Overview
This lecture covers the principles of axial and lateral resolution in ultrasound, focusing on their definitions, calculations, determining factors, and clinical implications for image accuracy.
Types of Resolution in Ultrasound
- Resolution is the ability to display detail accurately in ultrasound images.
- Types include spatial (pixel/scanline-based), contrast (gray shade differentiation), temporal (movement accuracy), elevational, lateral, and axial resolution.
- Higher resolution improves image accuracy and detail.
Axial Resolution (Section 10.1)
- Axial resolution is the ability to distinguish two reflectors parallel to the sound beam (front-to-back).
- Determined by spatial pulse length (SPL), calculated as SPL = number of cycles × wavelength.
- Axial resolution = SPL / 2; lower numerical values mean better resolution.
- Higher frequency and fewer cycles per pulse lead to shorter SPL and better axial resolution.
- Not directly adjustable on the machine; determined by transducer design and frequency selection.
- Synonyms: longitudinal, range, radial, and depth resolution.
Practice on Axial Resolution
- Fewer cycles per pulse and higher frequency both improve axial resolution.
- Shorter SPL provides better axial resolution.
- Need both number of cycles and wavelength/frequency to fully determine SPL and axial resolution.
Lateral Resolution (Section 10.2)
- Lateral resolution is the ability to distinguish two reflectors perpendicular to the sound beam (side-by-side).
- Determined by the beam width; formula: lateral resolution (mm) = beam width (mm).
- Best lateral resolution is at the beam's focus where it is narrowest.
- Lateral resolution changes with depth—best at focus, worse in the far field due to beam divergence.
- Higher frequency and larger diameter transducers reduce beam divergence in the far field, improving lateral resolution.
- Synonyms: lateral, angular, transverse, and azimuthal resolution.
Practice on Lateral Resolution
- Smaller transducer diameter at focus provides better lateral resolution.
- High frequency and large diameter transducers give better lateral resolution in the far field.
- Lateral resolution equals beam width at the given depth.
Focusing Techniques (Section 10.4)
- Focusing improves lateral resolution by narrowing the sound beam at specific depths.
- Three types:
- Acoustic lens (external, fixed),
- Curved element (internal, fixed),
- Electronic focusing (adjustable, used in arrays).
- Electronic focusing allows sonographers to adjust the focal point for optimal image detail.
Effects of Focusing (Section 10.5)
- Focused beams are narrower than the element in the near field.
- Focus is moved closer to the transducer, shortening the near zone.
- Beam diverges quickly after the focal zone.
- Focal zone is thinner and shorter, improving lateral resolution within that zone.
Key Terms & Definitions
- Spatial Pulse Length (SPL) — Physical length a sound pulse occupies in tissue.
- Axial Resolution — Minimum distance between two reflectors parallel to the beam to be seen as separate.
- Lateral Resolution — Minimum distance between two reflectors perpendicular to the beam to be seen as separate.
- Focus — Point of narrowest beam width, providing best lateral resolution.
- Divergence — Spreading of the beam in the far field, worsening lateral resolution.
- Electronic Focusing — Adjustable focus using array transducers for optimal imaging.
Action Items / Next Steps
- Complete workbook activities and practice problems on axial and lateral resolution.
- Review and understand formulas for SPL, axial, and lateral resolution.
- Use nerd check questions for self-assessment and flashcard study.