(RE
MRI) There are several approaches to speeding up the
MRI data acquisition process by repeating the
excitation by RF pulses in times short compared to T1, typically using small flip angles and
gradient echo refocusing. When TR is also on the order of or shorter than T2, the repeated RF pulses will tend to refocus
transverse magnetization remaining from prior excitations, setting up a condition of
steady state free precession, and a dependence of signal strength (and image
contrast) on both T1 and T2.
This can be modified in various ways, particularly:
1) to spoil the tendency to build up a steady state by reducing
coherence between excitations, e.g. by variation of the
phase or timing of consecutive RF pulses or of the strength of spoiler
gradient pulses, thus increasing the relative dependence of signal strength on T1 or
2) acquire the signal when it is
refocusing immediately prior to the next
RF pulse, thus increasing the relative dependence of signal strength on T2.
See also
Ultrafast Gradient Echo Sequence.
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