Unit 10: Axial and Lateral Resolution

Key Concepts

• Resolution: Describes how detail is displayed; higher resolution equates to more accurate displays.
• Types of Resolution in Ultrasound:
  • Spatial Resolution: Dependent on monitor pixels and scan lines.
  • Contrast Resolution: Ability to differentiate shades of gray.
  • Temporal Resolution: Accuracy of moving objects.
  • Elevational Resolution: Accuracy in thin slices of anatomy.
  • Lateral Resolution: Accuracy of side-by-side structures.
  • Axial Resolution: Accuracy of parallel reflectors to sound beam.

Axial Resolution (Section 10.1)

• Definition: The ability to accurately image reflectors parallel to the sound beam.
• Dependent on: Spatial Pulse Length (SPL).
• Calculation: Axial Resolution = SPL / 2.
• SPL Formula: SPL = Number of Cycles in Pulse × Wavelength.
• Importance of Numerical Value: Lower values indicate better resolution.
• Improving Axial Resolution:
  • Use high-frequency transducers for shorter wavelengths and SPLs.
  • Include backing material in transducer to reduce cycles per pulse.
• Synonyms: Longitudinal, Range, Radial, Depth Resolution.

Lateral Resolution (Section 10.2)

• Definition: Ability to image reflectors perpendicular to the sound beam.
• Dependent on: Beam Width.
• Calculation: Lateral Resolution = Beam Width.
• Beam Anatomy:
  • Beam narrows at the focus and diverges in far field.
  • Best lateral resolution occurs at the focus.
• Improving Lateral Resolution:
  • Smaller transducer diameter for near-field.
  • Larger transducer diameter for reduced far-field divergence.
  • High-frequency transducers for minimized divergence.
• Synonyms: Angular, Transverse, Azimuthal Resolution.

Clinical Application (Section 10.3)

• Choosing Equipment: Balance high frequency for detailed imaging with lower frequency for depth.
• Optimizing Image: Use high frequency and adjust focus.

Focusing Techniques (Section 10.4)

• Types of Focusing:
  • Lenses: External fixed focus, uses refraction.
  • Curved Elements: Internal fixed focus, uses curved PZT crystals.
  • Electronic Focusing: Adjustable, used in multi-element transducers (arrays).
• Electronic Focusing: Allows adjustment of the focus, improving image resolution by placing focus at or just below the area of interest.

Effects of Focusing (Section 10.5)

• Focused vs Unfocused Beams:
  • Focused beams have smaller diameters in near field.
  • Focused beams shorten near zone length, increase divergence after focal zone.
  • Reduced focal zone length and diameter in focused beams, improving lateral resolution.

Conclusion

• Axial resolution is generally better than lateral due to smaller SPL values.
• Focus on using high-frequency, few-cycle transducers to improve both axial and lateral resolution.
• Practice problems and review material to reinforce understanding.