The Physics of Ultrasound
Ultrasound sees with sound. A probe fires megahertz pressure waves into tissue and times the echoes — exactly like echolocation, except the bat is a crystal and the moth is your kidney.
Piezoelectric transducer
Quartz-like crystals expand and contract when voltage is applied, converting electrical pulses into ultrasound — and reverse the process to turn returning echoes back into signals. One crystal is both speaker and microphone.
Echoes at boundaries
Sound reflects where acoustic impedance (density × sound speed) changes — organ walls, bone, fluid boundaries. The echo delay gives depth (d = vt/2); the strength gives the boundary type.
Why the gel?
Air's impedance is ~4,000× lower than tissue's, so bare skin would reflect nearly everything. Gel matches impedances, letting sound cross into the body instead of bouncing off.
Doppler blood flow
Echoes from moving blood cells shift in frequency (Doppler effect). Color Doppler maps the shift to velocity — red toward, blue away — showing circulation in real time.
Frequency tradeoff
Higher frequency means finer detail but shallower penetration (more absorption). Obstetric scans use ~3–5 MHz for depth; eye or skin scans use 10+ MHz for resolution.
Try this yourself
- Watch an ultrasound video and spot the gel application — now you know it's impedance matching.
- Bats and dolphins do this naturally: compare their click frequencies to medical ultrasound.