Apparatus, consisting of a catheter having a distalend and a predefined outer diameter. The apparatus alsohas a plurality of elastic filaments, each filamenthaving at least one electrode fixed thereto and havingtwo ends fixed within the catheter distal end to hold thefilament as a loop. The loop intertwines with one or moreother loops of the other filaments so that the pluralityof the filaments forms an open lattice, which expandswhen uncompressed to a lattice diameter at least 5 timesgreater than the outer diameter of the catheter.
