3.0 T Diffusion Mr Imaging

Diffusion magnetic resonance imaging studies the diffusivity of water, i.e., the casual microscopic movement of water molecules induced by thermal energy. Unlike water, various tissue components (cell walls, membranes, intracellular organs, macromolecules) restrict the movement of molecules. Diffusion may be restricted in all directions (isotropic diffusion) or in one particular direction throughout the voxel (anisotropic diffusion) as in structured tissues (e.g. cerebral white matter). Molecular movement can be determined by MR imaging techniques using ultrafast sequences sensitized to movement. In particular, spin echo sequences are acquired by the echoplanar technique to which diffusion weighting is added to make them sensitive to movement. This feature is implemented by two strong fast bipolar, diffusion gradients applied symmetrically to the RF impulse at 180 degrees resulting in proton dephasing and subsequent rephasing. The water molecules diffusing freely during and after application of the first gradient (dephasing) are not completely restored during application of the second gradient (rephasing), unlike what occurs in stationary tissues. This process results in signal intensity changes. If the proton moves, the application of gradients will attenuate signal intensity, whereas if the proton moves only slightly or not at all, the application of gradients will not affect signal intensity. In addition, protons diffusing more slowly will be in greater phase coherence than those diffusing more quickly after the impulse at 180 degrees and hence undergo less signal attenuation.