Table 1. Doppler perfusion index—how to do it.

Procedure	How to do it
Prepare the patient	Overnight fasting; supine position
Measure the diameter of the common hepatic artery (AHC)	In transverse plane, find the celiac trunk and locate the common hepatic artery. Set the size of the Doppler window to encompass AHC. Set the measurement point so that the Doppler axis is as close as small as possible (should not be greater than 60 degrees to minimize error). Measure the diameter (in centimeters) of the common hepatic artery perpendicular to the longitudinal axis at the selected measurement point
Measure the cross sectional area of the common hepatic artery	Use built-in algorithm or calculate from the diameter of the AHC: cross sectional area $\left(S\right)={\left(\frac{d}{2}\right)}^2\pi$
Measure the velocity of blood flow in the common hepatic artery	At the same measurement point, position the Doppler cursor in the middle of the artery and record Doppler waves for at least 3 cardiac cycles (more cycles may be required if patient has arrhythmia). For each cycle determine the peak systolic velocity (PSV), end-diastolic velocity (EDV) and mean velocity (MV) in centimeters per second. Calculate mean PSV, EDV and MV across several cycles
Calculate resistance index (RI)	$RI=\frac{PSV-EDV}{PSV}$
Calculate blood flow in the common hepatic artery (fAHC)	Use built-in algorithm or calculate blood flow in mL/s: fAHC = MV*S
Measure the diameter of the portal vein (PV)	Locate the portal vein before its bifurcation (adjacent to common hepatic duct). Set the size of the Doppler window to encompass PV. Set the measurement point so that the Doppler axis is as close as small as possible (should not be greater than 60 degrees to minimize error). Measure the diameter (in centimeters) of the portal vein perpendicular to the longitudinal axis at the selected measurement point
Measure the cross sectional area of the portal vein	Use built-in algorithm or calculate from the diameter of the PV: cross sectional area $\left(S\right)={\left(\frac{d}{2}\right)}^2\pi$
Measure the velocity of blood flow in the portal vein	At the same measurement point, position the Doppler cursor in the middle of the portal vein and record Doppler waves for at least 3 cardiac cycles (more cycles may be required if patient has arrhythmia). For each cycle determine the mean velocity (MV) in centimeters per second. Calculate mean MV across several cycles (modern ultrasound machines will do that automatically)
Calculate blood flow in the portal vein (fPV)	Use built-in algorithm or calculate blood flow in mL/s: $fPV=MV\times S$
Calculate Doppler perfusion index (DPI)	$DPI=\frac{fAHC}{fAHC+fPV}$