This article presents the Core Expansion method to automatically detect border-zone corridors in MRI images of the left ventricle, to serve as guidance to Ventricular Tachycardia (VT) ablation. The method relies on the fact that the different gray level intensities of Delayed Contrast-Enhanced Magnetic Resonance Images (DEMRI) encode information about the fibrotic tissue. These differences in intensities among tissue types allow separating dense scar from healthy areas of the myocardium, and identify the border-zone region. After generating an onion-like layer-based 3D model of the left ventricle, the method detects potential corridors in the border-zone that can become electrical circuits of low conductivity. These circuits can be responsible for arrhythmic events. The method has been tested both in phantoms and patients. In patients there was a high degree of correlation between the channels detected and those visually identified by an expert on the MRI. Whenever electroanatomical maps were available post-intervention, the MRI detected channels were found to have a high degree of correlation with them.