A new method is proposed for modeling realistic heart activation using diffusion tensor imaging (DTI) scans based on biological properties of its tissues in addition to the working methodology of the DTI scanner. Modeling of the excitation propagation inside the heart is initiated by applying activation at only one pacing site at the root of the conduction network. The excitation propagation has been accomplished based on a proposed conduction system that is extracted from DTI heart scans and a modified version of monodomain reaction diffusion equation (RDE) that considers the heart inhomogeneous--anisotropic material. The Aliev--Panfilov method was used to model the reaction part of the equation and inhomogeneous--anisotropic diffusion to model the diffusion part in the equation. Variation in conduction speeds between the myocardium and the conduction system has been also taken into consideration. Employing the proposed conduction system and the modified RDE on the heart model shows (somewhat) realistic heart activation where the produced ventricular excitation propagation isochrones are similar to real measurements.