Inversion Recovery Sequence
Inversion Recovery Sequence
Inversion recovery pulse sequences are used to give heavy T1-weighting. In
addition, the STIR (short TI inversion recovery) sequence can be used for
fat suppression, where a relatively short inversion time is used to null
the fat signal while maintaining water and soft tissue signal. The basic
part of an inversion recovery sequence is a 180 degree RF pulse that inverts
the magnetization followed by a 90 degree RF pulse that brings the residual
longitudinal magnetization into the x-y or transverse plane where it can
be detected by an RF coil. In imaging, the signal is usually refocused with
a 180 degree pulse as in a spin echo sequence.The time between the initial
180 degree pulse and the 90 degree pulse is the inversion time (TI). A diagram
of the sequence is shown below.
With a TI of about 140 ms on a 1.5 T MRI machine, the fat signal is nulled
while the water proton signal is still present. This occurs because the T1
of fat is significantly smaller than the T1 of water. The diagram below shows
the magnetization of water (black arrow) and fat (red arrow) during the STIR
sequence used for fat suppression. One drawback of this sequence is the partial
loss of proton signal during the TI time. Also the TR time must be longer
than that of a spin echo sequence for recovery of longitudinal
magnetization.
