Angle correction. (a) Color duplex US image obtained with no angle correction shows how no meaningful velocity data can be obtained from the portal venous waveform because the computer automatically assigns an angle of 0° (cos 0° = 1). Without angle correction, the measured flow velocity is 18.0 cm/sec. (b) Color duplex US image obtained with correct definition of the angle between the transducer and the direction of portal venous flow demonstrates a flow velocity of 29.3 cm/sec. 
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