Vector Velocity Estimation Using Transverse Oscillation and Synthetic Aperture Imaging

Measuring high blood velocities in both the axial and lateral direction in the human heart often poses a challenge to doctors. A proposed way of tackling this issue is through transverse oscillation and interleaved synthetic aperture imaging. It is hypothesized that these two methods allow the estimation of high vector velocities from data already acquired without changing the pulse repetition frequency. The measurements in this study were performed using a 7MHz phased array transducer with 128 elements called STv7. The transducer was connected to a Verasonics Vantage 256 scanner. An interleaved synthetic aperture imaging sequence was used to estimate the velocity with 12 evenly distributed virtual sources along the transducer. The measurements were performed on a calibrated flow rig, where there was a circulation of a blood mimicking fluid. To compensate for the high Reynolds number and thus the turbulent flow in our system when the flow rate was above 100L/h, two pulse repetition frequencies, 625 Hz and 5000 Hz, were used instead and the flow rate remained constant. After conducting the experiment, for an angle of 90 and 75 degrees between the vessel and the probe, the mean maximum lateral velocity for the measurements with 5000Hz was 0.341 m/s with a 5% standard deviation and bias -2.19%. For the measurement with 625 Hz pulse repetition frequency it was 0.324 m/s with a 4% standard deviation and bias 3.09%. However, when the angle changed to 75 degrees, the mean lateral velocity components for the 5000 Hz were 0.324 m/s with a standard deviation of 14.5% and bias 3.14%. the axial component was 0.816 m/s with a standard deviation of 1.5% and bias 5.59%. For the 625 Hz at 75 degrees, the lateral velocity was 0.320 m/s with a standard deviation of 4.7% and bias 3.96 % but the axial component could not be estimated. This proves that measuring high velocities in the lateral direction is more effective and reliable than trying to measure axially.