Which statement about deep focus is true?

• A. A deeper focus is achieved by using a smaller diameter PZT.
• B. A deeper focus is achieved by using a larger diameter PZT.
• C. Focus depth is independent of PZT diameter.
• D. Focus depth depends only on frequency.

Answer: (B)

Deep focus comes from the aperture of the transducer. A larger diameter PZT acts as a bigger lens for the emitted ultrasound, increasing the numerical aperture of the beam. With a fixed frequency (and thus a fixed wavelength), this larger aperture allows the wavefronts to be shaped so they converge farther from the element, producing a deeper focal point. A smaller diameter, by contrast, limits how the wavefronts can bend and causes the focus to occur closer to the element, giving a shallower focus. The depth of focus isn’t determined by frequency alone; it also depends on the aperture size. So using a larger diameter PZT yields a deeper focus.