[Basic principles and technique of diffusion-weighted imaging and diffusion tensor imaging]

Due to their thermal energy, water molecules in tissue are in continuous random motion called diffusion. Water diffusion in pathologically modified tissue (e. g. ischemia, inflammation and neoplasia) is different from normal conditions. Diffusion-weighted magnetic resonance (MR) imaging (DWI) can measure the local strength and main direction of the diffusional motion in any picture element, thus providing diagnostic tissue information exceeding the morphological depiction.

Diffusion-weighted MR sequences are based on the echo planar imaging (EPI) technique which is very rapid but also susceptible to artefacts. Using especially strong magnetic field gradient pulses the MR signal is sensitized to microscopic motion of water molecules resulting in a unique image contrast in addition to T1 and T2. Local deviations of the diffusion strength from normal values indicate pathological processes. The DWI sequences can measure diffusion along any direction; however, in the clinical routine only directionally averaged DWI images (trace maps) are used. Diffusion tensor imaging (DTI) represents an advanced DWI method which specifically explores diffusional anisotropy in order to obtain additional information about tissue microstructure.

Diffusion-weighted MRI is an established technique for the assessment of pathological processes. Although DWI is mainly applied in stroke diagnostics, it is increasingly being used to detect and characterize various lesions in the brain as well as in the whole body. With new sequence techniques imaging artefacts can be significantly reduced. In addition, DTI allows the reconstruction and 3-dimensional visualization of tissue fibre structure. This method has proven to be clinically important primarily for the depiction of nerve tracts in the brain and spinal cord when planning surgical interventions and radiation therapy.