Careful interpretation of MR images, which include the location of the intracranial lesion and intensity of the signal, plays a key role in establishing a correct diagnosis. Most intracranial lesions produce a hyperintense signal due to a prolonged T2 relaxation, mostly as a result of high water content. 

Case of a 53-year-old female patient who presented a 3 week history of events consisting of headaches, metamorphopsia and left alien hand syndrome. Initial Brain MRI remarkable for decreased T2 signal intensity of the subcortical white matter along the right parietal occipital lobes. Subsequent Brain MRI with gadolinium sequences showed abnormal signal intensity of the cortex and subcortical white matter of the right parietal and occipital lobes, with associated contrast enhancement and restricted diffusion concerning for encephalitis. Lumbar puncture x2 negative for infectious etiology, cytology x2 negative. Systemic inflammatory and infectious work-up negative. Since autoimmune encephalopathy was suspected, patient received solumedrol 1gm for 5 days. Follow-up Brain MRI 1 week later remarkable for interval worsening in the degree of abnormal signal intensity and thickening of the cortex along the right parietal and occipital lobes, which together with the spectroscopy findings are highly concerning for underlying low grade primary brain tumor. Neurosurgery performed brain biopsy remarkable for GFAP partially positive, p53 focally positive, Ki67 positive, IDH wild type, diffuse astrocytoma WHO grade II.

Subcortical hypointensity on T2-weighted MRI scans are nonspecific findings for only a few conditions and rarely seen in cerebral primary tumors. Some authors conclude that subcortical low intensity  is a nonspecific sign of meningeal and cortical disease and that it may be caused by a transient increase in the amount of free radicals.