MRI diffusion-weighted imaging of the brain: contributions to image contrast from CSF signal reduction, use of a long echo time and diffusion effects.

The contributions of CSF signal reduction, use of a long echo time and diffusion weighting to the appearance of pulsed gradient spin echo (PGSE) images are analysed by reference to T2-weighted spin echo and T2-weighted fluid attenuated inversion recovery (FLAIR) pulse sequences. Both PGSE and T2-weighted FLAIR sequences reduce CSF signal and produce very heavy T2 weighting allowing the specific additional contribution produced by the diffusion weighting of PGSE sequences to be recognized. Considerable advantage accrues from CSF suppression with both PGSE and FLAIR sequences through reduction in partial volume effects and artefacts. The very heavy T2 weighting with both these pulse sequences highlights certain white matter tracts and provides high sensitivity to disease. The additional diffusion weighting with PGSE sequences can enhance or reduce white matter tract signals and may enhance or reduce lesion conspicuity relative to the FLAIR sequences. Many of the benefits attributed to the diffusion-weighted PGSE sequence may result from the reduction of the CSF signal and the heavy T2 weighting of the sequence without a contribution from diffusion effects. However, additional anatomical detail, sensitivity to myelination and increased lesion conspicuity may result from the diffusion weighting.