Inversion recovery sequences
Inversion recover sequences are a variant of Spin Echo sequences. They are used to null the signal from certain tissues e.g. fat in a STIR and fluid in a FLAIR by first applying a 180° RF pulse and then starting the cycle.
This flips the Mz through 180° to a negative value. As the Mz recovers, at some point it reaches zero before becoming positive again. If we apply our 90° RF pulse when the Mz is 0, at time TI (time to inversion), there is no magnetisation to create a Mxy signal. We have, in effect, nulled that signal.
As fat and fluid have different T1s and will reach Mz of zero at different times, we can select which tissue to null by select when to start the 90° RF pulse.
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Short Tau Inversion Recovery
Fat signal nulled by selecting short TI (130 ms)
FLuid Attenuated Inversion Recovery
Fluid signal nulled by selecting long TI (2500 ms)