In a move to extend the storage capabilities of magnetic storage systems beyond 1 Tb/in2, the use of patterned media has often been cited. Here, recorded domains are constrained by the geometry of the magnetic island and not the geometry of the recording head. Conventional two-dimensional readout modeling techniques, using the reciprocity integral, rely on the assumption that the across-track medium magnetization is uniform under the giant magnetoresistive replay head. However, in the case of a geometrically constrained medium this is not the case. This work investigates the effect that the island geometry has on the characteristics of the replay signal in perpendicular patterned magnetic media storage through the extension of the reciprocity integral to three dimensions. The paper describes replay pulses that offer different characteristics from those obtained by conventional two-dimensional techniques. The origins of these differences are explained by the variation in medium magnetization across the track.