High-resolution functional vascular assessment with ultrasound.

In order to improve the resolution of contrast-assisted imaging systems, we have created a high-frequency destruction/contrast replenishment imaging system with a spatial resolution of 160 microm x 160 microm. The system utilizes a 1-MHz cylindrically focused transducer for destruction and a 25-MHz spherically focused transducer for pulse/echo imaging. Speckle tracking and a clutter filter are applied across frames to remove the challenging physiologic motion artifacts that are obtained when imaging with a mechanically scanned transducer. Using a new estimation technique, flow constants proportional to absolute flow rate were estimated from B-mode time-intensity curves (TICs). The in vitro results indicate a correlation between the actual flow velocity and the estimated rate constant. In vivo images are presented showing blood perfusion in the ciliary processes and iris of the rabbit eye. The regions of interest (ROIs) from the ciliary processes yielded slower perfusion compared with the iris, as expected from vascular casts of the microcirculation in this region. Potential applications of this system include high-resolution perfusion assessment in small animals.