Decreasing the pulse length generally leads to which change in axial resolution?

• A. Worsening
• B. Not defined
• C. Improvement
• D. No change

Answer: (C)

Shorter pulse length improves axial resolution because axial resolution depends on the spatial pulse length, which is the distance the pulse occupies along the beam. When the pulse length is shorter, the spatial extent of the pulse is smaller, so two reflectors that lie close together along the beam can produce distinct echoes instead of merging. Since axial resolution is roughly SPL/2, reducing the spatial pulse length directly enhances the ability to distinguish features along the axis. This shortening can be achieved by using fewer cycles in the pulse or by increasing the frequency (which shortens wavelength), though higher frequency reduces penetration depth. So decreasing the pulse length generally leads to improvement in axial resolution.