Fig. 2.

The perfusion defect is modelled by a circular region of near-zero $\mathbf{K}$. Due to the fact that $\mathbf{K}$ is not exactly zero in this region, a very small flux passes through the defect region. This means that the path of least resistance to the flow is a compromise between distance travelled and resistance offered by the permeability field, and hence means that the circular boundary of the defect is not perfectly reproduced in the velocity field.