Infratentorial insensitivity of FLAIR

Axial FLAIR and T2w images of same patient at the same level show a lacunar infarct in right half of Pons visible only on T2 and not on FLAIR, attributed to relative low sensitivity of FLAIR for Posterior fossa lesions. 

It is common to have conflicting findings on MR from two different sequences of same patient.
The common example is lesions like lacunar infarcts or plaques of multiple sclerosis are visible on the Proton Density / T2w sequence but not on the FLAIR.
In practice, one should put a much higher value on the proton density / T2w sequence in the posterior fossa due to low sensitivity of FLAIR as far as infratentorial lesions are concerned.
It is not entirely clear why lesions like lacunar infarcts or plaques of multiple sclerosis below the tentorium are not well visualized on FLAIR.
This limitation of FLAIR can be dangerous at times when only FLAIR sequence is offered to radiologist for interpretation, the institutions where only axial FLAIR screening of Brain is commonly advised may be to save time or with intention to save patient’s money.

Multiple sclerosis: Case 2

Reference: Practical MR Physics and case file of MR artifacts and pitfalls, Alexander C. Mamourian, MD