Validation of ultrasound imaging velocimetry

Aim: Doppler ultrasound is standard for measurements of blood velocity. Aninherent limitation is that Doppler methods only measure the velocity parallelto the ultrasound beam. In Ultrasound Image Velocimetry (UIV) regions of two sequential B-mode images are cross-correlated to calculate 2-D velocity vectors. UIV results were compared with Doppler and transit-time flow measurements.Methods: In vitro experiments were performed in a pulsatile flow loop. Theworking fluid was water/glycerol with ultrasound contrast agent (microbubbles).The latex tube was imaged using an Ultrasonix RP500 and a novelimaging sequence was used to interleave two ultrasound frames, enabling ashort and variable (0.3-39 ms) interframe time separation (δt). A rabbit wasanaesthetised and imaged through the abdomen, with microbubbles administered via the ear vein. Radiofrequency data were post-processed offlineusing in-house code which calculates the local correlation between successiveframes, then sums correlation results for identical phases of all cardiaccycles.Results: Peak velocities >2 m/s were accurately measured across the entirefield-of-view in vitro, while peak systolic velocities in the rabbit were 0.99 m/sand 1.04 m/s with UIV and Doppler respectively. As δt was increased flow instability during deceleration caused the UIV velocity measurement to drop to zero. Comparing velocity measurements of decelerating flow with different values of δt leads to a new method for investigating flow instability.Conclusions: With short δt UIV and derived flow rates agreed excellently withDoppler and transit time flow rates.