Answer #23

The correct answer is D: Bone. Bone has the highest propagation velocity as shown in the table below.

Propagation velocity is defined as how fast a sound wave can travel through a medium. It's calculation is not straightforward but depends on both the stiffness and the density of the medium. The higher the stiffness the faster the velocity. The more dense the medium the lower the velocity. However the catch is that many materials which are quite dense are also very stiff and usually the stiffness factor (called bulk modulus) wins out.

The actual equation is

velocity = square root (bulk modulus/density)

Hence bone (which is denser but also much stiffer than air) has the highest propagation velocity. Air has the lowest propagation velocity since it is not stiff at all. Hence lung tissue which has a lot of air has a low propagation velocity as well. Fat has a slightly lower propagation velocity than blood.

In general, we use a soft tissue average of 1540m/sec for ultrasound. This is a handy number of keep in mind for other ultrasound physics questions.