A system for treating tissue includes a source of conductive and/or magnetic beads, a first member, e.g., a catheter or cannula, coupled to the source of magnetic beads, and a second member, e.g., a catheter or cannula, carrying a magnet on its distal end. The system is used for ablating or otherwise treating tissue within a target tissue region including a blood vessel contacting or passing therethrough. Magnetic beads are introduced into the target tissue region, e.g., using the first member, and a magnetic field is generated within the target tissue region, e.g., using the second member, to cause the magnetic beads to migrate towards a wall of the vessel. Energy is delivered into the target tissue region, e.g., to heat tissue therein, and the magnetic beads may attenuate or enhance treatment of tissue adjacent to the vessel.