The sealed bag contains a constant volume of coupling fluid that improves acoustical contact resulting in superior imaging. Within the bag, the probe achieves good acoustical coupling on irregular surfaces and stays flush with surfaces, even if the transducer is not parallel to the surface. The design permits the operator to change the shape of the bag at will from a long, slender bag for passing through the biopsy channel to a short, fat bag for imaging. Expanding the bag to a larger diameter provides the needed acoustic stand-off for imaging the mucosa layer.
Good coupling can be difficult to achieve when the surfaces of interest are curved or irregular.
For external imaging, the operator applies acoustical gels and maneuvers the transducer manually. In EUS, good coupling is more difficult to achieve because the operator cannot apply acoustical gels internally and can only manipulate the probe indirectly. Whereas some EUS techniques rely on using inflatable balloons or applying water to the imaging surface, such methods are susceptible to interfering air bubbles and require additional channels in the insertion tube. In addition, contact by water immersion cannot be used in gastrointestinal applications and does not work when the water disperses. Provides acoustic standoff.