Marieke van Rosmalen

General introduction and thesis outline 15 1 the eigenvalues ((λ 1 + λ 2 + λ 3 )/3). Anisotropy is expressed as the ‘fractional anisotropy’ (FA) and can be calculated using a mathematical formula that contains all eigenvalues. FA is a scalar value that ranges from 0 to 1 ( Figure 1.3 ). Pure water has isotropic diffusion properties, which means that the water molecules are equally likely to move in any direction, hence FA = 0. For tissues that have very strong anisotropy FA = 1, i.e. diffusion is restricted by the presence of cell membranes and there may be a preferential direction, for example along nerve fibers. As MD and FA are summary measures of the eigenvalues λ 1 , λ 2 and λ 3 , changes in MD and FA can be driven by changes in either AD or in RD. For example, an increase of AD or a decrease of RD both cause an increase of FA, and a decrease of AD or RD both cause a decrease of MD. Figure 1.3 Principles of diffusion parameters Isotropic diffusion (left): water molecules are equally likely to move in any direction and fractional anisotropy is 0. Anisotropic diffusion (right): water molecules move in a preferential direction and fractional anisotropy is 1. Other quantitative MRI techniques as T2 mapping and fat fraction analysis can provide information on T2 relaxation times and fat fraction percentage of a tissue. T2 mapping relies on the principle that different echo times result in different T2 contrasts in images. By plotting the signal intensity for different echo times an exponential decay curve can be constructed. The T2 relaxation time can be calculated as a constant of the fitted curve. In this way, the T2 relaxation time can be calculated for the tissue of interest. Fat fraction analysis relies on the fact that water and fat contain protons that can be measured using Dixon imaging (chemical shift imaging). Protons in fat rotate at a different Larmor frequency than protons in water. Aminimum of two images, i.e. one in phase and one out of