Today's Veterinary Practice

TVP_JUL-AUG2018

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PEER REVIEWED 56 JULY/AUGUST 2018 todaysveterinarypractice.com Pulsed-wave Doppler is used in both echocardiography and abdominal/vascular ultrasonography. In pulsed-wave Doppler, the pulse– echo principle (the same crystal transmits and receives the ultrasound wave) is used to measure blood flow velocity, direction, and pattern (laminar versus turbulent). The advantage of pulsed-wave Doppler is that the exact anatomic location of the returning velocity can be accurately recorded by specifying where the ultrasound transducer should "listen" for returning echoes along the line of Doppler interrogation. The specified area, known as a sampling gate, is displayed as two parallel lines along the interrogation line. There is no range ambiguity. A disadvantage of pulsed-wave Doppler is that it is limited by aliasing artifact for higher velocities (>2.0 m/sec; BOX 1 ). This clearly affects the utility of pulsed-wave Doppler in echocardiography, where pathologic velocities can easily exceed this limit; however, pulsed-wave Doppler is adequate for vascular evaluation in the abdomen. When pulsed-wave Doppler is used for abdominal vessels, the angle between the vessel being interrogated and the transducer at the skin's surface will never reach 0°. Therefore, these machines have a dial for angle correction. Angles > 60° result in sampling inaccuracy for peak flow velocity. Color Display Doppler In this form of pulse-echo Doppler, the returning frequencies are displayed based on a color display known as BART, in which blue indicates flow away from the transducer and red indicates flow toward the transducer. The color spectrum indicates the velocity range or scale, which is displayed on the image, typically on the right. Color Doppler displays the presence or absence, direction, velocity, and character (laminar versus turbulent) of blood flow. In this technique, the color Doppler button is engaged and a box (sample window) is overlaid on the grayscale image. Within this box, changes in frequency based on the presence of motion in the vessels result in a color display of the velocity for each pixel ( FIGURE 2 ). One available color display is called a variance map, in which velocities that exceed the pulse repetition frequency are displayed as green BOX 1. Aliasing in Spectral Pulsed-Wave Doppler Images Aliasing in spectral pulsed-wave Doppler images is an artifact that appears as a "wraparound" of the maximum velocities to the opposite side of the pulsed-wave spectral display in the y direction. The maximum velocity (up to 1.5 to 2.0 m/sec) that can be accurately recorded in pulsed-wave Doppler is a function of the sampling frequency, also called the pulse repetition frequency (PRF), and the depth of the interrogating sampling volume. For a given PRF, the maximum frequency shift that can be recorded without aliasing is equal to half the PRF. This maximum shift is known as the Nyquist limit. To reduce aliasing, the ultrasonographer may increase the PRF (often called scale on ultrasound machines), decrease the baseline, or reduce the depth of the sample volume to allow PRF to increase with the lack of depth constraints. If the PRF is set correctly, aliasing indicates high-velocity and/ or turbulent blood flow. The incident angle can be corrected by a sampling gate angle correction; however, an angle above 60° is not recommended. FIGURE 2. (A) Long-axis view of the right kidney in a normal cat using color Doppler. Note the red vessels indicating blood flow toward the transducer and the blue vessels indicating flow away from the transducer. (B) Color Doppler image of the caudal vena cava of a dog in dorsal recumbency as imaged from the right side in long axis. Aliasing (red, yellow, and orange) can be seen because the pulse repetition frequency or Doppler scale has been set too low at 6.0 cm/sec. (C) The same dog as in B. The Doppler scale has been adjusted correctly to 18.4 cm/sec to eliminate aliasing. B C A

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