Quantitative magnetic resonance measurements of porous media: radio frequency field mapping and selective pulse design
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Date
2015
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University of New Brunswick
Abstract
Magnetic resonance imaging (MRI) is a powerful tool for the non-destructive measurement of fluid content and fluid behavior in porous media. The essential factor in quantitative MRI of such systems is a reliable measure of fluid quantity in the pore space. Quantitative imaging is impaired in many cases by non-uniform B1 fields in the sample space.
In this thesis a novel method is described for mapping B1 inhomogeneities based on measurement of the B1 field employing centric-scan pure phase encode MRI measurements. The resultant B1 map is employed to correct B1 related non-uniformities in MR profiles, which leads to quantitative density profiling. The new B1 mapping technique is also employed to investigate B1 induced MRI artifacts by analyzing image distortions surrounding two geometrically identical metallic strips of aluminum and lead.
Quantitative density profiles may be acquired in porous media with a spatially selective adiabatic inversion pulse, which is immune to B1 field non-uniformities. The pulse is applied in the presence of a slice selective magnetic field gradient to restrict the field of view to a region of interest in which the B1 field is fairly uniform. This is advantageous in axial profiling of petroleum reservoir core samples and core plugs in which the sample of interest may be much longer than the natural field of view defined by the RF probe and region of constant magnetic field gradient. The adiabatic slice selection lends itself to a spatially selective T2 distribution measurement when a CPMG pulse sequence follows the slice selection. This method is an alternative to MRI-based techniques for T2 mapping in porous media when T2 is required to be measured at only a few positions along the sample, and a resolution of 5 mm is acceptable.
The above T2 distribution mapping method is compared with spin-echo SPI (SESPI) and DANTE-Z CPMG methods in terms of spatial resolution, minimum observable T2 and sensitivity.
Finally, multi-slice T2 measurement employing the longitudinal Hadamard encoding technique and adiabatic inversion pulses is discussed. The method has an inherent sensitivity advantage over corresponding slice-by-slice local T2 measurements.