Diffusion-weighted sequences

The aim of these diffusion-weighted sequences is to obtain images whose contrast is influenced by the differences in water molecule mobility. This is done by adding diffusion gradients during the preparatory phase of an imaging sequence, usually of the SE-EPI type (spin echo– ultrafast echo planar imaging preparation) that is T2 weighted (figure 13.4).
The diffusion gradients are strong and symmetrical in relation to the 180° rephasing pulse:

• the spins of the immobile water molecules between the applications of the two gradients are dephased by the first gradient and rephased by the second.

• the spins of the water molecules that move in the direction of the gradients, during the interval between the two gradient applications, will not be rephased by the second gradient: they dephase in relation to the hydrogen nuclei of the immobile water molecules.

Taking the entire population of water molecules in a voxel, the faster the water molecules diffuse, the more dephased they will be and the weaker the recorded signal (figures 13.5 and 13.6).
This technique allows diffusion weighting of any imaging sequence. The echo planar – spin echo sequence is generally preferred for its speed, which limits the (macroscopic) motion artifacts.
Parallel acquisition methods can be used to improve the quality of diffusion images by reducing sequence time, TE and certain artifacts.

The degree of diffusion weighting of the sequence, expressed as the b-factor (in s/mm2), depends on the characteristics of the diffusion gradients:

• gradient amplitude
• application time
• time between the two gradients

The sensitivity of these sequences is limited to diffusion in the direction of the gradients, so they must be repeated by applying diffusion gradients in at least 3 spatial directions. Diffusion magnitude, calculated from the 3 diffusion images thus obtained, renders the image weighted in global diffusion (trace image). Two diffusion sequences with different b-factors can be used to quantitatively measure the degree of molecular mobility, by calculating the apparent diffusion coefficient (ADC). ADC is represented in the form of a map, whose values (in mm2s-1) no longer depend on T2. An ADC hyposignal thus corresponds to a restriction in diffusion.

In current practice, diffusion imaging of the brain consists of an acquisition with a b-factor b = 0 s/mm2 (T2-weighted) and imaging with a b-factor = 1000 s/mm2 (with diffusion weighting).

Diffusion sequences are actually T2 weighted sequences, sensitized to diffusion by gradients. The contrast of the diffusion image will have both a diffusion and a T2 component, which must be taken into consideration in the interpretation. Namely, a hypersignal in the diffusion image with b = 1000 s/mm2 can either correspond to a diffusion restriction or to a lesion that is already in T2 hypersignal (T2-shine-through) (figure 13.7).