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PEER REVIEWED 54 JULY/AUGUST 2018 parallel to the area of interest or lines up with the flow of blood so that the angle of incidence of the ultrasound beam is 0°. If the incident angle is 90°, no flow will be displayed ( FIGURE 1 ). Two types of spectral Doppler techniques are used today: continuous wave and pulsed wave. Continuous-wave Doppler is used in echocardiography. It uses phased array transducers, which contain multiple crystals that independently and continuously transmit and receive ultrasound waves. The advantage of continuous-wave Doppler is that it can record high blood flow velocities (>6 m/ sec) with high fidelity (no aliasing artifact; BOX 1 ). It also gives the direction of blood flow relative to the transducer: flow directed toward the transducer is above the baseline (positive direction), and flow away from the transducer is below the baseline (negative direction). Its disadvantage is that flow velocities are recorded along the line of interrogation, and a specific anatomic location along that line is not accurately recorded. This is called range ambiguity. FIGURE 1. Diagram of Doppler ultrasonography and the Doppler shift frequency equation. The Doppler shift (f d ) is measured by the ultrasound machine as the difference between the transmitted (f t ) and reflected ultrasound frequency as a result of the motion of red blood cells. The equation shows that measurement of f d is related to the velocity (V) of blood flow. The speed of sound (c) in tissue is assumed to be a constant at 1540 m/sec for the purpose of this calculation. Note that the cos 90° is 0; therefore, if the angle of f t is 90°, the measured blood flow will be 0. Doppler Frequency (f d ) = 2 · f t · V · cos θ c f d Doppler shift c Speed of sound in tissue f t Transmitted beam V Velocity of the blood θ Angle between the transmitted ultrasound beam and flow direction f t V θ TABLE 1 Comparison of Pulsed-Wave, Color, and Power Doppler 1 PULSED-WAVE DOPPLER COLOR DOPPLER POWER DOPPLER Properties Examines the flow waveform within a specific part of the vessel being interrogated Provides a map of flow superimposed over an organ or structure Color brightness is related to the number of moving cells (presence of blood flow) Advantages Provides directional information about flow Provides directional information about flow (BART) Sensitive: can detect low-flow states in smaller-diameter vessels Provides velocity information and allows calculation of specific indices (resistive index in the kidneys, systolic:diastolic ratios, etc.) Provides velocity information and shows vascular distribution within the organ of interest Not angle dependent Assesses the laminar appearance of blood flow and any flow disturbances Detects turbulent flow, lack of flow, and high-flow velocities as seen with aliasing Free from aliasing Disadvantages Location only delivered in combination with additional images such as real-time B-mode Limited by respiratory influence (very difficult to obtain in a panting animal) No directional information Highly angle dependent so that flow perpendicular to the transducer is registered as zero. Should strive to keep all angle- corrected interrogations < 60° in the abdomen. Angle dependent; flow perpendicular to the transducer is recorded as zero No velocity information Lowers frame rate due to increased computer processing required No flow character information Poor temporal resolution Susceptible to noise and motion BART, blue away and red toward the transducer.

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