The reasoning
- Ultrasound = sound > 20 kHz; diagnostic range ~2–18 MHz (higher for ophthalmic and intravascular use; ultrasound biomicroscopy ~50 MHz).
- Piezoelectric effect (Pierre and Jacques Curie, 1880): crystals (lead zirconate titanate, PZT, most common; historically quartz) convert electricity ↔ pressure waves. The same crystal transmits and receives.
- Pulse–echo principle: depth = c × t / 2, with c in soft tissue assumed 1540 m/s.
- Echo strength depends on acoustic impedance mismatch (Z = density × velocity) at an interface. Soft tissue–air and soft tissue–bone mismatches reflect almost all sound; hence gel and the inability to image through bowel gas, lung or bone.
- Frequency vs resolution vs penetration: higher frequency = better axial resolution but more attenuation (less depth).