Acoustic Impedance

A pulse of sound leaving the transducer will travel into the patient until it encounters a change in acoustic impedance (Z) (Fig 1).  Acoustic impedance refers to the resistance of the tissue due to molecular movement. It is directly related to tissue density

At such an interface, a proportion of the sound energy is reflected back to the transducer and this return echo is detected. If the speed of sound is known and the time taken for the echo to return is measured, the depth of the reflecting interface can be calculated:

Distance = speed x time