Which statement about axial resolution is true?

• A. Longer pulse improves axial resolution
• B. Shorter pulse improves axial resolution
• C. Pulse duration does not affect axial resolution
• D. Higher frequency alone improves axial resolution

Answer: (B)

Axial resolution is determined by how long the ultrasound pulse lasts in tissue, i.e., the spatial pulse length. This length is the distance over which the pulse occupies space and is equal to the number of cycles in the pulse times the wavelength. When the pulse is shorter, echoes from closely spaced reflectors along the beam path return at more distinct times, allowing those reflectors to be resolved as separate structures. Therefore, shortening the pulse duration directly improves axial resolution.

Longer pulses smear echoes from nearby interfaces and degrade axial resolution, so that option is not correct. While higher frequency can contribute to a shorter wavelength and thus help, saying it improves axial resolution “alone” isn’t as precise as focusing on the actual pulse duration; the essential improvement comes from shortening the pulse itself. The statement that shorter pulse improves axial resolution best captures the fundamental relationship.