Introduction

Magnetic resonance imaging (MRI) is a relatively recent medical imaging modality. Although the physical phenomenon of nuclear magnetic resonance (NMR) has been known since the early 1940s [14, 15], its practical application to the field of medical imaging was only realized in 1973 when Paul C. Lauterbur made the first NMR image [16] by introducing gradients in the magnetic field. In 1974 Peter Mansfield presented the mathematical theory for fast scanning and image reconstruction, needed in clinical practice, and showed how extremely rapid imaging could be obtained by very fast gradient variations. Lauterbur and Mansfield shared the Nobel Prize in Medicine or Physiology in 2003.

A difficulty is that NMR cannot totally be explained using “classical” physics (i.e., the physical theories based on the laws of Newton and Maxwell). In 1905, Einstein demonstrated in his special theory of relativity that Newton's laws are only approximately valid. Later in the twentieth century the theory of quantum mechanics was developed to explain physical phenomena on the atomic and subatomic scale. A concise description of the basis of NMR, the property of spin angular momentum, needs the theory of quantum electrodynamics, which combines the special theory of relativity and quantum mechanics. This theory is beyond the scope of this text. A simplified discussion of NMR based on classical and quantum mechanics suffices to explain the principles of MRI.

Physics of the transmitted signal

In essence, MRI measures a magnetic property of tissue. The following section describes the behavior of a single particle with angular momentum and magnetic moment in an external magnetic field.