What does a matching layer typically have in terms of acoustic properties?

A matching layer is designed to optimize sound transmission between the ultrasound transducer's element and the tissue being examined. The primary role of this layer is to reduce the acoustic impedance mismatch between the transducer and the skin.

Acoustic impedance is a product of density and the speed of sound in a material. In medical ultrasound, the impedance of human tissue is typically around 1.38 Mrayls, while the impedance of piezoelectric materials used in transducers is significantly higher. If the matching layer has a lower impedance than skin, it helps reduce reflections and improves the efficiency of sound transmission into the body. This design maximizes the amount of ultrasound energy that enters the tissue, enhancing image quality.

The other options do not support the primary function of the matching layer. If the matching layer had a higher impedance than skin, it would worsen the impedance mismatch, leading to increased reflection and reduced transmission efficiency. Similarly, having the same impedance would not optimize transmission, and variable impedance could introduce inconsistencies depending on the frequency, which is not ideal for ultrasound applications. The correct answer highlights the importance of matching layer design in achieving optimal performance in ultrasound imaging.