A thesis submitted to the Faculty of Science, School of Statistics and Actuarial Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, January 25, 2017. / The classical Wishart distribution, was first derived byWishart (1928) as the distribution of the maximum likelihood estimator of the covariance matrix of the multivariate normal distribution. It is a matrix variate generalization of the gamma distribution. In high dimensional settings,Wishart distributions defined within the framework of graphical models are of particular importance. [No abstract provided. Information taken from introduction] / MT2017

Cone-beam computed tomography (CBCT) is an imaging technique used in conjunction with radiation therapy. CBCT is used to verify the position of tumours just prior to radiation treatment session. The accuracy of the radiation treatment of thoracic and upper abdominal tumours is heavily affected by respiratory movement. Blurring artefacts, due to the movement during a CBCT scanning, cause misregistration between the CBCT image and the planning image. There has been growing interest in the use of motion-compensated CBCT for correcting the breathing-induced artefacts. A wide range of iterative reconstruction methods have been developed for CBCT imaging. The direct motion compensation technique has been applied to algebraic reconstruction technique (ART), simultaneous ART (SART), ordered-subset SART (OS-SART) and conjugate gradient least squares (CGLS). In this thesis a dual modality imaging of electrical impedance tomography (EIT) and CBCT is proposed for the first time. This novel dual modality imaging uses the advantages of high temporal resolution of EIT imaging and high spatial resolution of the CBCT method. The main objective of this study is to combine CBCT with EIT imaging system for motion-compensated CBCT using experimental and computational phantoms. The EIT images were used for extracting motion for a motion-compensated CBCT imaging system. A simple motion extraction technique is used for extracting motion data from the low spatial resolution EIT images. This motion data is suitable for input into the direct motion-compensated CBCT. The performance of iterative algorithms for motion compensation was also studied. The dual modality CBCT-EIT is verified using experimental EIT system and computational CBCT phantom data.

The nervous system is required for human cognition, motor function, and sensory interaction. A complex network of neuronal connections, or synapses, carries out these behaviors, and defects in neural connectivity can result in developmental and degenerative diseases. In vertebrate nervous systems, synapses most commonly occur at axon terminals. Upon reaching their synaptic targets, growth cones lose their motility and become boutons specialized for neurotransmitter release. I am studying this process in R7 photoreceptors in the

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 179-181. / by Stephen Mark Paneitz. / Ph.D.

Introduction: Increasing the imaging demand in the dental field has lead to a dramatic increase in the number of CBCT machines in the U.S. market with a variety of new models and features, as well as different radiation exposures. These differences in exposure among the different CBCT machines and the potential for radiation accumulation over a life time are major concerns for aiming for a reduction in patients’ radiation exposure. Most of the studies have aimed to measure the radiation dose in different CBCT units with different field of views. Up to date, few studies have aimed to measure the radiation dose in different CBCT devices with similar fields of view. The aim of the study was to compare the dosimetry levels with relatively small FOV in different scan protocols in two CBCT units. Materials and methods: A 16-cm diameter PMMA phantom with 10-cm pencil ionization chamber were used to measure the radiation exposure from two CBCT devices: i-CAT FLX and CS9000. A smallest FOV in both CBCT (8 x 8 cm in the i-CAT FLX and 5 x 3.7 cm in the CS9000) was selected at different scan protocols. The scan settings included in the i-CAT FLX HD (120 kVp; 5 mA; 7.4 sec; 0.125, 0.250, 0.200 mm voxel sizes; 360° rotation) Quick HD (120 kVp; 5 mA; 4.1 sec; 0.200, 0.250 mm voxel sizes; 180° rotation), Quick+ (90 kVp; 3 mA; 2 sec; 0.300, 0.400 mm voxel sizes; 180° rotation) , Quick (120 kVp; 5 mA; 2 sec; 0.300, 0.400 mm voxel sizes; 180° rotation), and standard scans (120 kVp; 5 mA; 3.7 sec; 0.300, 0.400 mm voxel sizes; 360° rotation). In the CS9000 scan settings included voxel sizes (CS9000: 0.076 mm, 0.100 mm, and 0.200 mm), 80 kVp, 10 mA, 10.8 sec, and 360° rotation. The phantom was exposed three times at the same position to calculate the average measurement of dose by the ionization chamber. All the radiation exposure doses were read by one examiner. Results: The radiation exposure of the phantom slots in different resolutions and scan protocols in the both CBCT units ranged from 4.31 to 60.73 mR. There were statistically significant differences in radiation value between i-CAT FLX and CS9000 due to voxel size (P < 0.001). Each voxel size was significantly different from the other in both scanners, except between CS9000 0.076 and HD 0.125; CS9000 0.200 and HD 0.125; and CS9000 0.100 and HD 0.200. Also, there were no statistically significant differences between the voxel size within the same scan protocols in the i-CAT FLX, especially the Quick HD, Quick+, Quick, and Standard scans. Conclusion: The selection of x-ray parameters (mainly scan time), voxel size, and rotation angle have a significant radiation expsoure reduction in both the i-CAT FLX and CS9000 units and hence should be appropriately selected to minimize the radiation dose.

The objective of this research was to characterize the performance of a downflow bubblecontact (DBCA) hypolimnetic aerator — Speece Cone-. The effect of two key design factors, inlet water velocity and the ratio of gas flow rate to water flow rate on four standard units of measure was examined: (a) the Oxygen Transfer Coefficient, KLa, corrected to 20°C, KLa₂₀ (hr­-¹), (b) the Standard Oxygen Transfer Rate, SOTR (g0₂.hr­-¹) (c) the Standard Aeration Efficiency, SAE (gO₂kWhr­-¹), and (d) the Standard Oxygen Transfer Efficiency, SOTE (%). Two sources of oxygen, Pressure Swing Adsorption (PSA) oxygen (87% purity) and air, were compared. KLa₂₀, SOTR, and SAE increased with an increase in the ratio of gas flow rate to water flow rate for both air and oxygen, over a range of 0.5% to 5.0%; while SAE deceased. An increase in inlet water velocity resulted in a decrease in KLa, corrected to 20°C, SOTR, and SAE, but an increase in the SOTE. Treatments on air showed similar, but much less dramatic effect of the gas flow rate to water flow rate ratio and water inlet velocity on KLa₂₀, SOTE, SAE, and SOTE, when compared to treatments on PSA oxygen. The best performance was achieved with an inlet water velocity of 6.9-7.6 ms­-¹ and oxygen flow rate to water flow rate ratio of about 2.5%. At this combination, the SOTE was about 66-72%.

The static cone penetration tests are quite extensively used for carrying out in-situ geotechnical investigations both for onshore and offshore sites especially where the soil mass is expected to comprise of either soft to medium stiff clays or loose to medium dense sands. The wide use of the cone penetration tests (CPT) in geotechnical engineering has resulted in a great demand for developing necessary correlations between the cone penetration resistance and different engineering properties of soils. The successful interpretation of the cone penetration test data depends mainly on the various empirical correlations which are often derived with the help of a controlled testing in calibration chambers. The calibration chambers have been deployed in various sizes (diameter varying from 0.55 m to 2.10 m) by a number of researchers. It is quite an expensive and time consuming exercise to carry out controlled tests in a large size calibration chamber. The task becomes even much more difficult when a sample comprising of either silt or clay has to be prepared. As a result, most of the reported cone penetration tests in calibration chambers are mainly performed in a sandy material. Taking into account the various difficulties associated with performing tests in large calibration chambers, in the present study, it is attempted to make use of a miniature static cone penetrometer having a diameter of 19.5 mm. This cone was gradually penetrated at a uniform rate in a triaxial cell in which a soil sample of a given material was prepared; the diameter of the cone was intentionally chosen smaller so that the ratio of the diameter of the cell to that of the cone becomes a little larger. Two different diameters of the cells, namely, 91 mm and 140 mm, were used to explore the effect of the ratio of chamber (cell) size to that of the cone size. In addition, the rate of penetration rate was also varied from 0.6 mm/minute to 6.0 mm/minute (the maximum possible rate for the chosen triaxial machine with the larger cell) to examine the effect of the rate of the penetration of the miniature cone on the tip resistance. By using the chosen experimental setup, a large number of static miniature cone penetrometer tests were carried out on four different materials, namely, (i) clean sand, (ii) sand with 15% silt, (iii) sand with 25% silt, and (iv) sand with 15% fly ash. The cone tip resistance for each material was obtained for a wide range of three different relative densities. The effective vertical pressure (σv) for the tests on different samples was varied in between 100 kPa and 300 kPa. The variations of the tip resistance with axial deformation in all the cases were monitored so as to find the magnitude of the ultimate tip resistance. In contrast to the standard cone, the diameter of the piston shaft was intentionally kept a little smaller than that of the cone itself so as to restrict the development of the piston resistance. For each cell (chamber) size, two different sizes of the pistons were used to assess the resistance offered by the penetration of the piston shaft itself. It was noted that the resistance offered by the chosen piston shaft is not very substantial as compared to that of the cone tip itself. Most of the experimental observations noted from the present experiments were similar to those made by the penetration of the standard size cone in a large calibration chamber. The ultimate tip resistance of the cone was found to increase invariably with an increase in the magnitude of σv. An increase in the relative density of the soil mass leads to an increase in the value of qcu. For the same range of relative densities, an addition of fly ash in the sample of sand, leads to a considerable reduction in the magnitude of qcu. Even with the addition of 25% silt, the values of qcu were found to become generally lower as compared to clean sand and sand added with 15% silt. An employment of a larger ratio of the diameter of the cell to that of the miniature cone leads to an increased magnitude of qcu. An increase in the penetration rate from 0.6 mm/min to 6.0 mm/min, was found to cause a little increase in the magnitude of qcu especially for sand added with fly ash and silt. The effect of the penetration rate on the results was found to increase continuously with a reduction in the rate of penetration. At higher penetration rates, in a range closer to those normally employed in the field (20 mm/sec), it is expected that the rate of penetration of the cone will not have any substantial effects on the magnitude of qcu for clean sands. The magnitude of qcu obtained in this thesis at different values of σv for all the cases with the use of the miniature cone were compared with the two widely used correlations in literature. It is found that except for dense sands, in most of the cases, the present experimental data lie generally in between the two correlation curves from literature; for dense sands the measured values of qcu were found to be significantly lower than the chosen correlation curves. It was noted that with the use of the miniature cone penetrated in a given sample prepared in a triaxial cell, it is possible to obtain reasonably an accurate estimate of the tip resistance of the standard cone especially for loose to medium dense states of all the materials. Further, from the analysis of all the tests results, it was noted that approximately a linear correlation between qcu/σv and soil friction angle (φ) for different chosen materials exists provided the dependency of the φ on the stress level is taken into account. As compared to the standard cone penetrometer which is usually employed in the field, the miniature cone used in this study is expected to provide a little conservative estimate, of the tip resistance of the standard static cone penetrometer with reference to the different materials used in this study on account of the facts that (i) there is a reduced area behind the cone, (ii) the ratio of the diameter of the calibration chamber (cell) to that of cone is not very high, (iii) the chosen size of the cone is smaller than the standard cone, and (iv) the chosen penetration rate is much smaller than the standard rate of penetration. Further, in the case of clean sand, an attempt has also been made in this thesis, with the help of a number of direct shear tests at different stress levels, to generate an expression correlating peak friction angle, critical state friction angle, relative density of sand and vertical effective stress. A correlation has been generated with the help of which, the value of peak dilatancy angle can be obtained from the known values of peak friction angle and critical state friction angle. In confirmation with the available information in literature, this exercise on clean sand has clearly indicated that a decrease in the magnitude of vertical effective stress leads to an increase in the values of both peak friction angles and peak dilatancy angles.

Thesis (Th. M.)--Dallas Theological Seminary, 2000. / Includes bibliographical references (leaves 48-50).

The objective of this research was to characterize the performance of a downflow bubblecontact (DBCA) hypolimnetic aerator — Speece Cone-. The effect of two key design factors, inlet water velocity and the ratio of gas flow rate to water flow rate on four standard units of measure was examined: (a) the Oxygen Transfer Coefficient, KLa, corrected to 20°C, KLa₂₀ (hr­-¹), (b) the Standard Oxygen Transfer Rate, SOTR (g0₂.hr­-¹) (c) the Standard Aeration Efficiency, SAE (gO₂kWhr­-¹), and (d) the Standard Oxygen Transfer Efficiency, SOTE (%). Two sources of oxygen, Pressure Swing Adsorption (PSA) oxygen (87% purity) and air, were compared. KLa₂₀, SOTR, and SAE increased with an increase in the ratio of gas flow rate to water flow rate for both air and oxygen, over a range of 0.5% to 5.0%; while SAE deceased. An increase in inlet water velocity resulted in a decrease in KLa, corrected to 20°C, SOTR, and SAE, but an increase in the SOTE. Treatments on air showed similar, but much less dramatic effect of the gas flow rate to water flow rate ratio and water inlet velocity on KLa₂₀, SOTE, SAE, and SOTE, when compared to treatments on PSA oxygen. The best performance was achieved with an inlet water velocity of 6.9-7.6 ms­-¹ and oxygen flow rate to water flow rate ratio of about 2.5%. At this combination, the SOTE was about 66-72%.

No description available.