Introduction

MRI involves three kinds of magnetic field: the main field of the scanner, the gradients which are used for spatial localization, and the oscillating magnetic field of the RF pulses. There must be something in the body which has magnetic properties in order to interact with all these fields. So far we have deliberately avoided a detailed discussion of these properties, since we believe it's easier (and more useful practically) to understand the images first. However, the time has come to explore the protons which are essential for MRI, and on the way we will tackle some difficult concepts from quantum mechanics. We will discuss the relaxation mechanisms T1 and T2 in more detail, including a molecular model of tissues that is used to explain effects such as magnetization transfer. We will find that:

• hydrogen nuclei have a magnetic moment which interacts with the main field of the scanner;

• quantum mechanics controls the behaviour of the individual protons, but classical mechanics is used to describe the changes in a large collection of nuclei;

• excitation and relaxation of a collection of protons is described by the Bloch equations;

• spin-spin and spin-lattice relaxation mechanisms are due to dipole interactions and relaxation times depend on molecular motions within the tissues;

• relaxation times can be measured using special pulse sequences (but these are not commonly used in the clinical setting);

• we can use contrast agents to modify the relaxation times of tissues, usually to create a brighter signal from pathological tissues.