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PEER REVIEWED 60 JULY/AUGUST 2018 caudally to the trifurcation (origin of the left and right external iliac arteries and continuation of the caudal abdominal aorta) and cranially to the cranial abdominal efferent arteries of the aorta, including the renal arteries and veins, cranial mesenteric artery, and celiac artery ( FIGURE 4 ). NORMAL ULTRASONOGRAPHIC FEATURES OF ABDOMINAL VASCULATURE On routine abdominal ultrasonography, the aorta, caudal vena cava, portal system, and their branches/ tributaries should be visualized ( FIGURE 4 ). Aorta The aorta is best visualized in the caudodorsal abdomen, just to the left of midline, where it runs parallel to the caudal vena cava (on the right) from the level of L3 to L6. Cranial to L3, the caudal vena cava and the aorta diverge. The aorta is dorsal to the caudal vena cava throughout the length of the abdomen until the level of L6-L7, where the caudal vena cava and the common iliac veins move dorsal to the aorta and its caudal branches. The cranial mesenteric and celiac arteries can be identified as they leave the aorta cranial to the renal artery; their location is important for identifying the left adrenal gland ( FIGURE 4 ). Color Doppler is needed to identify smaller arteries. At the trifurcation of the caudal abdominal aorta, the medial iliac lymph nodes are adjacent to the lateral borders of the aorta and external iliac arteries on the right and left sides. The aortic diameter can be used as an internal reference to evaluate the renal size in dogs,2 which is an important parameter in the assessment of canine renal disease. One study indicated that the ratio of left renal length, measured in long-axis plane, and adjacent aortic luminal diameter (LK:Ao) should be between 5.5 and 9.1 in normal dogs.2 The aorta should be a straight, anechoic tube with no mineralization within its wall. The walls of the aorta will be discrete hyperechoic lines that do not change diameter until the trifurcation in the caudal abdomen. The normal pulsed-wave Doppler spectral display includes a triphasic spike, with peak velocities reached during ventricular systole. There is a short reversal of flow (signal drops below baseline) followed by a positive rebound from the compliance and elasticity of the ascending aorta and aortic arch (Windkessel effect; FIGURE 5A ). The laminar flow of the blood within the aorta should cause an area relatively void of signal in FIGURE 4. (A) Diagram of the portal vein, the caudal vena cava, and their main abdominal branches and tributaries. (B) Diagram of the aorta and its main abdominal branches. a, artery; v, vein. Caudal vena cava (blue) Gastroduodenal v. Right adrenal gland Right adrenal gland Right kidney Right kidney Right renal v. Pancreas Pancreas Ileocolic v. Cecum Cecum Duodenum Duodenum Urinary bladder Urinary bladder External iliac a. Cr. pancreaticoduodenal v. Heptic v. Liver Portal v. (green) Liver Stomach Spleen Spleen Left kidney Left kidney Cr. mesenteric v. Cr. mesenteric a. Left deep circumflex iliac a. Jejunal v. Left colic v. Colon Colon Left adrenal gland Left adrenal gland Celiac a. Stomach Aorta (red) Left phrenicoabdominal v. Splenic v. A B Dr. Elizabeth Huynh (2) Right renal a. Internal iliac a.

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