Impedance matching and matching layers

Impedance matching improves the transmission and reception of sound at the transducer-skin interface. 

Echoes are produced at interfaces where there is a difference in acoustic impedance.  The greater the difference in acoustic impedance, the stronger the echo.

At the transducer-skin interface there is a large difference in acoustic impedance because the transducer has a high acoustic impedance and the skin has a much lower acoustic impedance.  As a result, a large percentage of the transmitted sound would be reflected back into the transducer at the transducer-skin interface and very little sound would be transmitted into the patient.  To prevent this, impedance matching is employed.

Impedance matching uses material of intermediate impedance between the transducer and the skin.  This reduces the acoustic impedance difference and transmits more sound into the patient.

Two matching layer materials are used.

1. Gel is used between the skin and the transducer.  The acoustic impedance of gel is of intermediate impedance between the Z of the transducer and the Z of the skin.

2. Impedance matching layers are built into the transducer.  These are very thin layers of intermediate impedance material that are placed in front of the transducer crystals.  Modern arrays use multiple matching layers.  Each is 1/4 wavelength thick for each frequency in the pulse.

Impedance matching using gel and matching layers optimizes both the transmission and reception of sound across the transducer-skin interface.

To assist with your understanding, I have created two video lectures on this topic.