Which factor least influences axial resolution?

• A. Damping
• B. Beam width
• C. Frequency
• D. Pulse duration

Answer: (B)

Axial resolution is about distinguishing two reflectors that lie along the path of the ultrasound beam. It depends on the spatial pulse length—the physical length of the emitted pulse along the beam. Spatial pulse length equals the number of cycles in the pulse times the wavelength. A shorter spatial pulse length means the pulse occupies less distance along the beam, so two nearby interfaces can be resolved more clearly.

Damping shortens the pulse by reducing ringing, which lowers the number of cycles contributing to the pulse and thus reduces spatial pulse length. Higher frequency lowers the wavelength (since wavelength = c/f), so for a pulse with the same number of cycles the spatial length is shorter, improving axial resolution. Shorter pulse duration directly reduces the time—and consequently the spatial length—of the pulse, enhancing axial resolution.

Beam width, on the other hand, affects lateral resolution—the ability to distinguish objects side by side perpendicular to the beam. It doesn’t change how far along the beam two interfaces can be separated, so it has the least influence on axial resolution.