What is the primary purpose of matching layers at the front face of ultrasound transducers?

• A. They increase system impedance mismatch.
• B. They increase sensitivity but narrow bandwidth.
• C. They bridge the impedance gap between PZT and tissue, minimizing reflections and maximizing energy transfer, sensitivity, and bandwidth.
• D. They completely eliminate reflections.

Answer: (C)

The front-face matching layers are there to bridge the large acoustic impedance difference between the piezoelectric crystal and human tissue. When impedance is highly mismatched, most of the transmitted ultrasound is reflected back at the crystal–tissue boundary, reducing how much energy reaches the tissue and, consequently, the transducer’s sensitivity and usable bandwidth. By inserting layers with intermediate impedance and with thickness chosen to optimize energy transfer (often a quarter-wavelength at the center frequency), reflections are minimized and energy is transmitted more efficiently into tissue. This improves both sensitivity and bandwidth, enabling clearer signals over a wider frequency range. While reflections aren’t completely eliminated, the matching layers substantially reduce them and prevent increasing the mismatch.