An ablation catheter assembly including an elongate catheter body having a proximal portion, a distal portion and a lumen therethrough. A helical structure associated with the catheter distal portion carries a plurality of independently operable electrodes and is transformable between a low-profile configuration wherein a straightening element is positioned in the lumen and an expanded configuration wherein the straightening element is at least partially retracted from the spiral structure. When the helical structure is in the expanded configuration, a laterally offset tip portion extends distally therefrom.