The thickness range of PZT crystals in diagnostic imaging transducers is from:

• A. 0.2 to 1 mm
• B. 0.5 to 1.5 mm
• C. 1 to 3 mm
• D. 2 to 4 mm

Answer: (A)

In ultrasound transducers, the PZT crystal thickness sets the resonant (operating) frequency through thickness-mode resonance. The resonant frequency is roughly f ≈ v/(2t), where v is the acoustic velocity in the crystal (about 4000 m/s). To reach the diagnostic imaging range, which spans roughly a couple of megahertz up to around a dozen megahertz, the crystal must be on the order of tenths of a millimeter thick. For example, a 0.2 mm thickness gives about 10 MHz, while a 1 mm thickness gives about 2 MHz. That makes 0.2 to 1 mm a practical range for typical diagnostic transducers. If the crystal were thicker, the frequency would drop into lower, less useful bands; if it were thinner, it would push toward higher frequencies that are harder to generate efficiently and manufacture.