A medical device lead includes an insulative lead body, outer and inner conductive coils, and a flexible core assembly. The outer conductive coil extends through the lead body and is coupled to a first electrode at a distal end of the outer conductive coil. The inner conductive coil extends coaxially with the outer conductive coil, is coupled to a second electrode at a distal end of the inner conductive coil, and includes a central lumen. The flexible core assembly is disposed in the central lumen and is comprised of a material that has a saturation magnetization of at least about 1.5 T and a relative permeability of greater than one. The flexible core assembly includes a positioning interface configured for manipulation of the flexible core assembly such that the flexible core assembly translates through the central lumen during insertion and extraction of the flexible core assembly.