The invention describes methods for safely delivering ablation energy to a tissue, e.g., thrombus, in need of ablation therapy. The method uses a catheter adapted for IVUS imaging, ablation, and impedance measurements to monitor the impedance of a tissue receiving ablation energy. In an embodiment, a user may view an IVUS image of the tissue with impedance measurements to determine if it is safe to deliver additional energy. In another embodiment, a processor is configured to determine if it is safe to deliver additional ablation energy based upon the impedance measurement.
