Pulse-echo physics · reflection at tissue boundaries · A-mode trace & B-mode image
A pulsed transducer sends a short burst of sound into tissue and then listens. At every boundary between materials of different acoustic impedance (Z = ρ·c), part of the wave reflects back as an echo. The fraction of intensity reflected is the reflection coefficient R = ((Z₂−Z₁)/(Z₂+Z₁))². A large mismatch (e.g. soft tissue → bone or → air) reflects almost everything, leaving little energy to image deeper structures — this produces acoustic shadowing.
The depth of each echo is found from time-of-flight: d = c·t / 2 (the factor of two because sound travels there and back), with c ≈ 1540 m/s in soft tissue. The pulse also weakens with depth through attenuation, roughly 0.5 dB/cm/MHz, which is why higher frequencies cannot reach as deep.
This is the central trade-off in ultrasound: higher frequency → shorter wavelength → better axial resolution (≈ λ/2), but more attenuation → shallower penetration. That is why a vascular probe runs at 10–15 MHz for fine detail near the surface, while an abdominal probe drops to 2–5 MHz to reach deep organs. The right panel shows the A-mode trace (echo amplitude vs depth) and a B-mode image where echo strength is mapped to brightness across scan lines.