Systems and methods employing a guidewire for positioning and stabilizing external instruments deployed within the body

An assembly for treating a tissue region comprising

• a catheter tube having a distal end,
• an expandable structure comprising an array of spines arranged in a desired circumferential array to define a basket assembly projecting beyond the distal end of the catheter tube and including a far end,
• an array of electrodes carried by the spines, each electrode being selective movable between a retracted position withdrawn within the respective spine and an extended position extending outward of the respective spine, and
• a distal tail projecting beyond the far end of the basket assembly, the distal tail including a guidewire lumen that accommodates passage of a guidewire without threading the guidewire through the catheter tube, the distal tail including a rigid first member adapted to engage the array of spines to maintain the array of spines in the desired circumferential array, and a less rigid second member, the second member being sized and configured to overlap the first member, at least in part, to couple the first and second members, whereby the second member extends distally beyond the first member to provide a gradient of decreasing stiffness from a proximal region to a distal region.