Ultrasound Imaging
Definition
Seeing babies and hearts with sound — safe, real-time echoes at frequencies far above hearing.
Technical definition
A piezoelectric transducer emits MHz pulses; echo timing gives depth and Doppler shift gives blood velocity. No ionizing radiation is involved.
Example
Obstetric ultrasound at ~3–5 MHz images the fetus — the same piezoelectric effect as a BBQ lighter, run in reverse thousands of times per second.
Common confusion
Ultrasound can't see through bone or air well — the huge impedance mismatch reflects almost everything. That is why gel, not air, couples the probe to skin.