Introduction to diffusion MRI Robert Frost

Field gradients 𝜔(𝑥)=𝛾𝐵(𝑥)

Phase representation

Phase representation

Diffusion gradient

Reverse diffusion gradient “Refocus”

Diffusion parallel to gradient

Reverse diffusion gradient Not fully refocused

Diffusion perpendicular to gradient

Reverse diffusion gradient Refocusing of spins diffusing perpendicular to gradient

Diffusion encoding Measure a reduced signal when spins diffuse along the direction of the diffusion gradient Measure less signal if there is more diffusion Can measure diffusion in different directions by repeating the experiment with different diffusion gradient orientations

Water self-diffusion in the brain Diffusion coefficient, 𝐷~ 10 −3 𝑚𝑚 2 /𝑠 𝑟 𝑅𝑀𝑆 = 2𝐷𝑡 ~10𝜇𝑚 in 50ms anisotropic isotropic