Explain the term 'coupling medium' and why it is used in ultrasound imaging.

The extent to which incident ultrasound waves are reflected at a surface boundary is proportional to the difference in acoustic impedence between the two materials. Soft tissue (in the human body) has a very different impedence to air, so waves from a transducer would be almost entirely reflected at the boundary - this would leave very little transmitted intensity, and the resultant generated image would be dim and low contrast. A coupling medium displaces air at the boundary surface, and has an impedence much closer to that of soft tissue. This impedence matching greatly increases the intensity of the transmitted waves, since less is 'lost' in reflection.