Chapter 9 - MRI Instrumentation: Magnets, Gradient Coils, and Radiofrequency Coils

Abstract

Magnetic resonance (MR) imaging requires three different types of magnetic fields. First, a static and highly uniform magnetic field, known as the main field, is required to create the initial longitudinal magnetization in the object and to maintain Larmor precession of nuclear spins at constant angular frequency. Second, an “r.f.” field is required for excitation of the transverse magnetization in the object. Highly linear magnetic field gradients (that can be turned on and off during a relatively short time) are essential for spatial localization in the MR imaging. Because of the different functions and required characteristics of the main field, r.f. field, and gradient field, they are produced by different devices known as magnets, r.f. coils, and gradient coils, respectively. This chapter describes the main types and characteristics of these devices. The purpose of magnets used for MRI is to produce a static, strong, and homogeneous magnetic (main) field. The required characteristics of the main field used for NMR imaging and spectroscopy are discussed. Spatial localization in MRI is achieved through the use of three orthogonal magnetic field gradients. The main functions of r.f. coils are to excite magnetization in a sample and to receive the signal produced by the excited magnetization. These two functions may be performed by using different transmitting and receiving r.f. coils.