An expandable substantially spherical structure for deployment in a blood vessel or other body lumen, comprising: an open frame formed out of a closed loop of filament and configured to assume (i) a collapsed configuration in the form of a substantially two-dimensional elongated loop structure so as to facilitate insertion into the blood vessel or other body lumen, and (ii) an expanded configuration in the form of a three-dimensional substantially spherical structure so as to facilitate retention at a site in the blood vessel or other body lumen； and a flow-restricting face carried by the open frame； wherein the open frame is configured so as to permit substantially normal flow therethrough when the open frame is in its expanded configuration, and further wherein the flow-restricting face is configured so as to restrict flow therethrough.
