Inversion recovery sequences


 

Inversion recovery sequence

 

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.

 

Time to inversion

 

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.

 

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FRCR Physics Notes: Revision notes for the First FRCR Physics exam

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STIR

Short Tau Inversion Recovery

Fat signal nulled by selecting short TI (130 ms)

 

STIR

 

FLAIR

FLuid Attenuated Inversion Recovery

Fluid signal nulled by selecting long TI (2500 ms)

 

FLAIR

 

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