Diffusion MRI is a non-invasive imaging technique that probes tissue microstructure by measuring the random motion of water molecules. Because water mobility is influenced by cellular density, membrane integrity, extracellular space, and tissue organisation, diffusion MRI can provide sensitive information about tissue architecture that is not visible on conventional anatomical imaging.

Diffusion MRI acquisitions are typically performed over a range of b-values, which determine how strongly the images are sensitised to water motion. Lower b-values retain more overall signal and may include contributions from faster-moving components, whereas higher b-values provide greater sensitivity to hindered and restricted diffusion within tissue.

By measuring how signal changes across different b-values, diffusion MRI can derive quantitative parameters such as the apparent diffusion coefficient (ADC), a widely used measure of water mobility within tissues. Diffusion MRI is most commonly acquired using echo planar imaging (EPI), which allows rapid image collection and supports clinically feasible scan times. The choice of b-values and EPI approach is therefore a key part of diffusion MRI study design, as it influences image quality, quantitative analysis, and the physiological meaning of the derived parameters.

In the brain, diffusion MRI is particularly valuable because tissue microstructure is closely related to function and pathology. Water mobility in brain tissue is influenced by cellular organisation, axonal structure, myelination, oedema, and necrosis, meaning that diffusion measurements can provide important information about disease processes even when structural changes are subtle or absent on conventional MRI.

Diffusion MRI is widely used in neurology and neuro-oncology. It plays a central role in acute stroke imaging, where restricted diffusion can help identify early ischaemic injury. It is also used in brain tumours, inflammatory conditions, and neurodegenerative disease, where changes in diffusion may reflect altered cellularity, tissue disruption, or treatment response. In white matter, diffusion-based approaches can also support assessment of fibre organisation and connectivity.

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