How to perform diffusion-weighted imaging

Diffusion-weighted imaging (DWI) has become an invaluable tool in the manag ement of patients with stroke. DWI relies on detecting the random diffusion of water molecules. In normal tissues this movement may be restricted by t he presence of cellular structures, which provide a barrier to free movemen t. This occurs in myelinated white matter, where movement is restricted mor e across than along fibres. This directional dependence is termed anisotrop ic restricted diffusion. The diffusion of water molecules can be made the d ominant contrast mechanism within an image by applying large magnetic field gradients. The pulsed gradient spin echo (PGSE) sequence provides sensitiv ity to diffusion with gradient pulses either side of the 180 degrees refocu sing pulse. This sequence is generally heavily T2 weighted. In order to det ect normal anisotropic properties within the different components of the me dullary core, association, commissural and projection fibres, DWI must be p erformed with sensitisation in at least three directions. PGSE sequences ha ve been used to obtain the diffusion coefficient (D*), a measure of mobilit y at the molecular level within tissue. In acute infarction D* is increased ; in brain death it is decreased. Diffusion contrast needs to be optimised in relation to the highly T2-dependent nature of PGSE sequences. This also requires a more detailed knowledge of how D* changes in disease, but inform ation on nonischaemic neurological conditions is still very limited.