In a medical setting, a<unk>  is any substance that is used to enhance the visibility of structures or fluids within the body. An example of this is the use of a radiopaque substance during an x-ray exam to highlight features that would otherwise be less distinguishable from nearby tissue. Another example is the use of air bubbles during an echocardiogram to search for a shunt in the heart.
Contrast medium
Fluid
Radiocontrast
Fluence
Ionic liquid
Colloid
Biodiesel
Methylcellulose