RESUMO
The direct detection of hydrocarbon fluid and the discrimination of water through carbon-13 magnetic resonance imaging (MRI) would be a significant advance in many scientific fields including food, petrogeological, and environmental sciences. Carbon-13 MRI is a noninvasive analytical technique that has great potential for direct detection of hydrocarbons. However, the low natural abundance of carbon-13, low gyromagnetic ratio, and generically short transverse signal lifetimes in realistic porous media all conspire to hinder carbon-13 MRI. A multiple echo pure phase encode MRI technique introduced in this paper helps to overcome these limitations. As a pure phase encode technique, it is immune to artifacts arising from inhomogeneous B0 fields. It is also, by its nature, more quantitative than most MRI methods. Viscous hydrocarbon flow through a sand bed, a simple realistic porous medium, was used as our test system. Flow in this model system was driven by capillary suction. The detection limit, spatially resolved, was determined to be 26 mg.
RESUMO
The results of a magnetic resonance imaging (MRI) investigation concerning the effects of an aluminum honeycomb sandwich panel on the B1 and B0 fields and on subsequent image quality are presented. Although the sandwich panel structure, representative of an aircraft composite material, distorts B0 and attenuates B1, distortion-free imaging is possible using single point (constant time) imaging techniques. A new expression is derived for the error caused by gradient field distortion due to the heterogeneous magnetic susceptibility within a sample and this error is shown not to cause geometric distortion in the image. The origin of the B0 distortion in the sample under investigation was also examined. The graphite-epoxy 'skin' of the panel is the principal source of the B0 distortion. Successful imaging of these structures sets the stage for the development of methods for detecting moisture ingress and degradation within composite sandwich structures.