The push toward higher magnetic fields in MRI has consistently thrown up new challenges in hardware development. The recent development of a new generation of ultra-high field scanners for human imaging is no exception. The earch presented in this thesis aims to provide solutions to new technical challenges in radio-frequency probe design. All probe designs were developed for use at 7T on a Philips Acheiva full body scanner.

No description available.

Ferromagnetic metal films on semiconductors are considered to be one of the most likely candidates to achieve an efficient spin injection at room temperature, which is one of the essential requirements for spintronics devices. The work presented focused on a study of the ferromagnetic film-semiconductor heterostructures, especially their magneto-optical properties and spin injection.

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Electrical eddy currents can be induced inside biological tissue by time-varying magnetic fields according to Faraday's law of induction. These eddy currents are responsible for biological effects such as visual sensations in eyes called magnetophosphenes and they accelerate the healing process of fractured bones in magnetotherapy operation. Induced eddy currents also cause neuromuscular stimulation of cardiac muscle, shown as a disturbance in the electrocardiogram and respiratory disturbance shown as a brief period of apnoea (stopped breathing) and muscle contraction in the forearm and finger. Brain cortex also can be stimulated by pulsed magnetic fields. A transient decrease in blood flow in the human skin is seen as a result of exposing the skin to pulsed magnetic fields. To study the effects of time-varying magnetic field, a method is needed to assess and measure induced current densities. Many attempts have been made to find such a method, both theoretically and practically. A theoretical model with homogenous and isotropic concentric loops of tissue was suggested but biological tissues are neither homogenous nor isotropic. A Hall effect method using a slab of semiconductor was suggested for measurement of current densities inside tissues, but this method ignored disturbances in the current pathways inside the tissue as a result of differences in impedances between the semiconductor and the tissue. A cube substitution method using platinized conductive faces implanted in the tissue does not consider problems of alignment of the probes with the direction of isopotential lines or electrode-electrolyte impedance. Also, such electrodes measure only dc current. In a method using a three dimensional electrode to provide three-dimensional information, the author did not give evidence that these electrodes have a zero field distortion, and also did not give information about measurements made using his electrodes. None of the above methods provide a solid approach to the problems of measuring induced current densities. This thesis attempts to present a method of measuring induced current density. The method is capable of measuring both the magnitude and direction of induced current densities. It uses five point electrodes, four of them applied inside the tissue while the fifth one is just in electrical contact with the tissue. The method consists of a probe configuration system, an open-loop operational amplifier and a balanced semi-floating current driver. Leakage current, which goes to the ground and causes error, can be adjusted to be very low (about 0.01% of the total output current). A pair of Helmoltz coils was employed to provide a system for producing time-varying magnetic field. The core of the coil pair was shielded and grounded by a cut metal shield, to avoid any interference from time-varying electric field. The shield was also used as a metal incubator to keep biological samples at body temperature. The heat to the shield was supplied by a unit consisting of four power transistors, and a circuit of sensing, and controlling components. The method used in this study was tested by making measurements of eddy current densities induced in physiological saline solution as a model of a biological conducting fluid. The measurements were represented by arrows, each representing a single measurement, with the length of the arrow representing the magnitude of current density and the direction representing the direction of the induced current. Because electrically induced eddy currents are dependent on electric charge density available inside tissue, and therefore dependent on tissue electrical conductivity, this thesis presents a direct and simple method for measuring complex tissue electrical conductivity. The method uses the same five-electrode system and shares the same point electrode configurations and balanced semi-floating current driver as used for eddy current measurements. The method measures both real and imaginary components of tissue complex conductivity. Both systems are gathered into one box and their functions are separated by four toggle switches. Measurements of electrical induced current densities and complex electrical conductivities for body fluids and tissues have been carried out on saline solutions with different ionic concentrations, expired human whole blood, expired human plasma, human cerebrospinal fluid (CSF) and human urine. Solid tissue such as bovine cardiac muscle and liver were also examined. Current-to-field ratios were obtained for experiments in both fluid and tissues.

The presence of a moderately strong magnetic field, uniform and static, in the irradiated medium modifies the spatial distribution of events. The imposition of a magnetic field produces elecron helices, characterised by their radii and pitches. The differential and integral distribution of track lengths, corresponding to electrons slowing down in water in a magnetic field, have been computed as function of radii and pitches for 200 kVp X-rays and for ⁶⁰ Co gamma-rays. Theoretical work has shown that the probability of energy deposition in a smaller volume of the absorbing medium has been significantly increased as a result of the presence of a magnetic field during photon irradiation. The distributions of track lengths as function of electron radii and pitches have been studied in strong magnetic fields (1 - 20 Tesla). The trajectories of an electron moving in water for different emission angles (up to π/2) and for magnetic fields of 5 and 10 Tesla, have been computed. The data for stopping powers used in this study, cover electron energies of 30 eV to 1200 keV (initial energy). In the presence of a magnetic field, each electron spiral has enclosed a conical volume. As the magnetic field increases, so the volume enclosed by the spiral decrease resulting in a substantial increase in the number of hits (events) compared with events in the same volume in the absence of a magnetic field. The experimental work started with the study of the characteristics of a spherical walled proportional counter. The frequency density, y.f(y), energy probability density, y^2f(y), distributions and their averages overline Y_F and overline Y_D respectively, have been computed on the basis of the pulse height distribution of low-LET radiation. Gamma rays from ¹³⁷Cs and ⁶⁰Co have been used with and without a magnetic field. Transverse magnetic fields of 0.0304, 0.13, 0.24 and 0.34 Tesla as well as a longitudinal magnetic field of 0.0304 Tesla have been used in microdosimetric measurements. An average sphere diameter of 2 μm has been simulated for the purpose. In the presence of the transverse magnetic fields, an increase of up to ~ 45% and ~ 78% has been obtained in the values of overline Y_F and overline Y_D respectively for ^137Cs gamma rays. For ^60Co gamma rays the values of both overline Y_F and overline Y_D increase by up to about 97%. For the longitudinal magnetic field when compared with the corresponding transversal magnetic field, a substantial increase in the value of overline Y_F has been found for ^137Cs gamma rays and a less significant increase for ^60Co gamma rays. Also, a significant increase in overline Y_D has been obtained for both indirectly ionizing radiations.

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No description available.

Active magnetic shielding has been proposed to provide low-frequency magnetic field stability in the neutron electric dipole moment (nEDM) experiment planned for TRIUMF. A prototype active magnetic shielding system was constructed and tested at the University of Winnipeg. The system is capable of providing RMS shielding factors > 1000 for magnetic field perturbation frequencies ≤ 20 mHz, and > 100 for frequencies ≤ 0.5 Hz, and can reduce magnetic field variations on the order of tens of μT to the level of tens of nT. The achievable shielding factor was limited by the fi eld sampling rate limit of 400 Hz, and by the background fi eld noise floor of the laboratory. This represents good progress towards the eventual system for nEDM experiments, where low-frequency field drifts on the order of 100 nT require active shielding to the order of 1 nT.

Graduate / 0607