The present invention includes a method for determining optimal placement sites for internal defibrillators in pediatric and congenital heart defect patients. The method is executed by creating a personalized active heart-torso model. The model is created using imaging scans (e.g., low resolution clinical scans) and advanced image processing techniques. The image processing results in a heart-torso mesh model. The ventricular portion of the mesh incorporates cell membrane dynamics. The combined torso-active ventricular defibrillation model can be used for patient specific modeling of the defibrillation process and optimal defibrillator placement can be determined. This method could also be used to decrease the energy needed for a defibrillation shock, because of the optimized defibrillator placement.