A diagnostic device and method for assessing lesion formation by measuring temperature changes during endocardial ablation. Intracardiac echo catheter data is accurately mapped into a model maintained by a visualization, navigation, or mapping system using the position and orientation of the intracardiac echo catheter transducer within the model. For each point in the model, either a frequency shift or echo time shift is calculated from the intracardiac echo data to determine local temperature changes, and the local temperature changes are displayed within a rendering of the model for the user.
