DK2985_C000 1..28 - AlSharqia Echo Club

2Basic Principles of Doppler UltrasoundPIERRE-GUY CHASSOTCentre Hospitalier Universitaire Vaudois (CHUV), Lausanne, SwitzerlandI. Doppler Effect 23II. Instrumentation 25III. Aliasing 26IV. Continuous- and Pulsed-Wave Doppler 27V. Spectral Display 30VI. Color Doppler 31VII. Doppler Tissue Imaging 37VIII. Artifacts 37IX. Clinical Applications 39Acknowledgments 39References 39I. DOPPLER EFFECTThe Doppler effect is a well-known phenomenon: the soundof a train whistle has a higher pitch when the train is travelingtowards the listener than when moving away, althoughthe emitting frequency remains the same. In 1842, theAustrian physicist Johann-Christian Doppler, studying thedirection of movement of stars, described mathematicallythis shift of frequency of recorded waves, when a luminousor acoustic source, in relative motion, is compared with thestationary observer. The velocity of traveling waves (C),such as light, sound or ultrasound, is constant through adetermined medium, and depends on the characteristics ofthis medium. When the emitting source is in motion,regardless of its speed, the sound waves are “compressed”in front of the transmitter, which is “catching up” with thetransmitted waves it has produced [Fig. 2.1(A)]. After generatingan ultrasound wave, the source moves slightlybefore emitting the next wave. The two waves peaks arethen closer together when they reach the receiver (observer),where the wavelength is shortened and the frequencyincreased (1). This happens because the product of wavelength(l) and frequency ( f ) is constant:C ¼ f l (2:1)If the source moves away, the opposite occurs: the wavelengthincreases and the frequency decreases. The Dopplershift is the difference between the frequency generatedby the source ( f 0 ) and the frequency observed by thelistener ( f 1 ):Df ¼ f 1 f 0 (2:2)This shift is proportional to the ratio of the speed of theobject (v) to the velocity of the sound (c) and to the generatedfrequency ( f 0 ), but is independent of the amplitude ofthe wave:Df ¼ v f 0(2:3)cThe formula can be rearranged to determine the velocityof the object:v ¼ c Df(2:4)f 0The same phenomenon occurs if the moving object isthe target of an ultrasound wave emitted by a fixedsource: the emitted ultrasound wave and the echo reflectedwave returning to the transducer have different frequencies(Fig. 2.2). They are also linked by the previous Doppler23