Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Blood moving towards transducerproduces higher frequency echoes.Blood moving away from transducerproduces lower frequency echoes.FIGURE 16-45. Doppler ultrasound. Soundwaves reflected from a moving object arecompressed (higher frequency) when movingtoward the transducer, and expanded(lower frequency) when moving away fromthe transducer compared to the incidentsound wave frequency. The differencebetween the incident and returning frequenciesis called the Doppler shift frequency.velocity of the blood cells. When the sound waves and blood cells are not movingin parallel directions, the equation must be modified to account for less Dopplershift. The angle between the direction of blood flow and the direction of the soundis called the Doppler angle (Fig. 16-46). The component of the velocity vectordirected toward the transducer is the velocity along the vessel axis multiplied bythe cosine of the angle, cos (0). Without correction for this angular dependence,the smaller Doppler shift will cause an underestimate of the actual blood velocity.The Doppler angle joins the adjacent side and hypotenuse of a right triangle;therefore, the component of the blood velocity in the direction of the sound(adjacent side) is equal to the actual blood velocity (hypotenuse) multiplied bythe cosine of the angle (0). [Note: In physics, the term velocity refers to a vectorquantity, describing both the amount of distance traveled per unit time (speed) andthe direction of movement (such as blood flow). However, it is common practice inmedical ultrasound to use the term velocity instead of the proper term speed.] As thevelocity of blood cells (peak of ~200 em/see) is significantly less than the speed ofsound (154,000 em/see), the denominator can be simplified with extremely smallMeasuredfor velocity ~Doppler shiftDoppler shift forvelocity at e = 0°FIGURE 16-46. Geometry of Dopplerultrasound data acquisition. TheDoppler shift varies as a function ofangle (8) of the incident ultrasoundpulse and the axis of the blood vesselfor a fixed blood velocity v. The maximumDoppler shift occurs at anangle 8 = O. For a larger Dopplerangle (8), the measured Doppler shiftis less by a factor of cos(8), and velocityestimates are compensated by1/cos(8).