Metal-reinforced catheters have improved steerability and pushability. Currently, long (>wavelength/8) metallic-braided or metallic-tube-reinforced active MRI-compatible devices do not exist, due to risks of heating of surrounding tissue. Metallic-backbone devices may be possible if heating-sources are attenuated. Concentric-tube RF traps were miniaturized (“MBaluns)” by using leaky (loosely-wound) solenoids in-place of the external tube, providing transverse magnetic fields to attenuate (a) surface electric-fields on metals and (b) common-mode propagation on internal-cables. We validated (1) a 1.1mm outer-diameter (OD) active metallic-tube-based guidewire and (2) a 2.6mm OD metallic-braid-based electrophysiology catheter via; Electromagnetic modeling, Network-Analyzer electrical tests, phantom imaging and navigation into swine hearts.