What is Magnetic Resonance Imaging?

Magnetic resonance imaging (MRI) is a painless, noninvasive technique that creates anatomic images in any plane and that provides unique information on tissue characterization.  Paul Lauterbur, Ph.D. published the first magnetic resonance image in 1973.  Since that time, major technological advances, have been accompanied by the development of equipment that is now routinely used in clinical practice.  This method is particularly well suited to assessing disease states.

Magnetic resonance images are obtained by placing the patient or area of interest within a powerful, highly uniform, static magnetic field. Protons (hydrogen nuclei) within the patient align like small magnets in this field.  Radiofrequency pulses are then utilized to create an oscillating magnetic field perpendicular to the main field, from which the nuclei absorb energy and move out of alignment with the static field, in a state of excitation.  Time varying magnetic fields, called gradients, are used to encode for position and other parameters in the MRI image.  As the nuclei return from excitation to the equilibrium state, a radiofrequency signal is emitted from the body.  This signal is then transformed by a computer algorithm into diagnostic images.  Images based on different tissue characteristics can be obtained by varying the number and sequence of pulsed radiofrequency fields in order to take advantage of magnetic relaxation properties of the tissues.

Magnetic resonance images differ from those produced by x-rays: the latter are associated with absorption of x-ray energy, while MR images are based on proton density and proton relaxation dynamics.  These vary according to the tissue under examination and reflect its physical and chemical properties.