physics//wave//sound//acoustic impedance
Acoustic impedance says how hard a material resists being set in motion by a sound wave, and it decides how much of a wave crosses a boundary and how much comes back as an echo, which is the quantity every ultrasound scanner, sonar and transducer designer works with. It is the product of the density and the speed of sound in the material,
Acoustic impedance says how hard a material resists being set in motion by a sound wave, and it decides how much of a wave crosses a boundary and how much comes back as an echo, which is the quantity every ultrasound scanner, sonar and transducer designer works with. It is the product of the density and the speed of sound in the material,
Z=ρ c,Z = \rho\, c ,Z=ρc,
measured in rayl (Pa·s/m). Air is about 413 rayl at 20 °C, water about 1.5×1061.5\times10^{6}1.5×106, soft tissue about 1.63×1061.63\times10^{6}1.63×106 and bone about 7.8×1067.8\times10^{6}7.8×106.
At a flat boundary met head on, the share of the intensity that reflects depends only on the two impedances:
R=(Z2−Z1Z2+Z1)2.R = \left(\frac{Z_2 - Z_1}{Z_2 + Z_1}\right)^{2}.R=(Z2+Z1Z2−Z1)2.
Between soft tissue and bone this gives R≈0.43R \approx 0.43R≈0.43: close to half the energy comes back. Between soft tissue and air the ratio of impedances is about four thousand and RRR exceeds 0.99, so almost nothing crosses.
Echoes come from differences, not from materials.
Two soft tissues a few percent apart reflect less than one percent and still draw an image; a single layer of air or bone reflects nearly everything and hides what lies behind it.
Coupling gel exists because of this formula. Its impedance is close to that of skin, so it replaces the air layer that would otherwise send the whole pulse back into the probe.
The same mismatch protects the transducer and limits it: a piezoelectric crystal has a far higher impedance than tissue, so probes put intermediate layers on its face to pass the energy across in steps.
A strong reflector leaves a shadow. Behind bone or a gas pocket the scanner receives little or nothing, and an image of that region shows absence of signal, not absence of structure.
The formula covers a flat boundary at normal incidence; rough or curved surfaces scatter the wave in many directions, which is what gives tissue its speckled texture on a scan.