resonance imaging involves using strong magnetic fields and
radiofrequency (RF) energy to produce images based on the
hydrogen content (primarily water) of body tissues.
Magnetic resonance imaging relies on the
magnetic properties of atoms. When exposed to an external
magnetic field, atoms tend to align uniformly within that
field. RF energy is then directed at the atoms, knocking them
out of this alignment and causing them to spin. When the RF
pulse is discontinued, the atoms realign themselves with the
magnetic field, emitting RF energy as a signal. The response
time of this signal varies according to the density of atoms
in the tissue. A computer can process this signal and display
the results as a high-resolution image. Different substances
(e.g. bone, neurons, body fluids) are represented as different
brightnesses in this image.
imaging techniques (e.g.
tomography or CT), MRI can "see through" the body. MRI
has relatively less spatial resolution than CT, but better
Since the MRI scanner includes a strong magnet,
it is important that patients undergoing MRI remove all metal
objects such as jewelry or watches. Additionally, patients
with metal embedded in the body (e.g. aneurysm clip, pacemaker,
shrapnel, etc.) should not undergo MRI; the magnet can rip
the metal object free, causing bleeding and even death.
Otherwise, MRI has no known risks and eliminates
the danger from radiation in imaging techniques employing
X-ray (e.g. CT scanning). The magnetic fields and RF pulses
are imperceptible to the patient. The procedure does involve
lying within the confines of the scanner, which some patients
(especially those with claustrophobia) find uncomfortable.
Open MRI scanners have now been developed to alleviate this
Illustrated Guide to Diagnostic Tests, 2nd
edition. Springhouse Corporation, Springhouse PA, 1998
Article : "ARRESTING
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain