Send a pulse of sound too high to hear into the body, and time the echoes that bounce back from each boundary. How strong an echo is depends on how different the tissues are, measured by their acoustic impedance.
A piezoelectric transducer both produces and detects ultrasound. A pulse partly reflects at every boundary where the acoustic impedance Z = ρc changes; the fraction reflected is the intensity reflection coefficient Iᵣ/I₀ = (Z₁ − Z₂)² / (Z₁ + Z₂)². Timing the echoes gives the depth, and a coupling gel removes the air gap that would otherwise reflect almost everything.
Set the acoustic impedance of two media and watch how much of the pulse echoes back. Give them very different impedances and almost all reflects; match them and almost all passes through. This is exactly why a tissue-air gap is a near-perfect mirror, and why coupling gel matters.
The marks come from a few clear ideas.
The reflection coefficient depends on the difference in impedance, not the impedance itself; matched media (small Z₁ − Z₂) give almost no echo. When timing an echo to find depth, remember the pulse travels there and back, so d = ½ct. The coupling gel is needed because a thin layer of air between the probe and skin would reflect about 99.9% of the ultrasound.
Four quick checks on ultrasound. Each correct answer earns XP and lights this skill on your star map.
A piezoelectric transducer produces ultrasound because the crystal:
The specific acoustic impedance of a medium is given by:
The fraction of ultrasound intensity reflected at a boundary is largest when the two media have:
A coupling gel is smeared between the probe and the skin in order to:
In an A-scan, the horizontal axis is time, which you convert to depth with d = ½ct using the speed of sound in tissue (about 1500 m s⁻¹). The size of each echo tells you the impedance mismatch at that boundary; a strongly attenuated deep echo is small even at a big boundary.
This skill is now lit on your star map. Keep the chain going.