MR signal intensity of gray matter/white matter contrast and intracranial fat: effects of age and sex

Abstract Signal intensity (SI) values of gray- and white-matter brain regions of interest (ROIs) were obtained from T 2 - and proton density-weighted magnetic resonance (MR) images of 58 normal subjects aged 22–82 years (31 females, 52.3±18.8 years; 27 males, 54.1±18.1 years). Sampled ROIs included the caudate, putamen, thalamus, orbitofrontal gyrus, gyrus rectus, uncus, frontal white matter, anterior and posterior corpus callosum, cranial–cervical junction fat, and retroorbital fat. Effects of age and sex on SI were examined using repeated-measures analysis of covariance. For both T 2 - and proton density-weighted acquisitions, a significant inverse relationship between age and SI was observed for the ratio of all summed gray-matter ROIs divided by summed white-matter ROIs. This relationship was additionally observed for ratios of both subcortical gray/white matter and cortical gray/white matter. Females compared with males had significantly lower cortical gray/white matter ratios on T 2 -weighted scans. Differences in SI were observed between cranial–cervical junction fat and retroorbital fat on both acquisitions, with females showing significantly higher values for cranial–cervical junction fat and males showing higher values for retroorbital fat. Implications for brain morphometry, the use of fat as a reference standard, and other issues in neuroimaging are discussed.