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.

The transmitter (see also projector) is the component of the ultrasound machine that creates the impulses sent to the transducer to generate the sound waves. Also called the pulser.

See also Transmit Voltage Response and Transmit Current Response.

The transorbital window allow to insonate ophthalmic artery and ipsilateral carotid siphon through the eye. It is also possible to use the transorbital entrance to scan the middle and anterior cerebral arteries, if it is impossible through the transtemporal window.
The patient is asked to remove any contact lenses prior to examination and is instructed to close the eyes. The power output of the Doppler instrument has to be decreased to 10-20% to reduce the ultrasonic exposure of the eye.

See also Transcranial Doppler, Ultrasound Safety.