Anisotropy effects of the magnetic resonance signal of biological tissue.

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Magnetic Resonance Imaging (MRI) is a non-invasive and versatile imaging technique, which has seen tremendous success in research and diagnosis. With advanced MRI, one can probe biological tissues in ways that allow researchers to create quantitative maps of tissue properties. Characteristics assessed with MRI include the signal relaxation, the concentration of cell metabolites, and the amount and direction of water diffusion or blood flow. In recent years it has been shown that some MRI techniques are sensitive to the angle formed between the main axis of anisotropic tissues and the direction of the magnetic field. For instance, a systematic orientation dependency (OD) has been described in particular for images of R2* signal relaxation, attributed by current mathematical models to the contributions of both isotropic and anisotropic tissues. If understood well, these effects may be used to map early brain development or tissue damage and repair. ******The proposed program focuses on the investigation of the OD and its underlying origin across various MRI techniques. Our specific aims are: ******1. To investigate how the OD of R2* relaxation relates to venous blood oxygenation modulated via the inhalation of gas mixtures. Larger blood vessels run preferably in parallel with nerve fibre tracts of the brain's white matter. Therefore, deoxygenated blood in these vessels adds to the orientation dependency of R2* in white matter. Acquisition of R2* maps at varous degrees of venous blood oxygenation will allow us to investigate these effects in more detail. ******2. To investigate how cerebral blood flow and blood volume maps acquired with contrast-agent-based perfusion imaging are influenced by the orientation of white matter (WM). We will analyze perfusion maps acquired with gradient echo and spin echo scans to evaluate the dependency on MR data acquisition and perform numerical simulations of spatially anisotropic vascular networks. ******3. To investigate how myelin water maps depend on WM fibre orientation (both in vivo and ex vivo). ******4. To investigate how the OD of R2* relaxation evolves in the first months after birth. ******Since imaging of perfusion myelin water and blood oxygenation are used widely, it is vital to understand their limitations. Moreover, angle resolved analysis may allow for the detection of change in brain tissue where conventional analyses may fail.