What defines the best axial resolution that can be obtained with a given transducer?

• A. Spatial pulse length x 2
• B. Spatial pulse length x 4
• C. Spatial pulse length / 2
• D. Spatial pulse length / 4

Answer: (C)

Axial resolution is how well we can distinguish two reflectors that lie along the ultrasound beam, and it depends on how long the pulse is in tissue—the spatial pulse length. The shorter that pulse, the finer the detail we can resolve along the beam. The key rule is that two echoes must be separated by about half the length of the pulse to be distinguished, so the best axial resolution you can get from a given transducer is SPL divided by two. If you use a longer pulse (multiplying SPL), the minimum separable distance grows (worse resolution); dividing SPL by four would imply an unrealistically finer limit than the pulse itself typically allows. So the correct relationship is a spatial pulse length divided by two.