What do the crystals do when echoes reach them?

• A. Absorb the echoes
• B. Generate an electric current
• C. Change color
• D. Emit light

Answer: (B)

The crystals are using the piezoelectric effect in the ultrasound transducer. When echoes from tissues arrive at the crystal, the incoming sound waves mechanically deform the crystal. This deformation causes charge separation and creates an electrical current, which is then processed by the ultrasound system to form the image. In other words, the crystal converts the mechanical energy of the received echoes into an electrical signal.

Absorbing echoes isn’t what generates the signal here, and changing color or emitting light aren’t part of how these crystals work in this context.