Theory and design of radio frequency coils in magnetic resonance imaging.

摘要

Advances in Magnetic Resonance Imaging (MRI) depend upon the performance of the available hardware that include radio frequency (rf) coils. The rf coils are used in MRI system to transmit rf electromagnetic waves to flip proton intrinsic angular momenta (spins) from their original direction along the applied external static magnetic field and detect (or receive) their signals when the precessing spins relax back to that original direction.; There exists a variety of rf coils depending upon applications of interest. Uniformity of the field produced by the rf coils and the higher field sensitivity are commonly demanded for preferred rf coil performances. On the other hand, it has been difficult to theoretically analyze properties of rf coils that have complicated geometries, but it is essential to understand how the coils perform prior to manufacturing.; We have developed a method of moments to evaluate rf coil properties and implemented a set of numerical codes for analyzing the rf coils. Utilizing these tools, various coil structures, including circular and rectangular loops, birdcage coils with and without rf shields, half-birdcage coils and others, have been analyzed. The methodology can be applied to any rf coil geometry without limitation. Noteworthy is a novel patented hybrid technique, which has been developed by us, of the target field approach and the method of moments. With this technique, an optimized non-obvious rf coil current pattern is derived, satisfying a set of field constraints and rf coil properties. Thus a prototype coil can be virtually built within the framework of theoretical modeling, eliminating the need for trial-and-error laboratory work (virtual engineering). Such a coil has been built and tested, yielding good agreement with the analysis made by the method of moments. Another remarkable example is an open quadrature rf surface coil, which has been optimized with respect to the current distribution to yield two orthogonal magnetic field distributions leading to quadrature capability. A prototype coil has been constructed, and its performance has been compared with the theoretical results, verifying the validity of the modeling. Images have been obtained employing the prototype coil, showing excellent agreement with the calculated results.