Inversion recovery 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. The TI (time from initial inverting 180° pulse to the subsequent 90° pulse) is altered based upon the material that we want to null the signal from.
As fat and fluid have different T1s and will reach Mz of zero at different times, we can select which tissue to null by selecting when to start the 90° RF pulse.
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
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)