Ultrasound Image Velocimetry for High Spatiotemporal Resolution Blood Flow Velocity Field Mapping in Mice

Objective: Abnormal hemodynamics is thought to play an essential role in the development of cardiovascular diseases. Mouse models are widely used for elucidating the underlying mechanisms; however, their small size and high heart rates make it difficult to perform quantitative flow velocity field mapping with sufficient temporal resolution. Our objective was to develop a noninvasive method for quantitative flow field mapping in mice based on speckle-tracking from high-frequency ultrasound B-mode imaging. Methods: Ultrasound ECG-gated kilohertz visualization (EKV) was performed on a mouse-aorta-sized tubular flow phantom at frame rates up to 10,000 fps. Unexpected velocity underestimations were elucidated by simulating EKV reconstruction and performing ultrasound image velocimetry (UIV) in silico. A technique for error correction was developed and validated in vitro, and demonstrated in vivo. Results: In flow phantoms, EKV-UIV underestimated velocity in the beam lateral direction by 50%–70%. This was attributed to loss of speckle contiguity owing to EKV's retrospective strip-based reconstruction of the two-dimensional B-mode image. The proposed correction technique reduced the errors to <10% by accounting only for speckle movement within each image strip. A preliminary in vivo study showed that vortex shapes and near-wall expansion movement inside a mouse left ventricle were more aligned with physical expectations after correction. Conclusion: A novel technique was developed to quantitatively map blood flow with high spatiotemporal resolution. Further optimization will enable longitudinal studies in mice to gain insights on the role of local hemodynamic forces in the development of cardiovascular diseases.