Figure 4.

The error $\equiv \frac{\zeta (r)-\zeta (r\to 0)}{\zeta (r\to 0)}$ associated with using the analytic expression for the central voxel to approximate UISNR at positions away from the center. The error is shown as a function of the normalized distance $\frac{r}{a}$ from the sphere center. The dielectric properties were set to values in the average brain tissue. The dielectric sphere radius a was set to 8.5 cm, and the surface current radius b was set to 9.5 cm. The normalized distance $\frac{r}{a}$ was varied either along the (a) x–y plane or (b) z axis. The horizontal solid line indicates an error of 5%.