A time-harmonic inverse methodology for the design of RF coils in MRI

Abstract

An inverse methodology is described to assist in the design of radio-frequency (RF) coils for magnetic resonance imaging (MRI) applications. The time-harmonic electromagnetic Green's functions are used to calculate current on the coil and shield cylinders that will generate a specified internal magnetic field. Stream function techniques and the method of moments are then used to implement this theoretical current density into an RF coil. A novel asymmetric coil operating for a 4.5 T MRI machine was designed and constructed using this methodology and the results are presented.

A Genetic Algorithm/Method of Moments Approach to the Optimization of an RF Coil for MRI Applications --- Theoretical Considerations

“…We have previously shown that coils modeled in this produce experimental results in prototypes that conform closely to predictions [3].…”

“…where w is the diameter of the wire and C is the perimeter of the loop; G(r|r ) is the free-space Green's function; and J (r ) is the current flowing in the loop given by (3). Assuming the quasi-static case where current is uniformly distributed along the wire, we assign I j = 1 and G(r|r ) = 1/|r − r |.…”

“…The configuration of [3] characterized by two symmetrical sets of contours is adopted in this paper. Let visualise the surface of the cylinder in the plane defined by −90 point on the loop, η , is defined by the corresponding radial and angular coordinates η and α.…”

“…Organ specific designs require flexible geometric structures that are somewhat difficult to achieve using inverse methods. Although inverse methods have considerable promise in many applications [2][3][4], we explore the use of flexible forward methods for the optimization of an asymmetric RF coil, a difficult design problem.…”

Stream function approach for determining optimal surface currents

“…Dependent of the specific characteristics of the problem, a number of dedicated approaches have been developed. Recent examples include the design of antennas [8], gradient coils for MRI [11] and magnets for MRI [7]. These examples apply the stream function indirectly to get the conductor shape, after determining the (surface) current density, from which the stream function is constructed.…”