The spatial relation between the ultrasound beam and the image plane is such that reducing the spatial pulse length leads to which change in axial resolution?

• A. Axial resolution improves
• B. Axial resolution worsens
• C. No change
• D. Only lateral resolution changes

Answer: (B)

Shorter spatial pulse length improves axial resolution because axial resolution depends on how long the transmitted pulse is along the beam. When the pulse is long, echoes from reflectors that lie at similar depths within that pulse length arrive in quick succession and can blur together, making it hard to distinguish two closely spaced structures along the beam. Reducing the spatial pulse length shortens the distance over which a single pulse occupies space, so two reflectors that are closer along the beam can produce separate echoes. In practice, axial resolution is roughly half of the spatial pulse length, so decreasing SPL directly enhances our ability to resolve features along the depth direction. Lateral resolution, by contrast, is determined mainly by beam width, not the pulse length, so it isn’t directly affected by shortening the SPL.