Ultrasound waves are acoustic pressure waves that interact with propagation media. Ultrasound visualization is typically based on wave reflectance from regions with different acoustic properties. When a wave meets different acoustic media, part of it transmits further while others reflect. This happens due to different acoustic impedance. The ration between reflected and transmitted energy purely depends on the difference in acoustic impedance in both regions. As you can see each material can be characterized by its acoustic impedance Z which is equal to ultrasound speed v and material density r.