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.