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Accurate flow measurements with ultrasonic transit time technology depends on careful attention to several variables. These include:
Acoustical Coupling
Highest accuracy with ultrasonic transit-time flowprobes is achieved when the ultrasound signal is transmitted under uniform acoustic
conditions. This occurs when the acoustic properties of the coupling media and tissue are stable and most closely match the acoustic properties of the liquid being
measured. Since volume flow measurement with Transonic Systems' flowprobes is derived from a phase shift (the difference in upstream and downstream transit times) and is
impacted by changes in the acoustical velocity of the ultrasonic beam, discrete sources of error from acoustical mismatch can be eliminated by following these guidelines
in your experimental preparation.
Air
Air attenuates the probe's ultrasound signal and effectively blocks ultrasound transmission. With large air pockets in the
path of the ultrasound beam, little or no transmitted signal is received back to the flowprobe and accurate flow measurements cannot be made. Even small air bubbles
can compromise measurement accuracy. Therefore, all spaces between the vessel and probe must be filled with a suitable coupling agent.
Couplant
Media with lower acoustical velocity and impedance than blood are poor coupling agents for blood flow measurement with
current ultrasonic transit time flowprobes. These agents include saline, water, and NALCO 1181 mixed with saline. Aquasonic 100, an acoustic coupling agent used for
sonography proved to be only on the borderline of acceptability for use with transit time probes. Acoustically mismatched media cause reflections of the ultrasound at
the vessel boundary, can substantially change the acoustical beam direction within the probe, and impose uneven changes in the ultrasonic transit time. Measurements  may be unstable and unpredictable in both
positive and negative directions.
Fat
Fatty tissue also has a low acoustic velocity and affects the ultrasonic beam similarly. A
pad of fat on the vessel wall in the acoustic pathway of the ultrasonic beam can act like a lens, reflecting or defocusing the ultrasound
and altering the transit time nonuniformly.
Temperature
Temperature also effects the velocity of ultrasound and should be controlled for
the most accurate measurements. Acoustical velocity increases with temperature increase. However, transitions of the ultrasound beam from room
temperature coupling agent to body temperature vessel wall and blood will alter the transit time and may exacerbate errors from other sources.
Conclusion
Subtle phase shifts in the ultrasonic beam may be caused by inappropriate acoustic
conditions during the experiment and will affect the accuracy of the measurement. Acoustically tested and approved coupling agents listed below should be used with
Transonic Systems flowprobes. Fatty tissue should be carefully cleaned from the vessel where the probe is placed. Controlling temperature in the acute experiment
makes excellent physiological sense, in addition to being good acoustic practice. Transonic perivascular flowprobes are calibrated for measurements of blood at 37
degrees C and will give the most accurate readings if used within a +/- 2 - 3 degree range. Gels may be warmed on a heating plate and the probe itself should be allowed
to equilibrate to this temperature for about an hour prior to use.
Suitable Coupling Agents:
Surgilube by E. Fougera & Co. (available from most hospital supply stores)
HR Jelly from Mohawk Hospital Supply, Tel: 800-962-5660; 315-797-0570
TN #9: Acoustical Couplants for Acute Measurements
Optimizing Conditions
Choice of a Perivascular Probe
V-Series and A-Series flowprobes have increased accuracy and sensitivity
inherent to the flowprobe design; the effects of acoustic coupling mismatch are minimized. A-Series and V-Series probes should be used whenever compromised
or nonuniform conditions exist such as; for highly turbulent nonlaminar flow profiles (ascending aorta), for very small vessels (under 700 micromillimeters),
and for exceptions where the preferred acoustic coupling agents cannot be used.
Accuracy errors with R- and S-Series flowprobes will be minimized with the
correct vessel/probe fit and acoustic coupling choice. For acute applications, the vessel must fill at least 75% of the flowprobe lumen. A close or snug fit will
result in the least measurement variability. A close fit also lessens the amount of acoustic gel needed and minimizes its effect on the measurement.
Advantage of Chronic Implants
Many of the sources of error listed here are associated with acute use of
ultrasonic flowprobes and can be effectively eliminated when the probes are implanted for long term measurements and chronic protocols. No coupling gel is
required unless measurements are taken during the intra-operative procedure. Within 3 - 5 days during an animal's surgical recovery, the air spaces are filled
with fibrous connective tissue. This tissue is a good acoustic conductant and also serves to center the vessel in the most sensitive position of the probe. As
in acute applications, the vessel should be stripped of fatty tissue prior to flowprobe implantation and preventative measures should be taken to keep fat
from infiltrating into the acoustic pathway over time. In species or vessel sites predisposed to fatty tissue deposits, a thin sheet of silicone wrapped around the
outside of the probe and sutured to adjacent tissues at the time of implant will keep the probe fat free and also aid in stabilization.
In a chronic experimental design, the cardiovascular system will also be freed
from intraoperative stresses. Conscious measurements may be made without cardiovascular influences from anesthesia. Under these stable acoustic and
physiologic conditions, our customers have pushed the measurement capabilities of  Transonic flowprobes to record low flow states in difficult applications such as
bile flow in the cystic and common bile ducts in a dog model, and esophageal (amniotic fluid) flow in fetal lamb swallowing.
Scientific Protocol
While the ease of use of Transonic precalibrated flowprobes have earn plug and
play status, the rigors of scientific protocol should not be ignored. Transonic Systems specifies its probe for +/- 10% (A-, V-, R-Series, exception 1R) or +/-
15% absolute accuracy (S-Series). Careful attention to the above considerations will ensure that measurements reliably meet these standards.
Absolute accuracy may be further enhanced by in situ calibration of the flowprobes to validate the measurement under their specific conditions of use.
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