A catheter system for ablating a tissue portion of a body and visualising the ablation in real time, the system comprising a means for generating an optical imaging beam, a catheter including a catheter tip assembly comprising an array of first optical fibres for carrying the optical imaging beam and an ablating means, wherein the catheter tip assembly is adapted to direct said beam onto the tissue portion and capture a reflected portion of the optical imaging beam from the tissue portion. The system further includes a first switching means for switching the optical imaging beam between a plurality of the first optical fibres in the array and a means for processing the reflected portion of the optical imaging beam to account for variations in length between the first optical fibres.
