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PEER REVIEWED 58 JULY/AUGUST 2018 rather than aliasing and wrapping around into the opposite color column. Power Doppler displays only the presence or absence of blood flow. Some ultrasound units have directional power Doppler imaging in which the presence and direction of flow are recorded. These settings are not as sensitive as power Doppler without the directional information ( FIGURE 3 ). Applications All normal blood vessels possess certain ultrasonographic features. On grayscale ultrasonographic imaging, the vascular lumen should be anechoic. Slice thickness artifacts may result in spurious echoes within the lumen, but color Doppler interrogation of the vessel in both short- and long- axis planes helps differentiate between artifact and abnormality. When done with the correct machine settings, color Doppler interrogation of the vessels should result in a solid color within the lumen representative of flow in the expected direction toward (red) or away from (blue) the transducer ( FIGURE 2 ). It is important to note that, depending on the direction of the blood flow in the vessel relative to the transducer, flow in both arteries and veins may be displayed as red, blue, or both colors in the same vessel. For example, if the transducer is perpendicular to the aorta with the marker of the transducer pointing cranially, the flow in the aorta will be red on the left side of the display window and blue on the right side of the display window. PREPARATION AND SCANNING TECHNIQUE Examination of the abdominal vasculature is part of a comprehensive routine abdominal ultrasound examination. Before beginning the examination, clip the patient's hair and apply ultrasonic gel to the skin. The cranial abdominal organs should be evaluated for their normal vasculature (liver, spleen, and left kidney). The arteries surrounding the left adrenal gland are used to localize this gland. If the ultrasonographer scans the abdominal cavity in a clockwise fashion, the caudal abdominal vasculature examination can be reviewed after examining the urinary bladder and reproductive tract. Begin by angling the transducer from the left caudodorsal aspect of the left paralumbar space ventral to the hypaxial muscles to image the retroperitoneal cavity in long axis. The aorta and caudal vena cava will be seen dorsal to the urinary bladder and descending colon. When imaged from the left, the aorta will be dorsal and in the near field and the caudal vena cava will be ventral and in the far field. Next, place the transducer in a similar position on the right side of the patient, again in long axis with the patient and angled toward the retroperitoneal cavity dorsally. This window will show the caudal vena cava in the near field and the aorta in the far field. In addition to using anatomic landmarks to identify the aorta and caudal vena cava, the ultrasonographic and physiologic characteristics of these vessels may be considered: the aorta is pulsatile whereas the caudal vena cava is compressible (flattens easily when pressure is applied with the probe on the overlying abdominal wall). Complete evaluation of the aorta and caudal vena cava in both long- and short-axis views relative to the patient requires following their course FIGURE 3. (A) Long axis of the right kidney in a normal cat using power Doppler as seen in FIGURE 2A . This method detects presence of blood flow but gives no information on the direction, velocity, or turbulence of flow. Note that aliasing is not seen. (B) Directional power Doppler from a jejunal lymph node affected with lymphoma in a cat. This image provides directional information as well as the presence or absence of flow. Note the abnormally enlarged and infiltrated lymph node. B A

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