What is the definition or description of: doppler ultrasound?

Blood flow: Ultrasound machines use sound waves to detect density differences (masses, cysts). The doppler mode provides flow information: what direction? Is it blocked?

Duplex ultrasound: An ultrasound machine uses a black and white image in two dimensions and a pictorial representation of blood flow as a waveform and direction of blood flow by the use of color, to visualize body parts and blood vessels.