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.
This simulator models real pulse-echo physics: at every boundary between tissue layers of acoustic impedance Z = ρ·c, it computes the exact reflection coefficient R = ((Z₂−Z₁)/(Z₂+Z₁))², converts each echo's time-of-flight to depth via d = c·t/2 with c ≈ 1540 m/s, and attenuates the signal by roughly 0.5 dB/cm/MHz round-trip. The result feeds both an A-mode trace (raw echo amplitude vs depth) and an accumulating B-mode image (echo strength mapped to brightness across scan lines), letting you watch a real diagnostic image form column by column.
How ultrasound "sees" inside the body without any optics: sound reflects at every point where acoustic impedance changes, and the returning echo timing and strength are enough to reconstruct a depth-resolved image of internal tissue boundaries.
Choose a Preset (Abdominal, Vascular, Bone shadowing, Fluid cyst) to load a realistic tissue stack, or adjust Frequency, Gain/TGC, and Display depth directly; switch Mode between A+B, A-mode, and B-mode; watch the readout for Wavelength, Axial resolution, Penetration depth, Echo depths, and the Strongest mismatch boundary.
Bone has an acoustic impedance roughly 4-5× higher than soft tissue, so almost all sound energy reflects at the tissue-bone boundary — this is why bones cast a dark "acoustic shadow" on ultrasound images, hiding whatever lies directly behind them.
An echo forms whenever the sound pulse crosses a boundary between two materials with different acoustic impedance Z = ρ·c (density times speed of sound); the bigger the mismatch, the larger the fraction of the wave's energy that reflects back rather than continuing deeper.
It measures the round-trip time-of-flight of the echo and converts it to depth using d = c·t/2, where c is about 1540 m/s in soft tissue and the factor of 2 accounts for the sound travelling down to the boundary and back up to the transducer.
Higher frequency does shorten the wavelength and improve axial resolution (roughly λ/2), but attenuation also increases with frequency, so high-frequency pulses lose too much energy to reach deep structures — which is why vascular probes near the surface run at 10-15 MHz while abdominal probes reach deeper organs at only 2-5 MHz.
Bone's acoustic impedance is so much higher than soft tissue that the reflection coefficient at the tissue-bone boundary is very large, reflecting almost all the incoming energy and leaving almost none to continue past the bone and image whatever tissue lies behind it.
Because echoes from deeper structures have travelled further and lost more energy to attenuation, Gain/TGC boosts the displayed signal progressively with depth so that identical tissue boundaries appear equally bright regardless of how far down they are.