Which statement about the typical thickness of the diagnostic imaging transducer crystal is true?

• A. It is typically around 0.2 to 1 mm
• B. It is typically around 2 to 5 mm
• C. It is typically around 5 to 10 mm
• D. It is not related to frequency

Answer: (A)

The main concept here is that the transducer crystal thickness sets the resonant frequency used for diagnostic ultrasound. The crystal is a thin plate whose thickness is about half the acoustic wavelength in the crystal (t ≈ λ/2). With typical piezoelectric materials like PZT, the speed of sound is around 4000 m/s, so the wavelength in the crystal is roughly λ ≈ v/f. To achieve common diagnostic frequencies (about 2–10 MHz), the crystal thickness ends up in the ballpark of 0.2–1 mm. That’s why the statement saying the thickness is typically around 0.2 to 1 mm is correct.

If the crystal were 2–5 mm thick, the resonant frequency would be about 0.4–1 MHz, which is lower than what's normally used in diagnostic imaging. Thicker crystals push frequencies down further, which wouldn’t match clinical imaging ranges. And since thickness directly influences the resonant frequency, it is indeed related to frequency.