Target Audience: RF researchers and clinicians who want to understand RF coils in MRI systems

Objectives: Attendees will learn basics design methods and criterions of volume coils and surface coils used in MRI systems, and challenges encountered in creating homogeneous excitation, especially for body imaging with high and ultra-high field MRI systems, and existing solutions.

Introduction: RF coils are a front-end component that is required for spin excitation and MR signal reception in MRI systems. RF volume coils or surface coils are resonant at specific MR working frequencies. Volume coils are designed to create a uniform RF excitation within the imaging volume and surface coils are designed to maximize SNR of the RF reception and enable parallel imaging. In order to achieve optimal RF excitation and reception at the same time, usually the volume and surface coils are incorporated in one design, wherein the volume coil works as the transmit coil, and the surface coils work as the receive coils. In order to minimize the interference between the transmit and receive coils, decoupling technologies are needed to minimize coupling between the transmit and receive coil elements. In the course, we will discuss the principles and design methods of the volume and surface coils, as well as the decoupling technologies.

Volume coils: the volume coil is usually a cavity resonator having several electromagnetic modes, and the mode at the MRI working frequency is able to create a uniform excitation in an imaging volume. The most well-known cavity resonator used in MRI systems is the birdcage resonator invented by Hayes et al. (1). There are three types of birdcage resonators: lowpass birdcage, highpass birdcage and bandpass birdcage, we will go over the designs and mode structures of these three types of birdcage resonators and their applications in different MRI systems. Birdcage resonators have been widely used as body coils in 1.5T and 3T commercial MRI systems. The operating frequencies of body-size birdcage resonators, however, are lower than 300MHz/7T because the structure becomes self-resonant. Another cavity resonator, the TEM resonator invented by Vaughan et al. (2) was demonstrated to be able to extend the range of operation of a body-size resonator up to 300MHz, and we will go over the theory of the TEM resonator and its mode structures.

Surface coils the surface coil is a LC circuit resonating at a working frequency of the MRI system, it was first introduced by Ackerman (3), and extended to an array by Roemer (4). The surface coil is usually used as a receive coil in the MRI system. The main object of a receive coil is to maximize SNR. Therefore, minimizing coil losses, such as conductive loss, radiative loss and component loss, is important in the receive coil design. We will discuss methods to minimize the coil losses. Moreover, with the development of parallel imaging, multi-channel receive arrays gained more and more attention. A 96ch 3T head coil built by Wiggins et al (5) demonstrated to have a good performance with 3×3 acceleration. However, more coils covering a same region of interest means smaller coils, and more conductive and component losses. We will discuss how to balance the coil losses and the number of coil elements. At 7T and above, the body size is comparable to the wavelength, more and more far-field detectors such as dipoles were introduced in the high-field MRI systems as RF coils in recent years. We will go over these far-field detectors and their potential applications in the MRI systems and challenges.

Conclusion: Volume coils can generate homogeneous transmit field in the imaging volume but cannot provide sufficient SNR in most cases, while surface coils can provide high SNR but suffer from field inhomogeneity. A combination of these two types of coils can provide optimal excitation and reception at the same time.