When using ultrasound we are interested in the fraction of sound reflected at the boundaries between tissues since these reflections are used to make the ultrasound images. This fraction depends on the acoustic impedanceif each material. The impedance in turn depends on the densityand the speed of sound in the material

We can derive an equation for the fraction of sound at the boundaries of two tissues with different acoustic impedancesandIf the sound approaches the boundary at right angles with intensityand and the intensity of the reflected sound isthen

At the interface between muscle and bone we would expect a fractionto be reflected.

Notice that ifandare very different then most of the ultrasound will be reflected. This happens at an air tissue boundary (to reduce this during a scan, a gel with impedance very nearly equal to that with skin is smeared over the area of contact of the transducer with the skin) and conversely, between different soft tissues very little reflection will take place, which makes soft tissues difficult to differentiate on an ultrasound scan.

Lungs and bones are difficult to see for these reasons, and to image the heart, the sound must be directed between two ribs.