Focal areas of signal intensity (FASI), alternatively called focal abnormal signal intensity are bright areas on T2-weighted images commonly identified in the basal ganglia (often the globus pallidus), thalamus, brainstem (pons), cerebellum, and subcortical white matter in children with neurofibromatosis type 1 (NF1).
FASI areas are the most common neuroimaging feature in NF1 patients . A study showed a significant frequency (86%) of one or more FASI in children with NF1 . Patients younger than 10 commonly have an increase in either size or number lesions, but such an increase beyond 10 years of age raises concern for a neoplasm .
There is considerable debate about its real role within the NF1 spectrum and its potential relationship with cognitive dysfunction . The association between these focal areas of high signal and cognitive deficits remains controversial, wherein recent studies have found a favorable relationship with the presence, number and location of FASI areas . However, only the thalamic lesions seem to be strongly associated with cognitive impairment .
Di Paolo et al. published the findings of FASI areas on pathology study as characterized by spongiform myelinopathy or vacuolar change of myelin with no inflammatory reaction in the surrounding tissue and no frank demyelination. The high T2 signal was explained by the vacuoles being filled with water. It remained unclear why FASI areas sometimes regress . Further studies are needed to establish the real origin of FASI areas .
FASI lesions demonstrate bright signal on T2- and FLAIR-weighted images. They are usually isointense to hyperintense on T1, without contrast enhancement .
There is no mass effect, and they most commonly occur in the basal ganglia, brainstem, thalamus, optic tracts, cerebellum, and infrequently cerebral hemispheres .
MRS is usually normal, helping to distinguish them from tumors .