Diffusion MRI offers a non-invasive indirect method to study tissue microstructure. It is very useful for clinical evaluation of tissues pathology, such as cases of stroke, to detect tissue integrity changes and to compare the same structure between different populations. In addition, it allows visualization of microstructures, like white matter fiber organization, that later can be used for surgery planning.
Water molecules undergo random thermal motion, known as random walks, that is equal in all directions (i.e. isotropic). After time t, the mean square displacement of the water molecules by distance x is described by a Gaussian distribution and is proportional to diffusion coefficient D according to <x²>=2Dt. In biological tissues, the diffusion is affected by the cellular structure, and can be hindered or restricted (i.e. anisotropic).
Diffusion imaging is acquired using the pulse gradient spin echo sequence developed by Stejskal and Tanner in 1965 (1). A spin echo sequence with the addition of two diffusion-encoded gradients applied in the same direction, with the same magnitude and duration, G and δ respectively, and with Δ, the time interval between the two gradients. When applying a gradient it will induce a temporary change in the frequency of the spins, to create a phase shift. However, with diffusion, during time Δ, water molecules will change their position, which will cause changes in the spatial distribution of the spin’s phase shift that will not be cancel out by the second gradient. Thus, not all spins will align to create an echo, resulting in loss of signal. The signal loss will be greater with faster diffusion.
Diffusion imaging depends on number of factors such as b-value (by changing G, δ and Δ values) and gradient directions, to obtain detailed information about tissue microstructure and to create diffusion-weighted imaging (DWI) and ADC maps. While diffusion tensor imaging (DTI) provides a way to study and visualize tissue organization, such as white matter in the brain, by using the anisotropic nature of diffusion in tissues (2). DTI can be later applied to fiber tracking (tractography).

Acknowledgements

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