Welcome back! Today, we’re focusing on one of the most critical aspects of ultrasound imaging — spatial resolution. While temporal resolution captures motion, spatial resolution ensures clarity and detail of structures.
Let’s break down the three dimensions of spatial resolution: axial, lateral, and elevational.
🔍 What is Spatial Resolution?
Spatial resolution is the ability of an ultrasound system to distinguish two closely spaced objects as separate. It determines the sharpness and clarity of anatomical structures.
Three types of spatial resolution:
- Axial Resolution – Front to back
- Lateral Resolution – Side to side
- Elevational Resolution – Out-of-plane or slice thickness
1️⃣ Axial Resolution: Detail Along the Beam Axis
Definition: Ability to distinguish two structures that lie parallel to the beam (along the scan line).
📌 Formula:
Axial Resolution = SPL ÷ 2
(SPL = Spatial Pulse Length)
✅ Improves with:
- Higher frequency (shorter wavelength)
- Fewer cycles per pulse (shorter SPL)
Mnemonic: LARRD
- Longitudinal
- Axial
- Range
- Radial
- Depth
📈 Axial resolution is always better than lateral resolution.
2️⃣ Lateral Resolution: Detail Across the Beam
Definition: Ability to distinguish two structures that lie perpendicular to the beam (side-by-side).
📌 Determined by:
Beam Width
(Narrower beam = Better lateral resolution)
✅ Improves with:
- Focusing
- Higher frequency (within limits)
- Smaller sector width
Mnemonic: LATA
- Lateral
- Angular
- Transverse
- Azimuthal
3️⃣ Elevational Resolution: Slice Thickness
Definition: Ability to resolve structures perpendicular to the image plane (in 3D space). Also known as section thickness or out-of-plane resolution.
📌 Determined by:
- Crystal height (for 1D transducers)
- Fixed lens or mechanical focusing in elevation
✅ Best with: 1.5D array probes and high-frequency linear transducers
📘 SPI Exam Pearls
✅ Axial resolution is influenced by pulse length.
✅ Lateral resolution is influenced by beam width.
✅ Focusing enhances lateral resolution but only at the focal zone.
✅ Elevational resolution is generally poorest and causes artifacts like slice thickness artifact.
📝 Flashcard Quick Check
Q: What is the best method to improve lateral resolution in a specific region?
A: Focus the beam at the area of interest.
Q: What type of resolution improves with shorter spatial pulse length?
A: Axial resolution.
🛠️ Clinical Application Tip
- Axial resolution helps differentiate layers of the bowel wall or fetal membranes.
- Lateral resolution is crucial in detecting closely spaced cysts or nodules.
- Elevational resolution is vital in avoiding partial volume artifacts (e.g., pseudothick gallbladder wall).
🧠 Summary
| Type | Direction | Improved By | Mnemonic |
| Axial | Along the beam | Higher frequency, shorter SPL | LARRD |
| Lateral | Perpendicular | Focusing, narrow beam | LATA |
| Elevational | Out of plane | 1.5D arrays, beam shaping | – |
🔚 Conclusion
A strong grip on spatial resolution isn’t just a test strategy — it’s a clinical necessity. Master these dimensions to sharpen your images and your SPI performance.
Tagline:
“Sharper images, smarter scans: Conquer spatial resolution with confidence!”