The direct visualization of blood flow by real-time ultrasound: clinical observations and underlying mechanisms.

The observation of spontaneous low-amplitude echoes from the blood in the larger blood vessels has not been satisfactorily explained. In vitro experiments were performed to study flowing and stationary blood by ultrasound. In both cases numerous low-level echoes were seen in blood and suspensions of washed red cells. No echoes were observed in plasma or solutions of hemoglobin. It was concluded that the red blood cells is the most likely scattering agent responsible for the contrast effect observed clinically.