Ultraschall

Ultraschall (ultrasound) denotes sound waves and the corresponding imaging and measurement techniques that operate at frequencies above the range of human hearing, typically above about 20 kilohertz. The term is used for acoustic waves in air, liquids and solids, and for a set of methods that rely on the interaction of these waves with matter. In medicine, ultrasound is one of the main imaging modalities alongside X-ray-based methods.

Physically, ultrasound relies on piezoelectric transducers that convert electrical impulses into mechanical waves and back. The speed of sound and the attenuation depend on the medium; in soft tissue the approximate speed is 1540 m/s. Medical imaging commonly uses frequencies in the 1–15 MHz range; higher frequencies give better resolution but shallower penetration. The pulse-echo principle measures the time delay and amplitude of echoes to construct images. Doppler ultrasound uses a shifted frequency to quantify motion, notably blood flow. Focused beams can be used for therapy (high-intensity focused ultrasound, HIFU).

Medical use: Diagnostic ultrasound provides real-time visualization of organs and fetus, guidance of procedures, and assessment of flow with Doppler. Modes include B-mode two-dimensional imaging, M-mode for motion analysis, and newer 3D/4D imaging and contrast-enhanced ultrasound.

Industrial and other uses: In industry ultrasound is used for non-destructive testing, flaw detection in metals and composites, thickness measurements, and weld inspection. Underwater sonar employs longer wavelengths for navigation and object detection. Therapeutic uses include lithotripsy and ablative therapies.

Safety and regulation: Ultrasound is generally non-ionizing and considered safe when used appropriately, but excessive exposure can cause heating or mechanical effects. Safety standards and exposure guidelines are issued by organizations such as IEC and FDA, with the ALARA principle guiding use, particularly in fetal imaging and obstetrics.

History: The piezoelectric effect was discovered by the Curie brothers in 1880. Military sonar development in the early 20th century, notably by Paul Langevin, spurred broader ultrasound research. Medical ultrasound emerged in the 1940s–1950s with pioneers such as Karl Dussik and Ian Donald, and has since become a routine clinical tool.