KINETICS OF THE BLACK-HOLE PHENOMENON IN ULTRASOUND BACKSCATTERING MEASUREMENTS WITH RED-BLOOD-CELL AGGREGATION

The observation of a hypoechoic zone around the center of large tubes (the ''black hole'' phenomenon) in ultrasound backscattering measureme nts with red blood cell (RBC) aggregation was reported for the first t ime in 1989. Since then, a very limited number of studies tried to exp lain its complex mechanisms. In this study, blood models characterized by different RBC aggregation levels were prepared by diluting horse b lood plasma with a saline solution in different proportions. A laser r eflectometry technique was used to characterize the RBC aggregation ki netics and cohesion forces between RBCs for each blood sample. The blo od was circulated in a 12.7 mm diameter vertical tube. For each experi mental flow condition tested, 25 or 15 power Doppler ultrasound measur ements were performed across the tube with a 10-MHz system and insonat ion angles varying between 40 degrees to 70 degrees. For flow rates va rying between 100 and 1250 mL/min, the ''black hole'' was observed in most measurements performed with different aggregating RBC models. The ''black hole'' was more pronounced for RBCs with a high kinetics of a ggregation and measurements with increasing Doppler angles. Previous s tudies suggested that this phenomenon is due to tube entrance effects, and the reduction of RBC aggregation at very low shear rates around t he center of the tube. In the present study, the ''black hole'' was ob served for shear rates up to 25 s(-1). It is suggested that the struct ural organization and orientation of RBC rouleaux may participate in t he mechanism leading to the ''black hole'' phenomenon. A schematic rep resentation of the rheological behavior of horse RBCs in a large tube under steady flow is presented. (C) 1998 World Federation for Ultrasou nd in Medicine & Biology.