The video-intensity measurements were inversely and irreversibly related to ambient pressure changes (independent of velocity) in a tube model. However, video intensity varied inversely but reversibly with velocity (independent of pressure). This observation could not be explained simply by the "laminar flow" theory, by a change in transducer angulation, nor by a change in ultrasound imaging frame rate. This phenomenon was limited to Albunex microbubbles and was not observed with a contrast medium (corn starch) devoid of the acoustic properties of Albunex.