Transcutaneous Microvascular Imaging of Rat Liver and Kidney using Dropped-Frame Contrast-Free Super-Resolution Ultrasound

Super-resolution ultrasound imaging using erythrocytes (SURE) can create microvascular density and velocity images without injecting contrast agents, requiring only a few seconds of data acquisition. Previously, SURE was successfully applied to laparotomized rat kidneys and transcutaneous human lymph nodes; however, transcutaneous imaging of abdominal organs with significant motion remains challenging. This paper hypothesizes that microvascular density and velocity images of transcutaneous abdominal organs with substantial motion can be produced using SURE imaging when a dropped-frame method is employed. Three seconds of data from the kidney and liver of a Sprague-Dawley rat were acquired. Frames exhibiting substantial motion were automatically eliminated using a dropped-frame method based on transverse oscillation (TO). Subsequently, beamforming, motion compensation, echo cancellation, and erythrocyte peak detection and tracking were performed. The resulting microvascular density images of the liver and kidney, obtained using dropped-frame selection combined with TO motion compensation, showed improvements compared to traditional power Doppler imaging and SURE density images obtained without dropped frames. Additionally, the microvascular velocity image was successfully generated using the dropped-frame method, enabling the estimation of flow direction and velocity profiles, with example vessels of 53.3 and 77.8 µm in diameter exhibiting velocities of 4 and 10 mm/s, respectively, in this study.