Fluid-attenuated inversion recovery

Technique

By carefully choosing the inversion time (TI), the signal from any particular tissue can be nulled. The appropriate TI depends on the tissue via the formula:

$\text{TI}=\ln (2)\cdot T_1$

In other words, one should typically use a TI of around 70% of the T₁ value. In the case of CSF suppression, the long TI (inversion time) is adjusted to a zero crossing point for water (none of its signal is visible), so the signal of the CSF is theoretically "erased" from the image.

The standard T₂ variant of FLAIR uses a long TE to develop T₂-weighting and a long repetition time (TR) to increase signal to noise ratio (SNR), while the T₁ variant of FLAIR uses a short echo time (TE) and short TR to develop T₁-weighting. The TR also controls the SNR efficiency of the scan, which is optimal only for a particular T₁.