Doppler is an important technology in screening for blood flow in vessels or the movement of an anatomical structure.
The major concern for the physician is to make the false negative conclusion that a blood vessel is not a blood vessel when no flow is seen. Doppler technology allows the assessment of both velocity and directionality of blood flow. Accurate assessment of blood flow requires that the direction of the flow is parallel to the ultrasound beam. This is due to the Doppler equation.
In most cases, US imaging of blood vessels is performed along the short axis, and, therefore, the blood flow is perpendicular to the ultrasound beam. As the angle of the blood flow approaches 90 degrees, the cosine of this angle approaches zero thereby creating an artefactual non-detection of flow.
Two types of Doppler imaging modes are frequently set up on ultrasonic scanners, namely colour Doppler imaging (CDI) that allows estimation of the velocity of the blood flow and Power Doppler that provides more sensitive detection of low-flow compared to CDI.