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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.
1

A clinical comparison and analysis between conventional MLC based and solid compensator based IMRT treatment techniques [electronic resource] /

Khadija, Murshed. January 2009 (has links)
Thesis (M.S.)--University of Toledo, 2009. / "In partial fulfillment of the requirements for the degree of Master of Science in Biomedical Sciences." Title from title page of PDF document. Bibliography: p. 34-35.
2

Basic Study of Re-modulated Signal Quality for Fiber-to-the-home (FTTH)

Huang, Bo-hao 21 July 2009 (has links)
100Gbit/s transmission data rate is very attractive for the next generation Ethernet transport systems, and this kind of high-speed channel based WDM systems is very attractive for constructing cost-effective optical transport networks. However, it is quite difficult to achieve such high bit-rate by using the conventional amplitude shift keying (ASK) technology. For the subcarrier modulation of the orthogonal frequency division multiplexing (OFDM) scheme, it is possible to use various modulation formats. It is relatively easy to use such an advanced format in the microwave domain compared to the optical domain. Therefore, it is possible to increase the spectral efficiency relatively easily by using the OFDM technology. In this master thesis, 100 Gbit/s WDM system with OFDM is discussed. The performance of 8 channels 100Gbit/s OFDM system was evaluated. The simulation result showed that the BER of all channels were below 10-3 after 500 km transmission, and the performance could be improved by using the FEC. This result shows the possibility of 100Gbit/s transmission system using the OFDM technology. The major concern of WDM-PON system is the cost. For WDM-PON system, it needs several light sources for downstream signals and upstream signals. For the practical implementation of WDM-PON, it is essential to develop a low-cost light source in the optical network unit (ONU). In order to develop a simple efficient ONU, a promising solution is re-modulated the downstream signal from central office (CO) as the upstream signal at ONU using reflective semicounductor optical amplifier (RSOA) so that there is no need to setup an additional light source at the ONU side. For the conventional fiber-to-the-home (FTTH) system, the modulation format for both downstream signal and upstream signal is intensity modulation with direct detection (IM/DD). The demerit of the IM/DD scheme for the FTTH system is that the performance of the re-modulated upstream signal is limited by the interference of the downstream signal. In order to overcome this issue, the IM/DD format of downstream should be replaced by some different formats such as differential phase shift keying (DPSK). As there is no amplitude modulation for the DPSK downstream signal, the re-modulated signal will not be affected by the downstream signal. Therefore, this master thesis is focusing on comparing the re-modulated signal quality for both IM/DD downstream and DPSK downstream. The experimental results shows that the upstream signal of the DPSK downstream case shows better performance than the IM/DD downstream case, and the performance difference of the re-modulated signal is 17dB.
3

Comparison of IMRT delivery methods a thesis /

Markovic, Miljenko. January 2008 (has links)
Thesis (M.S.) --University of Texas Graduate School of Biomedical Sciences at San Antonio, 2008. / Vita. Includes bibliographical references.
4

Performance of multi-state Markov modulated queuing in ATM networks

Yousef, Sufian Yacoub Salameh January 1998 (has links)
No description available.
5

Application of the FMCW method to quasi-distributed absorption sensors

Zavrsnik, Miha January 2000 (has links)
We report on different addressing mechanisms for quasi-distributed absorption sensors based on the frequency modulated continuous wave (FMCW) method. The sensor units consist of open-path micro-optic cells constructed from GRIN lenses, each of differing lengths. Guided by initial simulations, two approaches are experimentally investigated and evaluated, namely reference arm addressing and coherence addressing. Reference arm addressing is accomplished by the selection of different length reference arms in a Michelson configuration where each reference arm corresponds to a certain sensing unit. Coherence addressing is achieved by the interferometric mixing of two signals originating from each cell (from the glass/air interfaces). For each method, we show theoretically and experimentally how individual cells can be addressed and the measured signals obtained by suitable choice of cell length, proper modulation of the source and appropriate signal processing. In order to improve sensitivity we present the theoretical analysis of a new scheme based on combining the (FMCW) technique with frequency modulation spectroscopy (FMS). Here we arrange for only one sideband of the rf-modulation to be attenuated by the absorption feature and a new signal, proportional to the absorbance, appears in the output spectrum at a frequency corresponding to the difference between the rf-modulation frequency and the beat frequency of a cell. The method is highly sensitive and applicable to a variety of chemical species with narrow absorption lines, such as in trace gas analysis. We present the mathematical analysis of the proposed method for single and multiple cell systems, using methane detection as an example.
6

Bearing current and shaft voltage measurement in electrical motors

Kolic, Edo January 2017 (has links)
Due to the fast-rising voltage pulses of pulse width modulated drives, the generated shaft voltage of electrical motors causes electrical discharge currents in bearings. These discharges can cause bearing failure leading to costly maintenance and unexpected production stops. To eliminate the raised shaft voltage, several techniques are used such as shaft grounding brushes, insulated bearings and conductive grease. The possibility to measure discharge activity on a PWM driven induction motor offers a tool for researchers to test different bearings in electric motors. In order to measure the shaft voltages and bearing currents, modifications of an induction motor are made firstly using 3D software and then on the physical motor. By insulating the bearing from the frame and attaching a copper ring to the bearings outer race bearing currents can be measured. The combined measurements of shaft voltage, bearing currents and frame currents shows the bearings conductive states at low speed, and insulated state at higher speed. Electrical discharge activity (DA) is observed as shaft voltage raises, resulting in a bearing current spike and shaft voltage drops rapidly while stator current is unaffected. Experimental tests were performed with a sample bearing at different operating conditions, such as operating speed, temperature, motor load, etc., to determine the effect of the common mode voltages on bearing currents and shaft voltages. At low temperatures between 23-26 ˚C and motor speeds above 1000 rpm EDM currents were observed. On temperatures above 40 ˚C no major EDM currents were observed regardless of rpm due to the bearing remaining in an ohmic conductive state. The modifications of the motor have shown to give reliable bearing current and shaft voltage measurements that can help in future research in this area.
7

Signal design and dynamics in FM bats : implications for echo processing

Boonman, Arjan Maarten January 2001 (has links)
No description available.
8

Reducing Complexity of Liver Cancer Intensity Modulated Radiotherapy

Lee, Mark Tiong Yew 15 February 2010 (has links)
Intensity modulated radiotherapy (IMRT) can potentially increase the dose delivered to liver tumours while sparing normal tissues from dose. More complex IMRT, with more modulation of the radiation beam is more susceptible to geometric and dosimetric uncertainties than simpler radiotherapy plans. Simple breath-hold liver IMRT using few radiation beam segments (<30) was investigated in 27 patients to determine the quality of treatment in terms of tumour dose coverage and normal tissue sparing as compared to index IMRT using >30 segments. In all 27 plans number of segments was reduced to <30 without compromising tumour coverage or normal tissue dose constraints, at the expense of dose conformity. Delivered tumour and normal tissue dose did not differ statistically between IMRT plans when accounting for treatment residual geometric error. This research supports considering the use of simple IMRT for treatment of liver cancer, except when loss of dose conformation is undesirable (i.e. very high doses).
9

Spin and orbital ordering in ternary transition metal oxides

Kimber, Simon A. J. January 2008 (has links)
Spin and orbital orderings are amongst the most important phenomena in the solid state chemistry of oxides. Physical property and powder neutron and X-ray diffraction measurements are reported for a range of mostly low dimensional ternary transition metal oxides which display spin or orbital order. Extensive studies of the physical properties and crystal structure of In2VO5 are reported. The structure of this material consists of one dimensional zig-zag chains of orbitally ordered S = 1/2 V4+. Magnetic susceptibility measurements show an unusual crossover from dominant ferromagnetic (θ = 17 K) to antiferromagnetic (θ = -70 K) exchange at 120 K, which is attributed to ferromagnetic dimerisation driven by magnetic frustration. The magnetic moment also increases from 1.81 to 2.2μB at the 120 K crossover. Heat capacity measurements confirm this scenario as the magnetic entropy tends towards 1/2 Rln3 below 120 K before approximating to Rln2 at high temperature. Synchrotron x-ray diffraction and high resolution neutron powder diffraction show no bulk structural changes, but the b axis, along which the VO6 chains run, shows an anomalous expansion below 120 K. At low temperatures, a downturn in the magnetic susceptibility is seen at 2.5 K, signifying a spin freezing transition. Heat capacity and powder neutron diffraction measurements show no evidence for long range magnetic order down to 0.42 K. The low dimensional brannerite materials MV2O6 (M = Mn, Co, Ni) were synthesised by a sol-gel method. Magnetic properties were investigated by magnetisation, powder neutron diffraction and in the case of CoV2O6, heat capacity measurements. The structure of these materials consists of linear chains of edge sharing MO6 octahedra. Monoclinic MnV2O6 is an isotropic antiferromagnet with TN = 20 K and a reduced magnetic coherence length due to 3 % Mn/V antisite disorder. The magnetic structure consists of ferromagnetic edge-sharing chains with k = (0,0,1/2) and a refined Mn moment of 4.77(7) μB. The triclinic materials CoV2O6 and NiV2O6 are also antiferromagnetic with TN = 7 and 14 K respectively and both show metamagnetic type transitions. Unusually, M(H) isotherms recorded below 5 K for CoV2O6 show a plateau at 1/3 of the saturation magnetisation. This feature, together with a long period modulated magnetic structure, is attributed to strong single ion (Ising) type anisotropy and nearest neighbour ferromagnetic exchange. Preliminary high pressure experiments on NiV2O6 have confirmed a previously reported transition to a columbite phase at 6 GPa and 900 °C. The high pressure polymorph is also antiferromagnetic with TN = 2.5 K. The previously uncharacterised perovskite, PbRuO3 has been prepared using high pressure/temperature synthesis techniques (10 GPa, 1000 °C). Synchrotron powder X-ray diffraction measurements show that the room temperature structure is orthorhombic, Pnma. A first order orbital ordering transition occurs at 75 K with an associated metal insulator transition. Below 75 K, the dxz orbitals are preferentially occupied and the structure is orthorhombic Imma. The transition may be driven by an increase in antiferroelectric Pb2+ displacements, whcih reach a peak at ~ 125 K. A further structural transition to a larger monoclinic cell is also identified at 9.7 K. The physical properties and crystal structures of two low dimensional lead manganese oxides have also been investigated. Acentric Pb2MnO4, which has a structure consisting of edge sharing chains, is antiferromagnetic with TN = 18 K. Powder neutron diffraction shows the magnetic structure consists of antiferromagnetic chains with k = (0,0,0) and a refined Mn moment of 2.74(2) μB. The crystal point group allows piezoelectricity and the magnetic point group symmetry allows piezomagnetism. We speculate that coupled magnetic and electric properties may be observed in this material. The layered material, Pb3Mn7O15, with a structure consisting of 1/2 filled Kagomé layers has also been studied. Single crystals were prepared by a flux growth method and polycrystalline material was prepared by the ceramic method. Powder neutron and synchrotron x-ray diffraction studies show that the single crystals are hexagonal and that the polycrystalline material is orthorhombic. Furthermore, heat capacity measurements show that the hexagonal single crystal material undergoes a glassy magnetic transition. In contrast, powder neutron diffraction shows that the orthorhombic polycrystalline material has coherent long range magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline material.
10

Investigations into static multileaf collimator based intensity modulated radiotherapy

Williams, Matthew John, Physics, Faculty of Science, UNSW January 2005 (has links)
Intensity Modulated Radiation Therapy (IMRT) is a modern radiotherapy treatment technique used to obtain highly conformal dose distributions. The delivery of IMRT is commonly achieved through the use of a multileaf collimator (MLC). One of the hindrances at present to the widespread use of IMRT is the increased time required for its planning, delivery and verification. In this thesis one particular method of MLC based IMRT, known as Static Multileaf Collimator based IMRT (SMLC-IMRT), has been studied along with methods for improving it???s delivery efficiency. The properties of an MLC commonly used in SMLC-IMRT have been characterised. The potential ramifications of these properties on the dosimetric accuracy of the delivered IMRT field were also investigated. An Interactive Leaf Sequencing (ILS) program was developed that allowed for the manipulation and processing of intensity maps using a variety of methods. The objective of each method was to improve the delivery efficiency whilst maintaining the dosimetric quality of the IMRT treatment. The different methods investigated were collimator angle optimisation, filtration, and intensity level optimisation. The collimator was optimised by identifying the angle at which the minimum monitor unit???s (MU???s) were required when using a sliding-window delivery method. A Savitzky-Golay filter was applied to random intensity maps and suitable filtration parameters identified for filtering clinical IMRT fields, and the intensity levels were optimised based on a deviation threshold. The deviation threshold identified the acceptable level of difference tolerable between the original and modified intensity map. Several IMRT cases were investigated and the impact of each the methods on MU???s, segments and dose distribution observed. As the complexity of IMRT fields increases the dosimetric impact of the MLC properties increases. Complex SMLC-IMRT fields require longer delivery times due to the increased number of MU???s and segments. Collimator optimisation was shown to be a fast and effective means of improving delivery efficiency with negligible dosimetric change to the optimised plan. Modifying intensity maps by applying a filter and optimising the intensity levels did reduce the complexity and improve the delivery efficiency, but also required a dosimetric compromise of the optimised plan.

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