We compared the effects of intravenous administration of 6 % hydroxyethyl starch ( maize-derived ) in 0.9 % saline ( Voluven ; Fresenius Kabi , Runcorn , United Kingdom ) and a `` balanced '' preparation of 6 % hydroxyethyl starch ( potato-derived ) [ Plasma Volume Redibag ( PVR ) ; Baxter Healthcare , Thetford , United Kingdom ] on renal blood flow velocity and renal cortical tissue perfusion in humans using magnetic resonance imaging .