Acoustic couplant is a material, usually a gel, which is placed in the acoustic window (i.e. the lumen) of a flowprobe in order to fill the gap between the flowprobe and the vessel and thereby complete the acoustic pathway. Acoustic couplant is used because it assists the propagation of acoustical waves between the vessel and the transducer, which otherwise would be poorly conducted through air. In flow studies acoustic couplant is chosen to best mimic the acoustic characteristics of biological tissue.
Different researchers have different preferences about the best acoustic couplant. In physical terms, the wave propagation properties of Surgilube and HR Lubricating Jelly are superior. Some researchers, however, prefer Nalco dissolved in saline. This yields a relatively stiff gel that stays in place over longer periods of time; this is significant in surgeries that can last in excess of several hours. Having to reapply acoustic couplant during an experiment is problematic because the procedure disturbs the positioning of the probe and takes time, etc. However, the acoustic properties of Nalco mixed with saline are comparatively poor and is not recommended by Transonic. Nalco can be mixed with Surgilube to obtain both qualities (acoustic wave propagation and durability). ADInstruments supply both Nalco 1181 and Surgilube. Please note alternative names that may be used for acoustic couplant include Ultrasonic couplant and Acoustical couplant.