An electroporation system comprising: a catheter comprising an electrode assembly configured to be positioned in a vein of a body , the vein defining a central axis, the electrode assembly comprising: a structure configured to at least partially contact the vein, and a plurality of electrodes distributed about the structure, each electrode of the plurality of electrodes configured to be individually energized; a direct current (DC) energy source coupled to the catheter and configured to selectively energize a subset of electrodes of the plurality of electrodes ; and a computing system coupled to the catheter and the DC energy source , the computing system configured to: detect respective positions of the plurality of electrodes within the vein, and select, based on the detected respective positions, the subset of electrodes from the plurality of electrodes to form a circumferential ring of energized electrodes that is concentric with the central axis of the vein, wherein to select the subset of electrodes, the computing system is configured to automatically select the subset of electrodes, wherein to automatically select the subset of electrodes, the computing system is configured to automatically determine, based on the detected respective positions, which of the plurality of electrodes form an approximately circular or ellipsoid closed path. a catheter comprising an electrode assembly configured to be positioned in a vein of a body , the vein defining a central axis, the electrode assembly comprising: a structure configured to at least partially contact the vein, and a plurality of electrodes distributed about the structure, each electrode of the plurality of electrodes configured to be individually energized; a structure configured to at least partially contact the vein, and a plurality of electrodes distributed about the structure, each electrode of the plurality of electrodes configured to be individually energized; a direct current (DC) energy source coupled to the catheter and configured to selectively energize a subset of electrodes of the plurality of electrodes ; and a computing system coupled to the catheter and the DC energy source , the computing system configured to: detect respective positions of the plurality of electrodes within the vein, and select, based on the detected respective positions, the subset of electrodes from the plurality of electrodes to form a circumferential ring of energized electrodes that is concentric with the central axis of the vein, detect respective positions of the plurality of electrodes within the vein, and select, based on the detected respective positions, the subset of electrodes from the plurality of electrodes to form a circumferential ring of energized electrodes that is concentric with the central axis of the vein, wherein to select the subset of electrodes, the computing system is configured to automatically select the subset of electrodes, wherein to automatically select the subset of electrodes, the computing system is configured to automatically determine, based on the detected respective positions, which of the plurality of electrodes form an approximately circular or ellipsoid closed path. <EOS>