The ability to resolve two separate images along (parallel to) the main axis of a sound beam

• Units: distance
• Determined by: spatial pulse length
  • Shorter pulses improve resolution
    • Reduce ringing (fewer cycles/pulse)
    • Increase frequency/decrease wavelength
  • Shorter pulse duration also improves resolution
• Not adjustable -> same at all depths
• Range: 0.1-1.0 mm -> the smaller, the better

Synonyms for axial resolution

• Longitudinal
• Axial
• Range
• Radial
• Depth

= SPL (mm) ÷ 2

= (wavelength (mm) * #cycles/pulse) ÷ 2

In soft tissue: = (0.77 * #cycles/pulse) ÷ frequency (MHz)

The ability to resolve two separate images that are side by side (perpendicular) to the main axis of a sound beam 

• Units: distance
• Determined by: sound beam diameter
  • Narrower beams improve resolution
• Adjustable depth by changing focus

Synonyms for lateral resolution

• Lateral
• Angular
• Transverse
• Azimuthal

Concentrates the sound energy into a narrower beam

Alters the beam by:

• Narrowing beam diameter in near field and focal zone
• Decreasing focal depth (shallower focus)
• Shrinking focal zone (the greater the extent, the smaller the zone)
• Increasing beam diameter in far zone

• Fixed aka conventional/mechanical
  • Focal depth and extent of focusing cannot be changed
  • Includes lens and curved active elements 
    • Single crystal transducers
  • Poorest lateral resolution

• Phased array aka adjustable/multiple focusing
  • Adjustable by electronics
  • Generally better lateral resolution

• External -> with a lens
• Internal -> with a curved active element
• Phased array -> with electronics of ultrasound system