Abstract: A method and system of ablating tissue under optical coherence tomography guidance including inserting an optical coherence tomography catheter into a patient's vasculature; navigating the catheter to a target site; imaging and mapping target tissue at the target site using the catheter; delivering a light-activated therapeutic agent into the target tissue; and illuminating the light-activated therapeutic agent with light emitted from the catheter, thereby activating the therapeutic agent and ablating the target tissue; establishing coordinates of the target tissue under computer tomography imaging whereby the catheter provides a marker visible under computer tomography imaging for establishing a positional relationship between the catheter and the target tissue, establishing a localized magnetic field in the target tissue on the basis of the coordinates obtained during the computer tomography imaging and where the light-activated therapeutic agent is magnetized and substantially retained within the target ti