Infographics / Ultrasound Physics

Acoustic impedance and reflection at interfaces

A beam meets a boundary between two media of acoustic impedance Z equals rho times c. It splits into a reflected echo and a transmitted beam, with the angle of reflection equal to the angle of incidence. The intensity reflection coefficient R equals the square of (Z2 minus Z1) over (Z2 plus Z1). Three bars show that a soft tissue to air interface reflects about 99.9 percent, soft tissue to bone about 45 percent, and two similar soft tissues (fat and muscle) only about 1 to 2 percent, so most energy is transmitted with a weak echo.

Acoustic impedance and reflection at a tissue boundary A beam meets a boundary between two media of acoustic impedance Z equals rho times c. It splits into a reflected echo and a transmitted beam, with the angle of reflection equal to the angle of incidence. The intensity reflection coefficient R equals the square of (Z2 minus Z1) over (Z2 plus Z1). Three bars show that a soft tissue to air interface reflects about 99.9 percent, soft tissue to bone about 45 percent, and two similar soft tissues (fat and muscle) only about 1 to 2 percent, so most energy is transmitted with a weak echo. Acoustic impedance and reflection at a boundary Z = ρc (unit: Rayl)   •   R = ((Z₂ − Z₁) / (Z₂ + Z₁))² medium 1 Z₁ = ρ₁c₁ medium 2 Z₂ = ρ₂c₂ normal θᵢ θᵣ incident reflected (echo) transmitted Angle of reflection = angle of incidence (specular); the impedance mismatch sets how much energy reflects. Fraction of intensity reflected vs transmitted (normal incidence) reflected transmitted soft tissue – air huge mismatch R ≈ 99.9% soft tissue – bone large mismatch R ≈ 45% soft tissue – soft tissue fat – muscle (similar Z) R ≈ 1–2% Coupling gel excludes the tissue–air gap, removing near-total reflection so the beam can enter the body.
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