Focal areas of signal intensity (FASI), alternatively called focal abnormal signal intensity or unidentified bright objects, 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.
FASI areas are the most common neuroimaging feature in neurofibromatosis type 1 (NF1) patients 1. A study showed a significant frequency (86%) of one or more FASIs in children with NF1 2. 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.10
There is considerable debate about its real role within the NF1 spectrum and its potential relationship with cognitive dysfunction 3. The association between these focal areas of high signal and cognitive deficits remains controversial, wherein recent studies have found a favourable relationship with the presence, number and location of FASI areas 3-6. However, only the thalamic lesions seem to be strongly associated with cognitive impairment 3-5.
Di Paolo et al. published the findings of FASI areas on pathology study as characterised 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 7. Further studies are needed to establish the real origin of FASI areas 1.
FASI lesions demonstrate bright signal on T2- and FLAIR-weighted images. They are usually isointense to hyperintense on T1, without contrast enhancement 9.
There is no mass effect, and they most commonly occur in the basal ganglia, brainstem, thalamus, optic tracts and cerebellum.
MRS is usually normal, helping to distinguish them from tumours 9.
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- 4. Goh WH, Khong PL, Leung CS et-al. T2-weighted hyperintensities (unidentified bright objects) in children with neurofibromatosis 1: their impact on cognitive function. J. Child Neurol. 2005;19 (11): 853-8. Pubmed citation
- 5. Moore BD, Slopis JM, Schomer D et-al. Neuropsychological significance of areas of high signal intensity on brain MRIs of children with neurofibromatosis. Neurology. 1996;46 (6): 1660-8. Pubmed citation
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- 8. FACR AGOMD. Osborns Brain. Lippincott Williams & Wilkins. ISBN:1931884218. Read it at Google Books - Find it at Amazon
- 9. Hegde AN, Mohan S, Lath N et-al. Differential diagnosis for bilateral abnormalities of the basal ganglia and thalamus. Radiographics. 2011;31 (1): 5-30. Radiographics (full text) - doi:10.1148/rg.311105041 - Pubmed citation
- 10. Sevick R, Barkovich A, Edwards M, et al. Evolution of white matter lesions in neurofibromatosis type 1: MR findings. Am J Roentgenol 1992; 159:171-175.