Figure 5.

(a) Simulations of $\Delta {\text{R}}_2^{*}$ for single vessels with varying radii and a constant CA concentration of 2 mM. With increasing volume fraction (i.e. increasing vessel radius), the angle dependency increases. However, there is no noticeable angle independent offset. Small negative values for $\Delta {\text{R}}_2^{*}$ at low angles are due to the fact that for the echo time used in this simulation (40 ms) the intravascular signal has decayed due to the CA. (b) Vessels with a radius of 34.2 µm at different CA concentrations. The angle dependency increases with increasing CA concentration. For very small angles there is a small negative effect. For this vessel size, the change in ${\text{R}}_2^{*}$ is about one order of magnitude smaller than in the measured data. (c) $\Delta {\text{R}}_2^{*}$ for a 1.75 × 1.75 mm² voxel with 100 isotropic background vessels as in Figure 1 and one major vessel with a radius of 34.2 µm. (d) $\Delta {\text{R}}_2^{*}$ for a background with 100 isotropic background vessels as in Figure 2 and one major vessel with a radius of 69.8 µm. While the offset is similar in both scenarios, the orientation dependency is more pronounced for the larger anisotropic vessel.