 2003 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references.
27

Super resolution technique and its potential usage in medical imaging

Chang, Yiu-chuen, 張耀泉 January 2014 (has links)
Purpose: Medical imaging systems are used to scan patients to obtain valuable information for diseases diagnosis and assisting treatment. An ideal scanner should be sensitive enough to detect any trace amount of abnormal tissue at its early stage. With the continuous development of high-tech treatment systems such as Tomotherapy (manufactured by Tomo HD), the high-resolution imaging system is favorable to reduce the damage of normal tissue due to the image guidance of Mega-voltage beam before treatment. In this study, a software approach was presented to improve image resolution without hardware upgrade of a scanner. Methodology A programming technique “Super Resolution Technique” was used and demonstrated in an example of CT. It utilized several similar images with known relative shifts between them. (They can be positional or angular shifted and taken at the same time frame as far as possible). Those images are of low resolution and can be reconstructed to form a higher resolution image. A Super Resolution program was written by MATLAB to prove the method. The experiments 1 to 4 were purely computer-based simulations and experiment 5 used a LightSpeed VCT scanner for real scans. For the computer-based experiments, a few low resolution images have been attempted and registration steps were explored for image reconstruction. A resolution target, USAF1951, was called from MATLAB and used to examine the resolving power before and after image processing based on Super Resolution algorithm. Image-image subtraction was used to compare pre-processing and post-processing images. The number of non-zero pixels was used to access the percentage of similarity. For the experiment using LightSpeed VCT scanner, a GE VCT QA phantom was used to test the performance of the technique. Result From the experiments using USAF1951, it was found that: the minimum resolvable line pairs had improved from family -1 element 6 to family 0 element 2 (2 elements improvement) after applying “Super Resolution Technique” as shown in the experiment 1. An xy directional shifting of the pre-processing images resulted in a better reconstructed image than x-axis shifting or y-axis shifting in terms of resolution, shown in the experiment 2. The experiment 3 concluded that the more the pre-processing images, the better the reconstructed image would be. The experiment 4 showed that the shifts of pre-processing images greater than the detector size could still result in a higher resolution image. The experiment 5 revealed that applying “Super Resolution Technique” to a real CT scanner could not give an obvious improvement in resolution, but the image background noise had reduced. Conclusion It was concluded that the “Super Resolution Technique” could improve the image resolution and reduce the background noise at expense of more imaging time and more dose from the additional view. In case of hardware upgrade of imaging device is not practicable, Super Resolution could help improve the image quality. / published_or_final_version / Medical Sciences / Master / Master of Medical Sciences
28

VISUAL PERCEPTION IN CORRELATED NOISE (MODELS).

MYERS, KYLE JEAN. January 1985 (has links)
This dissertation concerns the ability of human observers to perform detection tasks in medical images that contain structured noise. We shall show that physical measures of image quality, such as signal-to-noise ratio (SNR), resolution, modulation transfer function (MTF), and contrast, do not accurately predict how well an observer can detect lesions in an image. We have found that for images with equal pixel SNR, humans can detect a low contrast object more readily in images that have a low-pass noise structure, as opposed to a high-pass noise structure. This finding is important in the comparison of images generated by a classical pinhole imaging system with images generated by a computed tomography imager. We would like to have a figure of merit that accurately predicts a physician's ability to perform perceptual tasks. That is, we want a figure of merit for imaging systems that is more than an evaluation of the physician's performance, measured using human observers and an accepted method such as receiver operating characteristic (ROC) techniques. We want a figure of merit that we can calculate without requiring lengthy observer studies. To perform this calculation, we need a model of the imaging system hardware in cascade with a verified model of the human observer. We have chosen to approach this problem by modelling the human observer as an ideal observer. Our hypothesis is that the human observer acts approximately as an ideal-observer who does not have the ability to prewhiten the noise in an image. Without this ability, the ideal observer's detection performance for even a simple task is degraded substantially in correlated noise. This is just the effect that we have found for human observers. In search of a physiological explanation for a human observer's inability to do prewhitening, we shall investigate the detection capability of the ideal observer when a frequency-selective mechanism is invoked. This mechanism corresponds to the frequency channels known to exist in the human visual system. We shall show that the presence of such a mechanism can explain the degradation of human observer performance in correlated noise.
29

Acoustic imaging with blazed arrays and time-frequency beamforming

Thompson, Roger Lee, 1966- 26 July 2011 (has links)
Not available / text
30

Radially symmetrical coded apertures

Miller, ElRoy Lester, 1942- January 1978 (has links)
No description available.

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