If both diameter and frequency are increased, focal depth will be:

If both diameter and frequency are increased, focal depth will be:

• A. Shallower
• B. Deeper
• C. Unchanged
• D. Variable

Answer: (B)

The main idea is that focal depth in ultrasound is influenced by the transducer’s aperture and the wavelength of the sound. For a circular aperture, the near-field length (how far the beam stays collimated before diverging) is roughly NZL ≈ D^2 / (4λ). Here, D is the element diameter and λ is the wavelength (λ = c/f, with f the frequency). If you increase the diameter, D^2 grows, pushing NZL farther and making the focus occur deeper. If you also increase frequency, the wavelength λ becomes smaller, and dividing by a smaller λ makes NZL larger still. So both changes—larger diameter and higher frequency—work together to move the focal depth deeper into the tissue. Therefore, the focal depth will be deeper.

The main idea is that focal depth in ultrasound is influenced by the transducer’s aperture and the wavelength of the sound. For a circular aperture, the near-field length (how far the beam stays collimated before diverging) is roughly NZL ≈ D^2 / (4λ). Here, D is the element diameter and λ is the wavelength (λ = c/f, with f the frequency).

If you increase the diameter, D^2 grows, pushing NZL farther and making the focus occur deeper. If you also increase frequency, the wavelength λ becomes smaller, and dividing by a smaller λ makes NZL larger still. So both changes—larger diameter and higher frequency—work together to move the focal depth deeper into the tissue.

Therefore, the focal depth will be deeper.