The filtration of suspensions in the colloidal regime is part of many processes. It is necessary due to the widespread use of nano-size particles in many applications. In nano-scale particle systems, the huge specific surface of the particles is dominating the processes. The question is whether the well-known filter theory is applicable for such type of products. The experiments in this study were conducted by gas-driven filtration. The analysis of the filter cake was done globally according to the Darcy equation and locally by the use of magnetic resonance imaging (MRI) techniques. Due to the very high resistances during filtration for colloidal suspensions, comparatively thin Filter cakes with an approximate height of 0.5-3 mm were investigated. The overall porosities determined by the different techniques are in good agreement. The MRI technique allows an in situ measurement of the porosity gradient within a filter cake formed by compressible material.