Transonic HT331-FT - Appendix E: Theory of Operation

AU-OPR-AureFloFT-EN,
Rev H
43
Transit-Time Ultrasound
A Transonic
®
Perivascular Flowprobe (Fig. E.1) consists of a
Probe body which houses ultrasonic transducers and a xed
acoustic reector. The transducers are positioned on one
side of the vessel or tube under study and the reector is
positioned at a xed position between the two transducers
on the opposite side. Electronic ultrasonic circuitry directs a
Flowprobe through the following cycles:
UPSTREAM TRANSIT-TIME MEASUREMENT CYCLE
An electrical excitation causes the downstream transducer
to emit a plane wave of ultrasound. This ultrasonic wave
intersects the vessel or tubing under study in the upstream
direction, then bounces off the xed "acoustic reector." It
again intersects the vessel and is received by the upstream
transducer where it is converted into electrical signals. From
these signals, the Flowmeter derives an accurate measure
of the "transit time" it takes for the wave of ultrasound to
travel from one transducer to the other.
DOWNSTREAM TRANSIT-TIME MEASUREMENT CYCLE
The same transmit-receive sequence is repeated, but with the
transmitting and receiving functions of the transducers reversed
so that the ow under study is bisected by an ultrasonic wave
in the downstream direction. The Flowmeter again derives and
records from this transmit-receive sequence an accurate measure of
transit time it takes for the wave of ultrasound to travel from one
transducer to the other.
Just as the speed of a swimmer depends, in part, on water
currents, the transit time of ultrasound passing through a conduit
is affected by the motion of liquid owing through that vessel.
During the upstream cycle, the sound wave travels against ow and
total transit time is increased by a ow-dependent amount. During the
downstream cycle, the sound wave travels with the ow and total transit
time is decreased by the same ow-dependent amount. Using wide beam
ultrasonic illumination, the Flowmeter subtracts the downstream transit
times from the upstream transit times. This difference in the integrated
transit times is a measure of true volume ow.
WIDE BEAM ILLUMINATION
One ray of the ultrasonic beam undergoes a phase shift in transit time
proportional to the average velocity of the liquid times the path length over
which this velocity is encountered. With wide-beam ultrasonic illumination
(Fig. E.2), the receiving transducer integrates these velocity-chord
products over the vessel's full width and yields volume ow: average
velocity times the vessel's cross sectional area. Since the transit time
is sampled at all points across the vessel diameter, volume ow
measurement is independent of the ow velocity prole. Ultrasonic
beams which cross the acoustic window without intersecting the
vessel do not contribute to the volume ow integral. Volume ow
is therefore sensed by perivascular Probes even when the vessel is
smaller than the acoustic window (Fig. E.3).
Appendix E: Theory of Operation
Fig. E.1: Schematic views of a Transonic
®
perivascular
ultrasonic volume Flowsensor. Using wide beam
illumination, two transducers pass ultrasonic signals back
and forth, alternately intersecting the owing liquid in
upstream and downstream directions. The Flowmeter
derives an accurate measure of the “transit time” it takes
for the wave of ultrasound to travel from one transducer
to the other. The difference between the upstream and
downstream integrated transit times is a measure of
volume ow rather than velocity.
Fig. E.2: The vessel is placed within a beam that
fully and evenly illuminates the entire blood vessel.
The transit time of the wide beam then becomes a
function of the volume ow intersecting the beam,
independent of vessel dimensions.
Fig. E.3: The ultrasonic beam intersects the vessel
twice on its reective path. With each intersection,
the transit time through the vessel is modied by a
vector component of ow. The full transit time of the
ultrasonic beam senses the sum of these two vector
components, or ow. With misalignment (bottom),
one vector component of ow increases as the other
decreases, with little consequence to their sum.
Drost, C.J., "Vessel Diameter-Independent Volume Flow Measurements Using Ultrasound", Proceedings San Diego Biomedical
Symposium, 17, p.299-302, 1978. U.S. PATENT 4,227,407, 1980.