the world of radiology is the world of magic and gessing
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. 
Angle correction. (a) Color duplex US image obtained with a 30° corrected angle, which is too low, demonstrates a flow velocity of 21.3 cm/sec in the portal vein. This figure represents an underestimation of the true flow velocity. (b) Color duplex US image obtained with a 70° corrected angle, which is too high, demonstrates a flow velocity of 52.8 cm/sec in the portal vein, which represents an overestimation of flow velocity. Note that the measured flow velocity increases as the corrected angle increases. 
