Refraction

Refraction of a sound wave occurs if it travels between tissues with different propagation speeds. As the incident pulse or returning echo strikes an interface of different density or elasticity and therefore a different propagation speed, the direction of the wave changes according to Snell’s law ¹ :

• sin θ₁/sinθ₂ = c₁/c₂

Where:

• sin θ₁ = incident angle of the sound wave at the interface
• sin θ₂ = refraction angle of the sound wave in the new medium            
• c₁ = propagation speed in the initial medium
• c₂ = propagation speed in the new medium

Ultrasound machines assume all pulsed waves and returning echoes travel along a direct path, therefore refraction can cause refraction artifact ² .