The presented paper summarizes the problem of assessment of molecular diffusion by magnetic resonance (MR) and discuss several typical applications of diffusion imaging in the diagnosis e.g. of cerebral ischaemia, changes in patients with phenylketonuria or multiple sclerosis (MS). The images were measured using diffusion weighted (DW) spin echo Echo-planar Imaging (EPI) sequ- ence with subsequent correction of the geometrical distorsion of images and calculation of ADC maps (Apparent Diffusion Coefficient). Anisotropy of molecule diffusion in the white matter of the brain is demonstrated in a healthy brain, as well as approximate ADC values in three basic directions. The diagnosis of cerebral ischaemia by assessment of DW images is nowadays a well known method which makes it possible not only to visualize tissue changes already several minutes after the ischaemic attack but it can also differen- tiate the age of the ischaemia. Marked changes - a decline by as much as 40 % - were found in patients with phenylketonuria in areas typical by prolongation of T2. These changes are obviously associ- ated with changes in the myelinization of white matter, but typical demyelinization is not necessa- rily involved, as suggested by normal NAA values assessed in these areas by MR spectroscopy. Also in patients with MS changes of ADC were found but their magnitude and direction (increase or decrease) depended on the size of the affected areas and probably on their age.

        Key words: MR assessment of molecular diffusion in the brain - cerebral ischaemia - phenylketonu- ria, affection of the brain