RF Coils for MRI

J. Thomas Vaughan, John R. Griffiths, J. Thomas Vaughan, John R. Griffiths

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781118590454
• Edition: 1
• Topic: Medicine
• Subtopic: Radiology, Radiotherapy & Nuclear Medicine

The content of this volume has been added to eMagRes (formerly Encyclopedia of Magnetic Resonance) - the ultimate online resource for NMR and MRI. To date there is no single reference aimed at teaching the art of applications guided coil design for use in MRI. This RF Coils for MRI handbook is intended to become this reference.

Heretofore, much of the know-how of RF coil design is bottled up in various industry and academic laboratories around the world. Some of this information on coil technologies and applications techniques has been disseminated through the literature, while more of this knowledge has been withheld for competitive or proprietary advantage. Of the published works, the record of technology development is often incomplete and misleading, accurate referencing and attribution assignment being tantamount to admission of patent infringement in the commercial arena. Accordingly, the literature on RF coil design is fragmented and confusing. There are no texts and few courses offered to teach this material. Mastery of the art and science of RF coil design is perhaps best achieved through the learning that comes with a long career in the field at multiple places of employment…until now.

RF Coils for MRI combines the lifetime understanding and expertise of many of the senior designers in the field into a single, practical training manual. It informs the engineer on part numbers and sources of component materials, equipment, engineering services and consulting to enable anyone with electronics bench experience to build, test and interface a coil. The handbook teaches the MR system user how to safely and successfully implement the coil for its intended application. The comprehensive articles also include information required by the scientist or physician to predict respective experiment or clinical performance of a coil for a variety of common applications. It is expected that RF Coils for MRI becomes an important resource for engineers, technicians, scientists, and physicians wanting to safely and successfully buy or build and use MR coils in the clinic or laboratory. Similarly, this guidebook provides teaching material for students, fellows and residents wanting to better understand the theory and operation of RF coils.

Many of the articles have been written by the pioneers and developers of coils, arrays and probes, so this is all first hand information! The handbook serves as an expository guide for hands-on radiologists, radiographers, physicians, engineers, medical physicists, technologists, and for anyone with interests in building or selecting and using RF coils to achieve best clinical or experimental results.

About EMR Handbooks/ eMagRes Handbooks

The Encyclopedia of Magnetic Resonance (up to 2012) and eMagRes (from 2013 onward) publish a wide range of online articles on all aspects of magnetic resonance in physics, chemistry, biology and medicine. The existence of this large number of articles, written by experts in various fields, is enabling the publication of a series of EMR Handbooks/ eMagRes Handbooks on specific areas of NMR and MRI. The chapters of each of these handbooks will comprise a carefully chosen selection of articles from eMagRes. In consultation with the eMagRes Editorial Board, the EMR Handbooks/ eMagRes Handbooks are coherently planned in advance by specially-selected Editors, and new articles are written (together with updates of some already existing articles) to give appropriate complete coverage. The handbooks are intended to be of value and interest to research students, postdoctoral fellows and other researchers learning about the scientific area in question and undertaking relevant experiments, whether in academia or industry.

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PART F

Coil Modeling and Evaluation

Chapter 26

Radiofrequency MRI Coil Analysis: A Standard Procedure

Rostislav A. Lemdiasov¹ and Reinhold Ludwig²

26.1 INTRODUCTION

When designing radio frequency (RF) coils for magnetic resonance imaging,^1–3 there is often a need to compare competing coil/coil array architectures in terms of their field uniformity, field of view, and signal-to-noise ratio (SNR).Numerical simulations can accomplish this comparison effectively prior to manufacturing,and thereby save significant resources as well as time-to-market constraints. Assessing the most suitable design among many competing candidates for the intended applications, allows us efficiently proceed with prototyping and field testing.

Full-wave numerical methods including the finite difference time domain (FDTD), finite element method (FEM), and method of moments (MoM) have been widely used to simulate MRI coils and predict their performance.⁴. These computational methodsalone, however, are not sufficient to characterize an RF coil. In general,they have to becombined with proper post-processing steps to render a valid assessment of the coil.

For many design engineers, one of the major figures of merits in coil performance is a high SNR. Roemer et al.⁵ describe how one can maximize theSNR for a coil array. Signals received by individual channels of the receive coil array need to be combined with certain weights to achieve the highest SNR at a certain point in space within a particular spatial domain. The weights are calculated based on the magnetic B₁ field patterns of particular channels and also on the resistance matrix of the system. The authors propose that the computation of the resistance matrix iscarried out by integrating the electric fields over the load.

Wright and Wald⁶ concentrated on a problem of calculating the SNR for coil arrays. Unlike Roemer,⁵ their approach is more computationally tractable. However, the problem of calculating the resistance matrix is addressed in a similar manner as reported in Ref. 5.

For a complete coil analysis, it is important to determine the specific absorption rate (SAR) of the transmit coil. Rojas et al.⁷ use FEMLAB to compute both the SNR and SAR of a coil. Even though the details are not shown, the numerical method they developed can be used to study the performance of phased arrays.

Jiao and Jin⁸ used the MoM approach to simulate a birdcage RF coil. They demonstrated that an asymptotic waveform evaluation (AWE) method can be developed which yields solutions for a range of frequencies near the resonance point. This saves valuable resources since a full scale MoM simulation has to be performed only at a single frequency.

Among recent contributions to quantify SNR and SAR, we should also mention Paska et al.⁹ and Zhang et al.¹⁰. These authors prove that lumped components can be introduced into the coil after the full-wave analysis is performed. Tuning and matching of the coil is then accomplished as a post-processing step. Again, this saves significant amounts of time and computation resources because full-wave simulations are performed only once. Furthermore, the electric and magnetic fields can be computed as linear combinations of fields that are calculated during the full-wave simulation.

In a comprehensive article, Kozlov and Turner¹¹ describe in detail the process of simulating an RF coil. They state that full-wave simulations are to be performed before lumped elements are introduced into the coil model for tuning and matching. They establish a two-way link between 3D electromagnetic (full-wave) simulations and an RF circuit simulator. Figure 26.1, in their article, provides a basic work flow diagram that describes how an RF coil is simulated.

Finally, Lemdiasov et al.^12–13 outline a standard procedure for simulating RF coils based on the MoM method. As an example, they use a four-channel breast coil to demonstrate the steps involved in the simulation.

In this chapter, we expand upon the approach laid out in the previous study^12–13 by systematically developing a numerical design approach for RF coils. To make the procedure tractable, we focus on a two-channel coil as an example. The goal of this chapter is therefore to provide the reader with an in-depth understanding of the design process. Although we use the MoM method as the full-wave numerical modeling approach, other methods (FEM, FDTD) could be substituted just as efficiently.

26.2 STRUCTURE

As mentioned above, we consider as a generic example an array of two rectangular loops that share a detuning capacitor. Figure 26.1 depicts the coil geometry in air and over a dielectric load.

The loops are positioned on a cylindrical surface. Each of the loops contains one break to accommodate tuning capacitors, one break for a matching capacitor and cable, and one shared break for a decoupling capacitor. In total, there are five breaks inthe structure and consequently we define N_structure=5 to denote the number of ports. In the following, the structure is simulated at a frequency of 63.65 MHz corresponding to proton imaging at 1.5 T. We set the surface conductivity to σ = 7.95·10⁷ S m⁻¹ (copper). And we assume that each capacitor has a series resistance that is estimated based on the quality factor provided by manufacturers such as Voltron- ics or ATC. The coil is loaded by a cylindrical load (diameter 5”, height 6”) having properties of human muscle.

In the following, we outline the coil design flow as a sequence of well-defined steps that render the entire approach suitable for computerized evaluation. As a first step, we need to generate a triangular mesh of the coil surface and a tetrahedral mesh for the volumetric load (Figure 26.1). Since meshing is n...

Table of contents

• Cover
• Title
• Copyright
• Contributors
• Series Preface
• Volume Preface
• Contents
• Part A: Surface Coils
• Part B: Loop Arrays
• Part C: Volume Coils
• Part D: Special Purpose Coils
• Part E: Coil Interface Circuits
• Part F: Coil Modeling and Evaluation
• Part G: RF Safety
• Abbreviations and Acronyms
• Index