Objetivos pedagógicos

After reading this chapter, you should be able:

  • To describe magnetic properties of hydrogen nuclei: spin, precession, Larmor frequency
  • To present the origin of the net magnetization
  • To explain the nuclear magnetic resonance phenomenon
  • To differentiate spin-lattice relaxation from spin-spin relaxation
  • To define relaxation times T1 et T2

Pontos principais

Within a magnetic field B0, the sum of spins is a net magnetization aligned with B0. This macroscopic magnetization results from a slight excess of spins in parallel state and a null transverse magnetization due to spins being out of phase.

Precession frequency (Larmor frequency) of protons is proportional to field strength intensity.

A RF pulse that matches the precession frequency affects the spin equilibrium : there is an exchange of energy and a tip down of the net magnetization vector. Flip angle depends on intensity, waveform and duration of RF pulse.

Relaxation is the dynamic physical process in which the system of spins returns to equilibrium. Relaxation can be broken down into :

  • Recovery of longitudinal magnetization, aligned with B0, following an exponential curve characterized by time constant T1.
  • Decay of transverse magnetization, due to spins getting out of phase, according to an exponential curve characterized by time constant T2.


  1. Elster. Questions and answers in magnetic resonance imaging. 1994:ix, 278 p..
  2. McRobbie. MRI from picture to proton. 2003:xi, 359 p..
  3. NessAiver. All you really need to know about MRI physics. 1997.
  4. Kastler. Comprendre l'IRM. 2006.
  5. Gibby. Basic principles of magnetic resonance imaging. Neurosurgery clinics of North America. 2005 Jan;16(1):1-64.
  6. Pooley. AAPM/RSNA physics tutorial for residents: fundamental physics of MR imaging. Radiographics. 2005 Jul-Aug;25(4):1087-99.