Which of the following is associated with superior axial resolution?

• A. Longer pulse duration
• B. Shorter pulse duration
• C. More cycles per pulse
• D. Lower frequency

Answer: (B)

Axial resolution is the ability to separate two interfaces that lie along the direction of the beam. It improves when the pulse length is shorter because a shorter spatial pulse length lets the echoes from two closely spaced interfaces be distinguished. Spatial pulse length equals the number of cycles in the pulse times the wavelength; a shorter pulse duration typically means fewer cycles per pulse, which reduces the SPL and sharpens axial detail. In contrast, a longer pulse, more cycles per pulse, or a lower frequency all increase the SPL and worsen axial resolution. So, shorter pulse duration directly leads to superior axial resolution.