Sonography is the use of ultrasound for the examination of organic tissue. It is one of the most frequently used imaging techniques in medicine.
Sonography is based on the echo-principle. Directed ultrasound is sent out and then reflected by the onbject it is directed at. Different layers cause more or less reflection. The layering of the object can be determined from the return-times of the reflected ultrasound signals.
Similarly, but on an optical basis, operates the optical coherence tomography (OCT), and the white light interferometry. At the radio wave scale the echo-principle is used in radar.
The validity of sonography can be improved by application of the doppler-effect, that is used for example in (colored) doppler sonography.
(Picture: Messe Duesseldorf / Constanze Tillmann)