INTRODUCTION

In Chapter 4 the basic features of the nuclear magnetic resonance (NMR) experiment were introduced, and in this chapter the basic physics underlying NMR is presented in more detail. We begin with a review of the basic physics of magnetic fields, including how coils are used to detect the NMR signal and how gradient fields are produced for imaging. The dynamics of a magnetic dipole in a magnetic field, which is the central physics underlying NMR, is considered next in terms of the two important physical processes of precession and relaxation. Precession and relaxation have quite different characteristics; precession is a rotation of the magnetization without changing its magnitude, whereas relaxation creates and destroys magnetization. The interplay of these two processes leads to a rich variety of dynamical behaviour of the magnetization. In the final section the magnetic properties of matter are considered in terms of how the partial alignment of dipoles with the magnetic field creates additional fields in the body. These field variations due to magnetic susceptibility differences between tissues lead to unwanted distortions in magnetic resonance (MR) images, but such effects are also the basis for most of the functional magnetic resonance imaging (fMRI) techniques.

In trying to understand how NMR works, it is helpful to have an easily visualized model for the process. The physical picture presented here is a classical physics view, and yet the physics of a proton in a magnetic field is correctly described only by quantum mechanics. The source of the NMR phenomenon is that the proton possesses spin, and spin is intrinsically a quantum mechanical property.