This work describes a calculation procedure for expressing quantitatively the differences among MTFs determined for different directions and locations in the field for a given mammographic system. This procedure yields a numeric value named here as "MTF Quality Relative Index", which corresponds to a percentage value showing those differences. The index is calculated using equations derived on the basis of the main characteristics of typical MTF curves. Accordingly, the equations take into account parameters as the area under the curve, the first minimum of spatial frequency, and the spatial frequency corresponding to the -3 dB attenuation rate. The results of this index calculations - considering some different mammographic equipment evaluated by MTFs determined from computer simulation procedures - were compared to the relative sharpness of images of a particular test object given by the shadow-penumbra ratio. A comparison was performed for all systems in several field orientations and locations, showing that the index can express the radiographic system quality with a good rate of accuracy. As this figure of merit is directly related to the transfer function method, its main advantage is to have the same attributes as that quality evaluation approach. In addition, the index can provide easier interpretation because it allows image quality to be compared among different orientations and locations of the object in the field in a more objective way.