Which layer helps reduce impedance mismatch at the skin interface?

Which layer helps reduce impedance mismatch at the skin interface?

• A. Backing material
• B. Matching layer
• C. Lens
• D. Element

Answer: (B)

Impedance matching at the skin interface minimizes reflections and maximizes energy transfer from the transducer into the body. The matching layer sits between the piezoelectric element and the skin to bridge the big impedance gap between them. Its impedance is chosen to lie between that of the element and the tissue—ideally near the geometric mean—so more of the ultrasound energy is transmitted rather than reflected back. The layer is typically a quarter-wavelength thick at the operating frequency, which helps the reflections from the two interfaces cancel and improves overall transmission into the tissue. This purpose distinguishes it from the backing material, which dampens the element to shorten the pulse, and from the lens, which focuses the beam, or from the element itself, which is the active piezoelectric source.

Impedance matching at the skin interface minimizes reflections and maximizes energy transfer from the transducer into the body. The matching layer sits between the piezoelectric element and the skin to bridge the big impedance gap between them. Its impedance is chosen to lie between that of the element and the tissue—ideally near the geometric mean—so more of the ultrasound energy is transmitted rather than reflected back. The layer is typically a quarter-wavelength thick at the operating frequency, which helps the reflections from the two interfaces cancel and improves overall transmission into the tissue. This purpose distinguishes it from the backing material, which dampens the element to shorten the pulse, and from the lens, which focuses the beam, or from the element itself, which is the active piezoelectric source.