Apparatus comprising: a blood pump configured to be placed inside a blood vessel of a subject, the blood pump comprising: an impeller configured to pump blood by rotating; a support cage that, when disposed in a radially non-constrained configuration, is shaped to define: a narrow portion that is configured to be disposed around the impeller, and to maintain a separation between a wall of the blood vessel and the impeller, and a radial extension from the narrow portion of the support cage that extends radially outward with respect to the narrow portion of the support cage, the radial extension being configured to substantially maintain a longitudinal axis of the impeller in alignment with a local longitudinal axis of the blood vessel by contacting the wall of the blood vessel; characterized by a material coupled to the support cage, the material defining a hole therethrough in a vicinity of the impeller, the material being configured to occlude backflow of blood around an outside of the impeller, and to allow antegrade blood flow in the vicinity of the impeller. a blood pump configured to be placed inside a blood vessel of a subject, the blood pump comprising: an impeller configured to pump blood by rotating; a support cage that, when disposed in a radially non-constrained configuration, is shaped to define: a narrow portion that is configured to be disposed around the impeller, and to maintain a separation between a wall of the blood vessel and the impeller, and a radial extension from the narrow portion of the support cage that extends radially outward with respect to the narrow portion of the support cage, the radial extension being configured to substantially maintain a longitudinal axis of the impeller in alignment with a local longitudinal axis of the blood vessel by contacting the wall of the blood vessel; an impeller configured to pump blood by rotating; a support cage that, when disposed in a radially non-constrained configuration, is shaped to define: a narrow portion that is configured to be disposed around the impeller, and to maintain a separation between a wall of the blood vessel and the impeller, and a radial extension from the narrow portion of the support cage that extends radially outward with respect to the narrow portion of the support cage, the radial extension being configured to substantially maintain a longitudinal axis of the impeller in alignment with a local longitudinal axis of the blood vessel by contacting the wall of the blood vessel; a narrow portion that is configured to be disposed around the impeller, and to maintain a separation between a wall of the blood vessel and the impeller, and a radial extension from the narrow portion of the support cage that extends radially outward with respect to the narrow portion of the support cage, the radial extension being configured to substantially maintain a longitudinal axis of the impeller in alignment with a local longitudinal axis of the blood vessel by contacting the wall of the blood vessel; characterized by a material coupled to the support cage, the material defining a hole therethrough in a vicinity of the impeller, the material being configured to occlude backflow of blood around an outside of the impeller, and to allow antegrade blood flow in the vicinity of the impeller.