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Ultrasound Elastography — Comprehensive Overview

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1. Definition

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2. Underlying Physical Principle

Key Physical Concepts

Stress-Strain Relationship

• Soft tissue deforms more (high strain, low stiffness)
• Hard tissue deforms less (low strain, high stiffness)
• Shear waves travel faster through stiffer tissue

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3. Aim / Goals of Elastography

1. Non-invasive tissue characterization — reduce or replace the need for biopsy
2. Staging of fibrosis — especially hepatic, renal, splenic fibrosis
3. Differentiation of benign vs. malignant lesions — breast, thyroid, prostate, lymph nodes
4. Monitoring disease progression and treatment response
5. Assessment of inflammatory bowel disease — bowel wall stiffness
6. Guiding interventional procedures — targeting the stiffest zone for biopsy
7. Real-time intraoperative guidance

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4. General Process / Workflow

Patient Positioning
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Standard B-mode Ultrasound Acquisition
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Selection of Region of Interest (ROI)
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Application of Mechanical / Acoustic Stimulus
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Detection of Tissue Displacement / Shear Wave
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Signal Processing & Computation of Stiffness
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Color Map or Quantitative Value Output (kPa or m/s)
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Interpretation and Clinical Correlation

Steps in Detail

1. Patient preparation: Fasting (typically 4–6 hours for liver), comfortable positioning (supine or lateral decubitus)
2. ROI placement: Avoid large vessels, bile ducts, ribs, capsule zones
3. Stimulus delivery: Manual compression (strain) or automated acoustic pulse (shear wave)
4. Data acquisition: Multiple measurements (minimum 10 for liver), IQR/median ratio <30% for reliability
5. Output: Either a color-coded elastogram overlaid on B-mode or a numerical stiffness value (kPa)
6. Interpretation: Values compared to validated cutoffs for specific conditions

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5. Types of Ultrasound Elastography — In Detail

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5.1 Strain Elastography (SE) — Compression Elastography

Definition

Principle

• Apply gentle, repetitive compression with the transducer
• Measure displacement of tissue before and after compression using speckle tracking or cross-correlation algorithms
• Stiffer tissue = less displacement = low strain value
• Softer tissue = more displacement = high strain value

Technique & Process

1. Operator applies gentle rhythmic compression perpendicular to the skin (1–3% compression recommended)
2. Real-time RF (radiofrequency) signal data is analyzed frame-by-frame
3. A strain map is computed and color-coded (typically: red = soft, blue = hard, though vendor-dependent)
4. Stability indicator displayed (waveform at bottom of screen) to guide consistent compression
5. Strain Ratio calculated: stiffness of lesion relative to surrounding reference tissue (e.g., SR > 2.5 suggests malignancy in breast)

Output

• Qualitative color map (no absolute units)
• Semi-quantitative Strain Ratio (lesion vs. reference tissue)

Scoring Systems

• Tsukuba Score (1–5) for breast nodules:
  • 1: Uniform green (soft, benign)
  • 3: Mixed green/blue
  • 5: Entirely blue extending beyond lesion (malignant)

Advantages

• Widely available on standard ultrasound machines
• Real-time, no special probe required
• Inexpensive

Limitations

• Operator-dependent (requires consistent compression technique)
• Qualitative/semi-quantitative — no absolute stiffness values
• Cannot penetrate deeply (limited in obese patients)
• Not applicable to cystic or partially cystic lesions (no strain in fluid)
• Cannot be used if lesion is adjacent to bone or calcification

Clinical Applications

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5.2 Shear Wave Elastography (SWE)

Definition

E = 3ρVs² (E = stiffness in kPa; ρ = tissue density ≈ 1000 kg/m³; Vs = shear wave velocity in m/s)

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5.2.1 Transient Elastography (TE) — FibroScan®

Principle

• A mechanical vibrator in the probe tip generates a low-frequency (50 Hz) elastic wave
• An M-mode ultrasound beam tracks the propagation velocity of this shear wave through liver tissue
• Shear wave speed → Liver Stiffness Measurement (LSM) expressed in kPa (range: 2.5–75 kPa)

Technique & Process

1. Patient lies supine, right arm raised, right intercostal space exposed
2. Probe placed over right liver lobe (intercostal approach)
3. System auto-selects a 3 cm² cylindrical sample volume at 25–65 mm depth
4. Minimum 10 valid measurements required
5. IQR/median ratio < 30% for reliable result
6. Success rate and IQR displayed

Probes Available

LSM Cutoffs (Liver Fibrosis — HCV, per EASL)

Limitations

• Blind technique — no real-time B-mode guidance
• Unreliable in ascites, narrow intercostal spaces
• Confounded by: hepatic inflammation (elevated ALT), congestion, cholestasis, food intake
• Cannot characterize focal lesions

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5.2.2 Point Shear Wave Elastography (pSWE) — ARFI (Acoustic Radiation Force Impulse)

Principle

• A focused acoustic push pulse (ARFI — Acoustic Radiation Force Impulse) is delivered to a small ROI within the tissue
• This pulse displaces tissue laterally, generating shear waves
• Two tracking beams placed laterally detect the arrival time of the shear wave → calculate Shear Wave Velocity (SWV) in m/s or convert to kPa
• Integrated into standard B-mode ultrasound transducers (real-time imaging guidance)

Technique & Process

1. Real-time B-mode image obtained
2. Small ROI (sampling box, ~10 × 5 mm) placed in target tissue under visual guidance
3. ARFI push pulse fired
4. System calculates SWV (m/s); typically 5–10 measurements averaged
5. Can be performed in intercostal, subcostal, or any accessible window

Advantages over TE

• Real-time B-mode guidance — ROI placed with precision
• Applicable to focal lesions and diffuse disease
• Functional in patients with ascites
• Works through intercostal spaces with probe angulation

Output

• SWV in m/s (typical range: 0.5–4.4 m/s in liver)
• Can convert: kPa = SWV² × 3000 / 1000

Clinical Applications

• Liver fibrosis staging (validated against biopsy)
• Kidney stiffness (renal fibrosis, transplant rejection)
• Spleen stiffness (portal hypertension assessment)
• Prostate cancer detection

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5.2.3 Two-Dimensional Shear Wave Elastography (2D-SWE)

Principle

• Multiple ARFI push pulses delivered simultaneously across a wide area
• Using ultrafast ultrasound imaging (thousands of frames/second), shear wave propagation is captured across a 2D plane
• Results displayed as a real-time color-coded stiffness map overlaid on B-mode over a large ROI (up to 4×4 cm)

Technique & Process

1. B-mode image acquired
2. Large ROI box positioned over target area
3. Ultrafast imaging captures shear wave propagation in real time
4. Color map generated: typically blue = soft, red = hard (vendor-variable)
5. User places a virtual sampling box (Q-box) to obtain mean kPa value ± SD within region
6. Can sample multiple zones simultaneously

Advantages

• Spatial mapping of stiffness heterogeneity — detects variation within tissue
• Larger area coverage than pSWE
• Highly reproducible, less operator dependent
• Real-time visual feedback

Limitations

• Expensive equipment
• Depth limitations in obese patients
• Susceptible to motion artifacts
• Requires ultrafast hardware not available on all machines

Output

• Color elastogram (kPa scale, color bar displayed)
• Mean kPa ± SD per Q-box
• Statistical distribution over ROI

Key Applications

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5.3 Additional / Emerging Types

5.3.1 MR Elastography (MRE) — Hybrid

• Uses MRI with mechanical vibrations; not sonographic but shares principles
• Gold standard for liver fibrosis in many centers

5.3.2 Real-Time Tissue Elastography (RTE)

• Strain-based, integrated into Hitachi systems
• Provides color overlay with live B-mode; used extensively in Japan

5.3.3 Supersonic Shear Imaging (SSI)

• Brand name: Aixplorer® (SuperSonic Imagine)
• Multiple simultaneous acoustic push pulses generating a "Mach cone" shear wave
• Enables ultrafast 2D-SWE with high spatial resolution

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6. Comparative Summary Table

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7. Confounding Factors and Pitfalls

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8. Clinical Utility Summary

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Key Takeaways

• Elastography measures tissue stiffness, which correlates with pathological changes (fibrosis, malignancy, inflammation)
• Two major categories: Strain Elastography (qualitative, operator-dependent) and Shear Wave Elastography (quantitative, reproducible)
• Shear wave speed is the fundamental measurable: faster propagation = stiffer tissue → higher kPa value
• Transient elastography (FibroScan) remains the most validated tool for liver fibrosis staging globally
• 2D-SWE offers the highest spatial resolution and is increasingly preferred for focal lesion characterization
• Results must always be interpreted in clinical context, accounting for confounders and limitations of each modality

Disease changes tissue stiffness

• Normal and abnormal tissues have different elasticity.
• Fibrosis, tumors, and chronic inflammation usually make tissue stiffer.
• Some lesions remain soft.

What elastography does

1. Apply force to tissue
   • Manual compression (strain elastography), or
   • Acoustic push pulse/mechanical vibration (shear wave methods)
2. Measure tissue response
   • Either amount of deformation (strain), or
   • Speed of generated shear waves
3. Convert response to stiffness map/value
   • Soft tissue: deforms more, shear waves travel slower
   • Hard tissue: deforms less, shear waves travel faster

Physics summary

• In shear wave elastography, tissue stiffness is related to shear wave velocity: [ E \approx 3\rho V_s^2 ] Where:
• (E) = elastic (Young’s) modulus (kPa)
• (\rho) = tissue density
• (V_s) = shear wave speed (m/s)

Two practical principles used clinically

• Strain elastography: relative stiffness (qualitative/semi-quantitative)
• Shear wave elastography: absolute stiffness (quantitative, m/s or kPa)