I. Introduction
Medical ultrasound is extensively used for studying blood flow dynamics in the human body by using color flow mapping. The technique displays a color image of the flow superimposed on an anatomic B-mode image. The velocity component along the ultrasound beam direction is measured, and a flow transverse to the beam is not displayed. This is shown in Fig. 1 in which flow in a carotid artery and jugular vein is displayed. The image was acquired with a convex array with radial beam lines, and the angle between flow direction and ultrasound beam therefore changes over the image. Notice the change of estimated flow direction around the dashed line in both vessels caused by the change of angle between the flow and the ultrasound beam. This angle dependence is one of the main limitations of current ultrasound color flow systems, because most vessels are parallel to the skin surface and it is therefore a problem to achieve a sufficiently small angle between the flow and the beam. Also, the flow is often not parallel to the vessel surface, and it is therefore difficult, if not impossible, to estimate the correct angle and compensate for it [1].
Color flow image of the carotid artery and the jugular vein scanned with a convex array transducer. Notice the change of the angle between the ultrasound beam and the velocity vector around the dashed line.