Duplex ultrasonography (more commonly but less correctly known as duplex ultrasound) is a form of medical ultrasonography that incorporates two elements:
Grayscale ultrasound to visualize the structure or architecture of the body part. No motion or bloodflow is assessed. This is the way plaque is directly imaged in a blood vessel, with the reader typically commenting on cross-sectional narrowing (greater than 70% is typically considered worthy of treatment).
Color-doppler ultrasound to visualize the flow or movement of a structure, typically used to image blood within an artery. Blood flow velocities increase through a region of narrowing, like a finger pressing up against the end of a running garden hose. Increased velocities indicate a region of narrowing or resistance (velocities greater than 250 cm/s are typically considered worthy of treatment).
Both displays are presented on the same screen (“duplex”) as overlapping images to facilitate interpretation.