Which factor most effectively improves axial resolution in a B-mode image?

• A. Increased aperture
• B. Increased bandwidth
• C. Focusing with an acoustic lens
• D. Increased dynamic range

Answer: (B)

Axial resolution hinges on how long the emitted pulse is in space. The shorter the spatial pulse length, the better you can distinguish two reflectors that lie one after another along the beam axis. Spatial pulse length depends on how long the pulse lasts in time, which relates to the range of frequencies (bandwidth) the transducer emits. A broader bandwidth creates a shorter pulse in time, reducing the spatial length of the pulse. That directly improves axial resolution because echoes from closely spaced objects along the axis become separable.

The other options mostly influence how the beam behaves in the plane of imaging rather than along the axis: increasing aperture or using focusing improves lateral resolution, while increasing dynamic range affects contrast resolution.