A new method for removing the so-called "shielding" effect inherent in a pore-size distribution of a porous medium obtained from the mercury porosimetry intrusion curve is presented. The methodology employs data from complementary pore structural characterisation methods, such as nitrogen adsorption BET, gravimetric analysis and NMR relaxometry, which are insensitive to the shielding effect, to correct the pore-size distribution for bidisperse materials obtained from mercury porosimetry. The characterisation of the macropore network of an alumina tablet obtained using the new methodology has been compared with the results of a study of a similar sample using microscopy carried out by earlier workers. NMR imaging studies have placed constraints on the volume of the sample which is necessary to study using microscopy in order to obtain statistically representative results. In conjunction with NMR spin density and spin-lattice relaxation time images the proposed deshielding method may be used to provide a spatially resolved pore-size distribution not possible with conventional methods.