A device that is a flexible tube designed for insertion within the blood vessels for the purpose of determining blood-flow velocity by measuring the ultrasonic frequency shift between the transmitted and reflected signals (Doppler principle). It typically has a ceramic crystal attached at or near its distal tip that is capable of both transmitting and receiving ultrasound signals [20 megahertz (MHz) frequency is typical]. The proximal end is connected to a Doppler blood flowmeter unit where the measurements are displayed. It is typically used for measurement of the coronary haemodynamics (e.g., blood mean and peak velocity) during coronary procedures. This is a single-use device.