ultrasound

Ultrasound refers to sound waves with frequencies above the upper limit of human hearing, typically above 20 kilohertz. It is used for imaging, measurement, and treatment because high-frequency waves can propagate through materials and biological tissue and reflect at interfaces.

Waves are generated and detected by transducers that use the piezoelectric effect: electric voltage causes a crystal to expand and contract, producing sound, and echoes are converted back into electrical signals. In soft tissue, sound travels at about 1540 meters per second, allowing distance estimates from echo travel time. Imaging typically uses 2–15 MHz; higher frequencies provide better resolution but less penetration.

Medical imaging uses include obstetric and gynecologic imaging, abdominal and pelvic scans, and cardiology. Doppler ultrasound measures flow by detecting frequency shifts from moving red blood cells. Modern equipment can produce real-time 2D and 3D images, and some systems offer four-dimensional imaging.

Outside medicine, ultrasound is used in nondestructive testing of materials, flaw detection, weld inspection, and industrial process monitoring. Therapeutic applications include focused ultrasound for tissue ablation, lithotripsy for kidney stones, and ultrasound-assisted drug delivery.

Safety considerations emphasize that ultrasound is generally safe when used appropriately, but high-intensity exposure can cause heating or cavitation. Operators follow ALARA principles to minimize exposure. Image quality can be limited by patient body habitus, gas, bone, and machine settings.

The modern use of ultrasound imaging arose in the mid-20th century, building on the piezoelectric effect discovered in the 1880s. Pioneers such as Ian Donald contributed to widespread clinical use by the 1960s and 1970s.