The matching layer is ______ thick.

• A. One-quarter wavelength
• B. One-half wavelength
• C. Full wavelength
• D. One-eighth wavelength

Answer: (A)

The matching layer being one-quarter of a wavelength thick uses the idea of acoustic impedance transformation to maximize energy transfer from the transducer to tissue. When the layer thickness is t = λ_m/4 (λ_m is the wavelength in the matching material, λ_m = c_m/f), the layer acts as a transformer that makes the impedance seen at the front face close to the geometric mean of the piezoelectric element and tissue impedances. This arrangement causes the reflections at the two boundaries to cancel out at the operating frequency, so more of the incident energy is transmitted into tissue rather than reflected back.

If the thickness were different, the phase relationships would not produce the same cancellation and impedance transformation, leading to greater reflections and reduced transmission. A half-wavelength, full-wavelength, or eighth-wavelength layer would not provide the effective impedance match at the target frequency, so they would be less optimal for transmitting energy into tissue.