A pulse of ultrasound travels through one tissue and meets another. How much echoes back depends on the mismatch in acoustic impedance Z = ρc: the bigger the difference between Z₁ and Z₂, the more is reflected. The fraction reflected is the intensity reflection coefficient, Iᵣ/I₀ = (Z₁ − Z₂)² / (Z₁ + Z₂)².
MissionMake the boundary reflect almost everything by giving the two media very different impedances, then almost nothing by matching them.Streak 0Best 0
A large impedance mismatch, such as soft tissue against air, causes the pulse to be:
impedance Z₁ / Z₂1.6 / 7.8 (×10⁶ kg m⁻² s⁻¹)
reflection coefficient Iᵣ/I₀40%
transmitted60%
A bigger impedance mismatch reflects more. A tissue-air gap reflects almost everything, which is why coupling gel (matched to tissue) is smeared on the skin.