(TOF) Time of flight is a ultrasound technique used for calculating of the distance to a target by using the timing of the returning echo from the target and the speed of sound in the medium between the target and the sensor. Time of flight is used in echo location and ultrasonic flowmeters.

A flowmeter derives an accurate measure of the transit time ; transit time (or time of flight) is the difference in time for an ultrasonic pulse to travel with the direction of flow and against it. With bi-directional illumination it is the time taken for an ultrasound pulse to travel from one transducer to another. This time is used to measure blood flow rate. The difference between the upstream and downstream measured transit times calculates the volume flow.

See also Echo Ranging.

(TCR) The transmit current response is the level of the acoustic output referenced to one meter per one ampere input.

See also Transmit Voltage Response.

(TVR) The transmit voltage response is the level of the acoustic output referenced to one meter per one volt input.

See also Transmit Current Response, Doppler Velocity Signal, Analog Output Signal, and Digital to Analog Converter.

(UCA / USCA) Ultrasonic contrast agents, also called ultrasound contrast agents, are encapsulated bubbles on the order of 1-10 μm in diameter. These gas bubbles are injected into the blood stream in order to increase blood/ tissue contrast during an ultrasonogram. These microbubbles are filled with air or a gas with a lower solubility in blood than air, such as perfluorochemicals. The microbubble shell consists of albumin, phospholipid, or other material and encapsulates the gas core. Due to this construction, ultrasonic contrast agents are highly compressible, and have a high echogenicity.

See also Ultrasound Contrast Agent Safety.