During an ultrasound scan, an air gap between the probe and skin produces a strong echo and a poor image of deeper tissues. Adding gel reduces this echo and improves the image.
Which explains the improvement?
More ultrasound is reflected at the surface, so more enters the body.
Less ultrasound is transmitted at the surface, so deeper echoes increase.
More ultrasound is absorbed at the surface, so deeper echoes increase.
Less ultrasound is reflected at the surface, so more enters the body.