Age-related total gray matter and white matter changes in normal adult brain. Part I: volumetric MR imaging analysis

Abstract

Background and purpose: A technique of segmenting total gray matter (GM) and total white matter (WM) in human brain is now available. We investigated the effects of age and sex on total fractional GM (%GM) and total fractional WM (%WM) volumes by using volumetric MR imaging in healthy adults.

Methods: Fifty-four healthy volunteers (22 men, 32 women) aged 20-86 years underwent dual-echo fast spin-echo MR imaging. Total GM, total WM, and intracranial space volumes were segmented by using MR image-based computerized semiautomated software. Volumes were normalized as a percentage of intracranial volume (%GM and %WM) to adjust for variations in head size. Age and sex effects were then assessed.

Results: Both %GM and %WM in the intracranial space were significantly less in older subjects (> or =50 years) than in younger subjects (<50 years) (P <.0001 and P =.02, respectively). Consistently, %GM decreased linearly with age, beginning in the youngest subjects. %WM decreased in a quadratic fashion, with a greater rate beginning only in adult midlife. Although larger GM volumes were observed in men before adjustments for cranium size, no significant differences in %GM or %WM were observed between the sexes.

Conclusion: GM volume loss appears to be a constant, linear function of age throughout adult life, whereas WM volume loss seems to be delayed until middle adult life. Both appear to be independent of sex. Quantitative analysis of %GM and %WM volumes can improve our understanding of brain atrophy due to normal aging; this knowledge may be valuable in distinguishing atrophy of disease patterns from characteristics of the normal aging process.

Fig 1.

Segmented images from one section based on various MR images. The normalized volume measure is tissue volume (E or F) relative to the total intracranial volume (C). A, Original dual-echo FSE proton density-weighted image. B, T2-weighted MR image. C, Total intracranial volume image. D, CSF volume image. E, GM volume image. F, WM volume image.

Fig 2.

Regression analysis of fractional brain tissue volume estimates on age in 54 healthy adult subjects. Linear and weighted constrained quadratic models are presented; these indicate the age-related volume estimates throughout adulthood in normal brains. A, %GM. B, %WM. C, GM/WM ratio.

Fig 3.

Regression analysis of fractional volume estimates on age in the brain tissues of healthy male and female subjects. Despite the lack of statistical power, some trends seem to be present. A, Faster decrease in %GM in male subjects with aging (solid predicted line). B, Generally higher %WM volume in female subjects (dotted predicted line).