Quantification of the biological age of the brain using neuroimaging

The cosmetic and behavioural aspects of ageing become increasingly apparent with the passing years. The individual variability in physical ageing can be immediately observed in people’s face, posture, voice and gait. In contrast, the pace at which our brains age is less obvious, only becoming apparent once substantial neurodegeneration manifests through cognitive decline and dementia. Therefore, a more timely and precise assessment of brain ageing is needed so its determinants and mechanisms can be more effectively identified and ultimately optimised. This chapter describes new approaches aimed at quantifying the biological age of the brain, so-called ‘brain age’; reviews how brain age can be contrasted to chronological age to index risk of premature brain ageing; and explores how brain age can be used to investigate genetic, environmental, health, and lifestyle factors contributing to accelerated ageing. Particular attention is given to the statistical approaches underpinning brain age, evaluating their validity and limitations. The developing brain-age literature covering diverse populations, all stages of life, health and psychopathology, humans and animals, is critically and comprehensively presented. Finally, gaps in our knowledge and unresolved methodological issues are summarised, alongside proposing future directions and highlighting opportunities for further research in this promising and exciting field.