Objective To develop a direct,noninvasive ultrasound-guided method for measuring flow velocity in small vessels using a harmonic ultrasound flow velocity (HUFV) technique.Methods In vitro,experiments were designed to mimick blood flow at different velocities inside tubes with diameter of 200μm and 1mm.Harmonic ultrasound imaging was used to track the movement of ultrasound microbubbles at a high frame rate.The movement of individual bubbles in the tubing was observed and recorded.The distance of the individual ultrasound signal traveled in successive frames was measured using commercially available software and the mean flow velocity was calculated.In vivo, experiments were carried out by choosing normal rabbits.The blood flow velocities are measured in the superficial blood vessels of the ears of rabbit by using high frequency ultrasound machine.The normal flow velocity is measured as well the changed velocity under the effect of drugs are also measured.After the experiment completion the histological section of the ear blood vessel is sent for HE stain and pathological exam to measure the diameter of the blood vessel.Results In the vessel phantom,the mean flow velocities detected with our proposed technique were found to be (37.03± 2.45)mm/s, (24.40±2.10)mm/s and (4.86±0.45)mm/s, while the actual flow velocities in the tube were set at 37.14mm/s, 21.01mm/s and 4.35mm/s, respectively.No significant differences were noted between the actual and measured velocities.In vivo,the flow velocities were calculated to be 5.38mm/s to 20.82mm/s.After injecting the drug dopamine hydrochloride, the blood flow was very fast, and the calculated velocity as a result was calculated to be high.As the drug started its work,the flow velocity increased and was recorded at 17.76 mm/s.Then As the rabbits blood pressure is lowered by applying cold water,the flow velocity started falling down and was recorded at 10.67mm/s and at 4.39mm/s with progressing time.After some time under normal condition the rabbits flow velocity was recorded at 7.63mm/s.Conclusion This study represents an initial attempt to demonstrate that the blood flow velocity can be modulated and manipulated by means of drugs.Results suggest that HUFV can accurately determine the velocity in blood vessels.With proper development and automation,this approach may eventually become a valuable noninvasive tool in medical research and the clinic.