Higher frequency ultrasound provides better axial resolution because it shortens the spatial pulse length.

• A. True
• B. False
• C. Only in water
• D. It worsens with higher frequency

Answer: (A)

The key idea is that axial resolution—the ability to distinguish two echoes along the direction of the sound beam—depends on spatial pulse length. A shorter spatial pulse length means echoes from two nearby reflectors will appear as two separate signals rather than one blurred one.

Spatial pulse length (SPL) is the product of the number of cycles in a pulse and the wavelength: SPL = n × λ. The wavelength λ is the speed of sound c divided by the frequency f (λ = c/f). So, for a given medium and a given number of cycles, increasing frequency decreases the wavelength and thus shortens the SPL. Since axial resolution is roughly half the SPL, a shorter SPL yields a smaller minimum separable distance along the beam axis.

This explanation applies in tissue as well as in water; the speed of sound differs between media, but the relationship remains. The trade-off is that higher frequency also increases attenuation, reducing penetration depth, but within the imaging depth the higher frequency provides better axial resolution.

So, higher frequency improves axial resolution because it shortens the spatial pulse length.