To improve axial resolution, which statement is most accurate?

• A. Longer spatial pulse length (SPL)
• B. Shorter spatial pulse length (SPL)
• C. Higher frequency alone does not affect SPL
• D. Increased attenuation

Answer: (B)

Axial resolution comes from how long the ultrasound pulse is in space. The shorter the spatial pulse length, the better we can distinguish two reflectors that lie along the beam path. Spatial pulse length equals the number of cycles in the pulse times the wavelength (SPL = n × λ). Since wavelength is inversely related to frequency (λ = c/f), using a higher frequency reduces the wavelength, and even with the same number of cycles, the SPL becomes shorter. A shorter SPL directly improves axial resolution.

So the best statement is that a shorter spatial pulse length improves axial resolution. Longer SPL would worsen resolution. Saying higher frequency alone does not affect SPL isn’t accurate because higher frequency reduces SPL through the shorter wavelength. Increased attenuation doesn’t improve axial resolution; it mainly affects how deep the signal can go and how strong it is, not the separation of reflectors along the beam.