Water diffusion at a molecular scale in-vivo can only be studied by diffusion-weighted MRI. In clinical oncology, diffusion weighted imaging (DWI) is applied to detect and characterize malignant tumors as the small size of tumor cells and their high number result in measurable restriction of water diffusion. This effect may be used for assessment of therapy response during chemotherapy. Besides that, DWI can be applied to investigate the characteristic microscopic structures of tissues, as during the random motion of molecules (generally water molecules) their displacement due to diffusion is hindered by the microscopic architecture of the tissue. The image contrast in DWI is related to the rapidity of the molecular diffusion in tissues. In areas of slow diffusion the signal attenuation is smaller than in the surrounding tissue. In a diffusion-weighted image these areas are hyperintense, in the corresponding map of the apparent diffusion coefficient these areas are hypointense. An example is visible in the following figure showing a coloractal liver metastasis with strong diffusion restriction compared to the surrounding normal liver tissue.
Diffusion Tensor Imaging
Due to the microscopic architecture of tissues, the molecular water diffusion may depend on the diffusion direction (e.g. in the cerebral white matter and the spinal cord). When the tissue presents direction-dependent features, the molecular diffusion cannot be fully described by a scalar, but a tensor formalism must be introduced. The collection of MR techniques enabling for both, the computation of the diffusion tensor and the visualization of scalar indexes derived from it, is called Diffusion Tensor Imaging (DTI).
One of our research interests is the development of new modalities for the acquisition of diffusion-weighted MR images in areas of strong field inhomogeneities, such as the human spinal cord. We were the first group to report differences in the DTI properties of the lateral and the dorsal white matter tracts of the spinal cord.
The mean diffusivity highlights (white arrow) atrophy and demyeliniation of the corticospinal tract in ALS patients.
Boss A, Kolb A, Hofmann M, Bisdas S, Nägele T, Ernemann U, Stegger L, Rossi C, Schlemmer HP, Pfannenberg C, Reimold M, Claussen CD, Pichler BJ, Klose U. Diffusion tensor imaging in a human PET/MR hybrid system. Invest Radiol. 45(5):270-4. (2010)
Rossi C, Boss A, Steidle G, Martiriosan P, Klose U, Capuani S, Claussen C, Maraviglia B, Schick F. Molecular diffusion anisotropy in white and gray matter of the human spinal cord. J. Magn. Reson. Imaging 2008; 27: 476-482.
Rossi C, Boss A, Martirosian P, Steidle G, Capuani S, Claussen CD, Maraviglia B, Schick F. Influence of steady background gradients on the accuracy of molecular diffusion anisotropy measurements. Magn. Reson. Imaging 2008; 26: 1250-1258.
Rossi C, Boss A, Lindig TM, Martirosian P, Steidle G, Maetzler W, Claussen CD, Klose U, Schick F. Diffusion tensor imaging of the spinal cord at 1.5 and 3.0 Tesla. Fortschr. Roentgenstr. 2007; 179: 219-224.
Department of Radiology
University Hospital of Zürich