Flow & Bubble Detection

Transit time is the name given to the “phase delay” between a pair of opposing Ultrasonic transmitters and receivers.  Pair one will measure the upstream transit time and pair two will measure the downstream transit time.
The transit time of the ultrasonic signal is decreased when travelling downstream with the blood flow and increased when travelling upstream against the blood flow.  The phase delay between the integrated transit times is a measure of flowing blood.

If the transit times were equal in both the upstream and downstream sensing systems this would indicate that blood flow was static.

Unlike blood flow, which is detected by changes in signal phase, emboli or gross Emboli (bubbles) is measured by detecting reductions in Ultrasonic signal strength.  Gas as opposed to flowing blood is a poor conductor of ultrasonic energy and as emboli pass through the sensing elements the ultrasonic signal is reduced. 

The level of emboli volume within the flowing blood will be dependent on the level of signal reduction multiplied by the number of signal reduction events.