Contrast ultrasound analysis of blood circulation in the human spleen parenchyma. (A) Enhancement of the ultrasound signal intensity in the spleen of a human volunteer receiving a constant perfusion of Sonovue microbubbles. (B) The ultrasound-induced decrease of signal intensity⁴³  in a subcapsular zone (white line in panel A) was studied for at least 8 seconds. The best fit to the resulting experimental signal-time curve (EC) was obtained with a bi-exponential curve (TC), as shown on this typical example (the correlation co-efficient R² was > 0.96 in all 16 volunteers). (C) Bi-exponential mathematical model of signal intensity vs time N(t) = V₁(C₀exp(−β₁t)) + V₂(B + (C₀−B)exp(−β₂t)), where the first and second terms correspond to a slow- and a rapid-flow compartment, respectively. (D) Schematic representation of the blood circulation in the human spleen parenchyma derived from these observations.