FLAIR imaging
Water is very bright on T2 weighted imaging and it can obscure lesions that are situated just adjacent to it. This becomes a problem in conditions such as multiple sclerosis in which periventricular lesions are part of the diagnostic criteria. To remove this problem we can perform a FLAIR sequence or fluid attenuation inversion recovery. In FLAIR the signal from freely moving water, e.g. in the ventricles and CSF spaces, is nulled and becomes dark.
This is also useful for differentiating things that are T2-bright because they contain water from other T2-bright lesions. For example, in the image below the lesion in the right middle cranial fossa could be a tumour or a cyst. FLAIR shows that the signal is completely nulled and so this must be water in a cyst.
FLAIR is also very helpful at highlighting infiltration of the CSF with anything other than fluid e.g. blood, pus, protein, tumor cells. In these cases looking at the sulci for high signal, i.e. incompletely nulled signal, is a good indicator that there is something within the CSF that is altering the signal.
Cons of FLAIR
FLAIR is a very useful sequence but it can produce inferior images, particularly within the posterior fossa such as:
- pulsation and flow void artifact of the CSF
- poorer quality imaging of the cerebellum than T2
Summary
- FLAIR nulls signal from free fluid
- FLAIR is useful for:
- periventricular lesions
- distinguishing free fluid from other causes of hyperintensity on T2WI
- looking for subarachnoid infiltration with blood, pus and cell