1) The Doppler effect in ultrasound refers to the change in frequency between emitted ultrasound waves and reflected waves when the ultrasound source or target is in motion.
2) Christian Doppler discovered the Doppler effect in 1845 while studying the color changes of stars.
3) In medical ultrasound, Doppler is used to analyze blood flow and tissue motion in the heart, vessels, and other organs by detecting changes in frequency of reflected ultrasound waves.