© RSNA, 2007
A Graphical Simulator for Teaching Basic and Advanced MR Imaging Techniques
Movie 1 narration transcript (click to play). This animation
illustrates precession, the rotating frame of reference, and RF field
interactions. You first see the precession of the magnetization in a
magnetic field. When the field is varied, the precession frequency
changes. We now add an RF field. The role of this is to exert a push
on the magnetization as shown by the red bar. At present this push
does not have much of an effect. The reason for this becomes clear
if we change to a rotating frame of reference that follows the
precession. We now see that the inconsistent push of the radio waves
only makes the magnetization wiggle slightly. We now adjust the
radio frequency so the radio waves consistently push the
magnetization as to make it rotate. Back in the stationary frame of
reference, the situation looks like this. You see that the resonant
radio wave field pushes the magnetization in synchrony with the
Larmor precession.
Movie 2 narration transcript (click to play). This animation illustrates excitation and relaxation. The
magnetization is initially in thermal equlibrium along the magnetic
field. When a resonant RF field is turned on, the magnetization is
rotated into the transverse plane and radio waves are being emitted
from the body. The situation shown here is not lasting, however. Due
to nuclear interactions, the transverse magnetization will decay
away on a time scale T2, that was here set to 3 seconds. Still the
situation shown is only partially realistic, as nuclear interactions
will also drive the magnetization back to thermal
equilibrium. We set T1 to 5 seconds and watch the return of the
magnetization. Typically multiple excitations will occur during an
MR measurement. In between, the magnetization returns towards
equlibrium.
Movie 3 narration transcript (click to play). This
animation presents dephasing and shows how it can be reversed by
use of a spin-echo sequence. We start viewing the equilibium
magnetization in the rotating frame of reference. After excitation,
the isochromates dephase due to inhomogeneity. This causes a signal
loss, that is complete when the distribution is uniform in the
transverse plane. The dephasing can be reversed by use of a
refocusing pulse that rotates the distribution 180° around a
transverse axis. Subsequently, field inhomogeneity brings the
different contributions to the magnetization back in phase and the
signal loss is reversed. Repeated use of 180° pulses gives
additional refocusings.
Movie 4 narration transcript (click to play). This
animation demonstrates how k-space imaging works. You see a row of
isochromates in a gradient field. After excitation, the isochromates
dephase and the signal is lost. Notice that the wavelength of the
phase roll decreases as time passes. We now repeat the experiment
with a structured object having equidistant holes. Again the signal
is initially lost due to dephasing. Waiting a bit longer, however,
the situation changes. The signal is recovered when the wavelength
approaches the distance between holes in the structured object. In
this situation, all isochromates are, more or less, pointing in the
same direction. That happens now. Subsequently, the signal is lost
again. The signal is therefore found to reflect object structure.
Finally, the anaglyph mode is demonstrated. This enhances
visibility when 3D goggles are worn.
