(
STIR) Also called Short
Tau (
t) (
inversion time)
Inversion Recovery.
STIR is a
fat suppression technique with an
inversion time t =
T1 ln2 where the signal of fat is zero (
T1 is the
spin lattice relaxation time of the component that should be suppressed). To distinguish two tissue components with this technique, the
T1 values must be different.
Fluid Attenuation Inversion Recovery (
FLAIR) is a similar technique to suppress water.
Inversion recovery doubles the distance spins will recover, allowing more time for
T1 differences. A 180° preparation pulse inverts the net
magnetization to the negative
longitudinal magnetization prior to the 90°
excitation pulse.
This specialized application of the
inversion recovery sequence set the
inversion time (
t) of the sequence at 0.69 times the
T1 of fat. The
T1 of fat at 1.5 Tesla is approximately 250 with a null point of 170 ms while at 0.5 Tesla its 215 with a 148 ms null point. At the moment of
excitation, about 120 to 170 ms after the 180°
inversion pulse (depending of the
magnetic field) the
magnetization of the fat signal has just risen to zero from its original, negative, value and no fat signal is available to be flipped into the transverse plane.
When deciding on the optimal
T1 time, factors to be considered include not only the main
field strength, but also the tissue to be suppressed and the anatomy. In comparison to a conventional
spin echo where tissues with a short
T1 are bright due to faster recovery, fat signal is reversed or darkened.
Because body fluids have both a long
T1 and a long
T2, it is evident that
STIR offers the possibility of extremely sensitive detection of body fluid. This is of course, only true for stationary fluid such as edema, as the MRI signal of flowing fluids is governed by other factors.
See also
Fat Suppression and
Inversion Recovery Sequence.
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