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Doppler ultrasound is a technique for making non-invasive velocity measurements of blood flow. Christian Doppler was the first to describe the frequency shift that occurs when sound or light is emitted from a moving source and the effect now bears his name. For the velocity measurement of blood, ultrasound is transmitted into a vessel and the sound that is reflected from the blood is detected. Because the blood is moving, the sound undergoes a frequency (Doppler) shift that is described by the Doppler equation: F = Fo (c + v cos(q))/ (c - v cos(q)) where: c is the acoustic velocity in blood, i.e., 1.54 F o is the transmitted frequency; q is the Doppler angle; v is the velocity of the blood. ... There are three main techniques for making Doppler ultrasound velocity measurements of blood flow; continuous wave Doppler, pulsed Doppler, and color Doppler. Pulsed Doppler can be referred to as pulsed Doppler, spectral Doppler, or duplex Doppler. source: http://www.ob-ultrasound.net/doppler_a.html

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