Advancing pure phase encoded MRI measurement of flow
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Date
2021-12
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University of New Brunswick
Abstract
Magnetic resonance imaging (MRI) is a non-optical and non-invasive measurement technique that employs magnetic field gradients to discriminate nuclei based on position and create images of a sample. A large range of research has been performed using MRI since its inception, but the work in this thesis focuses primarily on the use of MRI to study fluid flow. MRI measurements can be made sensitive to the motion of nuclei by applying magnetic field gradients with appropriate amplitude and duration. A primary concern for motion-sensitized MRI measurements is the difficulty of ensuring that the magnetic field gradients experienced by the sample are as intended. Eddy currents alter the magnetic field gradient waveform experienced by the sample and velocity values that are calculated using the input gradient waveform are incorrect.
This thesis addresses the issue by applying the pre-equalization correction method, which uses measured information about the system impulse response to tailor the input gradient waveform to give a desired output waveform. A pre-equalized version of the motion-sensitized SPRITE measurement was used to create velocity maps of water flowing through a pipe constriction and results were contrasted with previous correction techniques. Following this, a new approach was developed that uses a repeating, linearly-ramped magnetic field gradient waveform to minimize the effect of eddy currents in a motion-sensitized measurement by greatly reducing the number of gradient-amplitudes switches while still providing accurate velocity information. The linear ramp waveform was used to create velocity maps of water flowing through a pipe constriction and the results were contrasted with those from previous measurement methods. In addition, this thesis demonstrates the application of motion-sensitized SPRITE measurements to the study of foam flow. Foam flow is especially interesting to study because MRI can be made sensitive to the motion of the gas and liquid phases independently. Velocity maps of the gas and liquid phases of foam flow through a pipe constriction were created. Measured velocity maps were compared to simulations of foam flow using a Herschel-Bulkley viscosity model, showing consistent results.