Axial resolution is determined by which parameter?

• A. Lateral beam width
• B. Transducer frequency
• C. Spatial Pulse Length
• D. Frame rate

Answer: (C)

Axial resolution is determined by the length of the ultrasound pulse along the beam axis. The shorter the pulse in space, the better you can distinguish two reflectors that are close together in depth. This distance is described by the spatial pulse length (SPL), which is the number of cycles in the pulse multiplied by the wavelength. Since axial resolution is effectively half of the SPL, reducing SPL—by using fewer cycles in the pulse or a higher frequency (shorter wavelength)—improves axial resolution. Lateral beam width affects lateral resolution, and frame rate affects temporal resolution, so they don’t determine axial resolution directly.