The technique of MRI has become an essential tool in the evaluation of neurological disease. Not only is it an important adjunct to CT but in many cases it is the imaging method of choice.

MRI has its roots in the pioneering work of Bloch, Purcell and others in the first half of the century (Bloch, Hansen & Packard, 1946; Purcell, Torry & Pound, 1946). However, more than 40 years elapsed between the first description of the phenomenon of nuclear magnetic resonance (NMR) and the production of the first images of human anatomy by groups at the University of Nottingham in 1976 and 1977. The development and widespread clinical dissemination of these devices occurred in the 1980s.

Basic physics of MRI

Human tissue contains significant amounts of water, proteins, lipids and other macromolecules, which are abundant in hydrogen atoms the nuclei of which are positively charged particles called protons. MRI relies on the fact that protons are magnetically active.

Protons can be thought of as spinning around their own internal axis (Fig. 1.21). One of the fundamental features of electromagnetism is that a moving charge creates its own small magnetic field. In the case of a spinning proton, this self-produced magnetic field is referred to as its magnetic moment.

When patients are placed in a strong magnetic field, the spinning protons inside their bodies can take up one of only two possible orientations.