Measuring blood flow acutely with ultrasonic flowmeters is always a problem. Air forms an effective barrier to the passage of ultrasound in the 1 to 20 MHz frequency range. Even a thin layer of bubbles coating the surface of a crystal element can block the signal and prevent proper operation of a flowmeter. This is true for Doppler flowmeters, as well as ultrasonic Transit Time Volume flowmeters.

Air is not a problem for chronic implants because any air trapped in the transducer at the time of implantation is quickly absorbed and the sound path filled with body fluid and tissue.

Many approaches have been utilized to accomplish good sonic coupling during acute procedures including: water based acoustic gel, non-heparinized blood clot, flooding the transducer area with saline, etc. None of these approaches is truly satisfactory, as the coupling medium flows or moves away from the transducer during the procedure, causing the flow signal to be degraded or lost.

If the voids in the transducer bodies, between the crystal element and the wall of the blood vessel, could be filled with a sound conducting material which has sound conduction properties similar to that of water and also has good mechanical properties, the majority of the air problems would be solved. This material would need to remain semi-solid at body temperature and also when exposed to body fluids.

There is a readily available material that has these desirable characteristics and it is easily worked with in the field. The material is Agar, which is available in powdered form from laboratory supply houses. Agar is a gelling by-product of seaweed and when a small amount of powder is added to near boiling water, then allowed to cool, it forms a strong rubbery gel. This gel is approximately 98% water by weight and conducts sound with virtually no attenuation at the frequencies of interest. The gel is available in several shear strengths and is easy to prepare.

By doing simple preparation to flow transducers, a user can greatly simplify the acute measurement of blood flow. The procedure is slightly different for the ART² and SVT², but the goal is the same, fill the void with agar without trapping any air bubbles.

Agar can be purchased from a number of sources such as Sigma Chemical Co. We use Sigma - High Gel Strength (Part Number A6924). We mix this agar in a ratio of 22gm / liter. To make a large batch of liquid agar solution we use a 'double-boiler' kitchen cooking pot. This has a lower chamber partially filled with water and an upper chamber, the bottom of which is partially submerged in the hot water. The water in the lower chamber is heated to almost boiling (90 C) and a measured amount of water placed in the upper chamber. When the water in the upper chamber reaches approximately 90 degrees Centigrade the powdered agar is added while stirring to avoid lumps. Once all the agar is in solution, the mixture should be cooked for 5 to 10 minutes. It is important not to overheat the mixture or it will scorch.

Small batches of agar solution may be made in a similar manner by placing a glass test tube with a measured amount of water into a cup of gently boiling water on an electric hot plate.

The Triton Technology Inc. Small Vessel Transit Time probes (200-367) can be easily filled with the agar mixture to improve acoustic coupling for acute applications and avoid trapped air bubbles.

The cavities in the 200-367 probe bodies may be filled by simply inverting the probe body and carefully filling the voids with warm agar solution up to the level of overflowing. Care should be taken to fill in such a way that all the air is displaced from the cavity as it fills. An even better approach is to slide a rod or drill of the correct external diameter into the lumen from the end and mold the agar around the rod. The rod should be wiped with something to act as a 'mold release agent', (cooking oil or ?) prior to insertion. Allow the agar to set up until firm. Placing the unit in a refrigerator can accelerate this process. To remove the rod, twist the rod while extracting from the probe. The result should be a hemi-cylinder cavity of the correct vessel diameter molded into the agar.

Inspect the cavities for air or visible bubbles. If bubbles are seen, carefully remove the agar and begin again. Excess agar should be easily trimmed away and the probe is ready for use. To use the probe, wet it with saline or blood prior to placing it around the blood vessel. Another approach is to use a very small amount of acoustic gel to 'wet' the inside of the probe cavity and provide good coupling. Filled transducers may be stored for long periods of time by keeping the agar damp or wet, preferably in a refrigerator.

The Triton Technology Inc. Large Vessel - Active Redirection Transit Time (ARTT) probes can also be readily filled with agar to avoid air bubbles and provide good coupling in acute preparations.

The Large probes require a larger volume of agar to fill the larger cavities and thus a larger amount of agar solution must be prepared. The goal is to totally fill the cavities with agar, displacing all the air and air bubbles. Ideally the finished surface of the agar will be level with the inside of the probe body and the surface will appear totally smooth cylinder, when viewed through the end of the lumen.

We have found one simple way accomplish these goals. First, prepare a cup of warm agar and totally submerge the complete assembled probe body into the cup slowly, purging all the air as it is lowered. Next, visually inspect all the cavities of the submerged probe for air / bubbles. Any bubbles are 'teased out' with a wooden toothpick. We next insert a small rolled-up piece of Mylar plastic sheet ( approx. 0.004 inches thick) into the lumen of the submerged probe. When the Mylar is released it tries to un-roll and expands to fill the lumen of the probe. The entire cup is placed in the refrigerator and allowed to gel.

When the agar is gelled, the entire 'plug' of agar is removed from the cup. Carefully pull-off the agar from the outside of the probe body. The removed agar can be placed back in the 'double-boiler' and re-used. Next twist the Mylar plastic tube and pull it out of the lumen. (This agar can also be recycled.). The four cavities of the probe should all be completely filled with agar and no air / bubbles should remain. Each cavity should be filled up to the surface of the lumen interior. The probe is now ready for acute use. The interior surface should be wetted with acoustic gel, etc., at the time of use. Filled transducers can be stored for long periods of time by keeping the lumen damp or wet, preferably in a refrigerator.