| Tissue | ρ (kg/m³) | c (m/s) | Z (MRayl) | Impedance (relative) | Reflection vs Blood |
|---|
At 1540 m/s, round-trip travel to 1 cm depth takes 13 μs. The machine assumes exactly this. Every echo is time-stamped and converted to depth by this single constant — which is why speed artifacts from fat tissue shift structures deeper than reality.
Blood and the surrounding soft tissue have very similar acoustic impedance (1.61 vs 1.63 MRayl). Almost no reflection occurs within the vessel lumen — so it appears anechoic (black) on screen. The bright walls are the tissue-blood boundaries.
Tissue Z ≈ 1.6 MRayl. Air Z ≈ 0.0004 MRayl. Reflection coefficient approaches 1.0 — nearly total reflection. No signal penetrates. This is why gel is non-negotiable: it eliminates the skin-air interface entirely.
The grayscale value of any pixel encodes the amplitude of the returning echo, which directly encodes the acoustic impedance mismatch at that boundary. White = large mismatch. Black = no mismatch (or no structure). The image is literally a map of impedance gradients.