Figure 1. Directvs indirect methods of hepatic perfusion index (HPI) estimation. The shaded area represents the liver enhancement curve, while the dotted and dashed lines depict the arterial and portal contributions, respectively. The liver curve is separated into arterial and portal phases according to the time of peak splenic enhancement, and the peak gradients are superimposed as heavy black lines. For both methods the hepatic arterial perfusion is derived from the peak gradient of the arterial phase of the liver curve. (a) The "indirect" method calculates the portal perfusion from the peak enhancement of the portal phase of the liver curve, while (b) the "direct" method first subtracts the arterial component and then calculates the portal venous perfusion from the peak gradient of the derived portal liver curve.