Acoustic Radiation Force Impulse (ARFI)

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Acoustic Radiation Force Impulse (ARFI) imaging is an ultrasound-based elastography technique that evaluates tissue stiffness (elasticity) by generating and measuring shear wave velocity (SWV) using focused acoustic pulses.

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It provides a quantitative assessment of tissue stiffness — expressed in meters per second (m/s) or converted to kilopascals (kPa) — and is widely used in liver fibrosis staging, as well as in the evaluation of kidneys, spleen, pancreas, thyroid, and lymph nodes.

Physical Principle

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ARFI uses short-duration, high-intensity acoustic “push” pulses generated by the ultrasound transducer to displace tissue locally.

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The acoustic radiation force produces microscopic tissue displacements (1–10 μm).
This generates transverse shear waves that propagate laterally through the tissue.
The system then measures the velocity of shear wave propagation using ultrafast tracking pulses.

The stiffer the tissue → the faster the shear wave travels.

Key Equation

$$ E = 3 \rho c_s^2 $$

Where:

E = Young’s modulus (elasticity, in kPa)
ρ = Tissue density (≈ 1000 kg/m³)
cₛ = Shear wave velocity (m/s)

Thus, higher shear wave velocity (SWV) = higher tissue stiffness (fibrosis, tumor, inflammation).

Technique Overview

Step	Description
Mode of operation	ARFI integrated into B-mode ultrasound system (Siemens “Virtual Touch”, GE “ElastPQ”, Canon “ShearWave”, etc.)
Probe	Convex (for abdomen) or linear (for superficial organs) transducer
Push pulse	Short (≈ 0.3–0.5 ms) acoustic burst focused at a specific depth
Tracking pulse	Follows tissue displacement and measures shear wave velocity
Region of Interest (ROI)	Small fixed box (typically 5×10 mm) within target organ
Measurement unit	m/s (shear wave velocity) or converted to kPa
Typical measurement depth	2–6 cm (for liver, kidney, spleen)
Number of acquisitions	5–10 valid measurements averaged (median used for interpretation)