What are two characteristics of low-frequency pulsed wave imaging transducers?

• A. Thicker PZT crystals; PZT with higher speeds
• B. Thinner PZT crystals; PZT with higher speeds
• C. Thicker PZT crystals; PZT with lower speeds
• D. Thinner PZT crystals; PZT with lower speeds

Answer: (C)

Low-frequency pulsed-wave imaging transducers achieve their lower frequency by using a thicker piezoelectric element and a slower acoustic velocity in the material. For thickness-mode transducers, the resonant frequency is roughly the speed of sound in the PZT divided by twice the crystal thickness (f ≈ v / (2t)). Increasing the thickness lowers f, and using a PZT with a lower wave speed lowers f as well. Together, thicker PZT crystals and a slower PZT material give the desired low frequency for deeper penetration. Using thicker crystals with a higher speed would increase frequency, and thinner crystals generally raise frequency, so the combination of thicker crystals and lower speeds best explains low-frequency design.