Importance of Acoustical Coupling in Accuracy
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 observing the following guidelines.
Air attenuates the probe’s ultrasound signal and effectively blocks ultrasound transmission. With large air pockets in the path of the ultrasound beam, the flowprobe receives little or no transmitted signal and accurate flow measurements are not possible. Even small air bubbles can compromise measurement accuracy. Therefore, all spaces between the vessel and probe must be filled with a suitable coupling agent (Fig. 1).
Fig. 1. Upper Graphic shows a perivascular flow probe without acoustic couplant. Bottom graphic shows the same flow probe with acoustic couplant filling spaces between the probe and the vessel.
The best acoustic couplant is Surgilube (E Fougera & Co.) because it matches the acoustic properties of blood. 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.
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.
Temperature also effects the velocity of ultrasound and should be controlled for the most accurate measurements. Acoustical velocity increases with temperature increase. 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.
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 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 37ºC 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.