MRI
Also known as: magnetic resonance imaging, brain scan, neuroimaging
A non-invasive imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the brain's soft tissue structure.
Magnetic resonance imaging (MRI) is the workhorse of structural neuroimaging. Unlike CT or X-ray, MRI emits no ionising radiation — it works by aligning hydrogen atoms in tissue with a powerful magnetic field, then measuring the radio signals they emit as they relax.
Because the brain is mostly water and fat, different tissue types (grey matter, white matter, cerebrospinal fluid) relax at different rates. By tuning the scan parameters, researchers and clinicians can selectively contrast these compartments and reveal structure at sub-millimetre resolution.
Types of MRI relevant to brain ageing
MRI is not a single modality but a family of acquisition protocols, each sensitive to different tissue properties:
- Structural MRI — captures gross anatomy; the basis for brain age estimation
- T1-weighted MRI — the standard for grey matter morphometry (cortical thickness, volume)
- T2-weighted / FLAIR — highlights fluid and white matter lesions
- Diffusion MRI (DTI) — maps white matter tract integrity
- Functional MRI (fMRI) — tracks blood-oxygen-level signals as a proxy for neural activity
- MR spectroscopy — measures metabolite concentrations in a region of interest
In the context of brain age
Brain age models almost always start from a structural MRI scan — most commonly a T1-weighted sequence. From that single scan, software such as FreeSurfer or FastSurfer extracts hundreds of morphometric features (thickness, volume, surface area, curvature) that are then fed to a predictive model.
What MRI cannot directly measure
MRI is an indirect measure. It captures anatomy, not cells. A thinning cortex on MRI reflects the net result of many processes — neuronal atrophy, synaptic pruning, glial changes, vascular injury — without distinguishing between them. This is one reason brain age estimates are described as a summary biomarker rather than a specific disease marker.
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Related terms
T1-weighted MRI
The standard MRI sequence for brain morphometry. Grey matter appears dark, white matter bright, and cerebrospinal fluid very dark — making tissue boundaries easy to measure.
Cortical thickness
The thickness of the brain's outer grey-matter ribbon (the cortex), measured millimetre-by-millimetre across its surface from a structural MRI.
Brain age
An estimate of how old your brain appears structurally compared to a healthy reference population, derived from a single MRI scan.
FreeSurfer / FastSurfer
Software pipelines that automatically segment the brain from an MRI scan and extract morphometric measurements such as cortical thickness and regional volume.