Production and use of ultrasound
Seeing with sound
- An ultrasound scan shows a baby before birth — using sound, not radiation.
- It sends pulses in and listens for the echoes.
- It's safe, cheap, and shows soft tissue well.
The piezo-electric transducer
- A piezo-electric crystal changes shape when a p.d. is applied (it emits ultrasound).
- Squeezed by a returning wave, it makes an e.m.f. (it detects). One crystal does both jobs.
A piezo-electric crystal:
Both effects let one crystal send ultrasound (apply p.d.) and detect it (read the e.m.f.).
Pulse-echo imaging
- Send a short pulse; each tissue boundary reflects part of it back.
- The time delay gives the depth: $d = \dfrac{ct}{2}$. A coupling gel pushes out air so the sound can enter.
An echo returns after $2.0 \times 10^{-4}\ \text{s}$; the speed of sound is $1500\ \dfrac{\text{m}}{\text{s}}$. How deep is the boundary?
$d = \dfrac{ct}{2} = \dfrac{1500 \times 2.0 \times 10^{-4}}{2} = 0.15\ \text{m}$ (the pulse travels there and back).
The coupling gel between the probe and the skin:
Without it, the huge skin–air impedance difference would reflect almost all the ultrasound at the surface.
Impedance and reflection
- Acoustic impedance $Z = \rho c$ (density × sound speed).
- A big impedance difference reflects almost all the sound (skin–air — hence the gel); very similar impedances give no echo.
Acoustic impedance Z = density × ____.
$Z = \rho c$ — density times the speed of sound in the medium.
Two media with very similar acoustic impedance give almost no echo.
The reflected fraction depends on the impedance difference; similar impedances reflect very little, so the boundary is hard to see.
You've got it
- a piezo-electric transducer both sends and detects ultrasound
- pulse-echo depth $d = \dfrac{ct}{2}$; gel removes the skin–air reflection
- acoustic impedance $Z = \rho c$; a boundary needs an impedance difference to give an echo