Increasing which of the following will result in improved axial resolution?

• A. Pulse duration
• B. Pulse length
• C. Frequency
• D. Period

Answer: (C)

Axial resolution is tied to how short the emitted pulse is in space, known as the spatial pulse length. Spatial pulse length equals the number of cycles in the pulse times the wavelength. Shorter SPL means better axial resolution. Since wavelength is the speed of sound divided by frequency, increasing frequency makes the wavelength shorter. If you keep the same number of cycles, a higher frequency shortens the SPL, improving axial resolution. Conversely, increasing pulse duration or pulse length lengthens the pulse in space and would worsen resolution, while increasing the period (which lowers frequency) lengthens the wavelength and also worsens resolution. Keep in mind higher frequency also reduces penetration due to greater attenuation, so there’s a trade-off.