Figure 1. Diagramatic representation of the distribution of contrast media within a voxel. Contrast molecules (black dots) will enter within the plasma and their delivery will be controlled by the plasma concentration (C_p) and by the flow of blood through the voxel (F). Leakage will occur into the extravascular extracellular space whose fractional value is expressed as the variable v_e. Current models assume that contrast leakage does not occur into the intracellular space (v_i). The leakage of contrast will be governed by the concentration difference between the plasma and the extracellular extravascular space and by the permeability and surface area of the capillary endothelia which is expressed as permeability–surface area product (PS).