Hybrid numerical techniques for the modelling of radiofrequency coils in MRI

Abstract

Radiofrequency (RF) coils for use in MRI can have a significant effect on both the signal-to-noise-ratio of MR images and the specific absorption rate inside the biological sample. In the past, prototypes were constructed and tested to investigate the performance of the RF coils and often required several iterations to achieve an acceptable result. However, with the advancement in computational electromagnetic techniques, RF coil modelling has now become the modus operandi of coil design because it can produce accurate numerical results, thus reducing the time and effort spent in designing and prototyping RF coils. Two hybrid methods -method of moments (MoM)/finite difference time domain (FDTD) and MoM/finite element method (FEM) - for RF coil modelling are presented herein. The paper provides a brief overview of FDTD, FEM and MoM. It discusses the hybridisation of these methods and how they are integrated to form versatile techniques. The numerical results obtained from these hybrid methods are compared with experimental results from prototype coils over a range of operating frequencies. The methods are then applied to the design of a new type of phased-array coil - the rotary phased array. From these comparisons, it can be seen that the numerical methods provide a useful aid for the design and optimisation of RF coils for use in MRI.

An orthogonal‐based decoupling method for MRI phased array coil design

“…To demonstrate the feasibility of the proposed design method, a 2 T 3‐element orthogonal knee coil array was designed and constructed. Prior to constructing the prototype, a hybrid method of moments (MoM)/finite element method (FEM) numerical program, FEKO (available from EM Software & Systems‐SA Pty Ltd, Technopark, Stellenbosch, South Africa; http://www.feko.info), was used to accurately model and analyse the performances of the orthogonal knee coil array . Figure a shows the modelled orthogonal knee coil array loaded with a homogenous cylindrical phantom 140 mm in diameter with dielectric properties of σ = 0.6 S/m and ε r = 48.6, which approximates to a human limb.…”

An Improved Cylindrical FDTD Algorithm and Its Application to Field-Tissue Interaction Study in MRI

“…The use of a cylindrical FDTD algorithm for modelling MRI associated problems allows us to easily create any EM sources, without the complicated 3-D segmentation and discretization of MRI coils [11], which is prone to introducing deviations in the desired source. Moreover, many conventional designs are reaching boundaries of allowable performance due to the omission of the central tenant-patient in the design process.…”

In vivo sensitivity estimation and imaging acceleration with rotating RF coil arrays at 7 Tesla

“…The Method of Moments (MOM) [13,18,[47][48][49][50], used for calculating the EM field, is efficient for homogeneous loads, but it is not feasible to calculate the heterogeneous dielectric loads due to the complexity of calculating the Green function [51]. The Finite Element Method (FEM) [52], discretising heterogeneous subjects into tetrahedral or hexahedral elements, is capable of representing complex heterogeneous subjects smoothly and providing a more accurate solution.…”