Objective: To develop a direct, noninvasive ultrasound-guided method for measuring flow velocity in small vessels using a Harmonic Ultrasound Flow Velocity technique.
Methods: In vitro, experiments were designed to mimicking blood flow at different velocities inside tubes with diameters of 1mm and 200µm. Harmonic ultrasound imaging was used to track the movement of ultrasound microbubbles at a high frame rate. The distance of the individual ultrasound signal traveled in successive frames was measured and the mean flow velocity was calculated. In vivo, the flow velocity was measured in a subcutaneous colorectal carcinoma tumor model in nine rats. A similar method was employed to calculate the blood flow velocity inside the tumor vessels. The biggest internal diameter of the tumor vessels in each tumor was confirmed on hematoxylin and eosin (H&E) stained tumor sections.
Results: In vitro, the mean flow velocities detected with our proposed technique were found to be 37.03±2.45mm/s, 24.40±2.10mm/s and 4.86±0.45mm/ s , while the actual flow velocities 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 in tumors were calculated to be 5.38mm/s to 20.82mm/s. The mean size for the largest vessels inside the tumor was 40.82 ± 11.17 μm.
Conclusion: This study represents an initial attempt to demonstrate the feasibility of a new technique for blood velocity measurement in small diameter vessels. With proper development, this approach may eventually become a valuable noninvasive tool in medical research and the clinic.