Angle of insonation. (a, b) Color duplex US images of the anterior branch of the right portal vein obtained with the transducer positioned in an intercostal (a) and subcostal (b) location depict flow as moving toward and away from the transducer, respectively. (c) On a color duplex US image obtained with the transducer positioned perpendicular to flow (arrow), no color is assigned, yielding a false finding of absent flow. The angle of insonation of the vein depends entirely on the position of the transducer. 
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