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67 JULY/AUGUST 2017 ■ TVPJOURNAL.COM IMAGING ESSENTIALS iliac, and sacral) and visceral nodes (hepatic, splenic, gastric, pancreaticoduodenal, jejunal, ileocolic, and colic). 3,4 A number of abdominal lymph nodes are not routinely seen on ultrasonography. Abdominal lymph nodes vary dramatically in size and shape depending on the age of the animal and the location of the node. 12 Assessment of abdominal lymph nodes requires the sonographer to understand the normal anatomic location of the individual nodes as well as the regional anatomy, particularly the vascular anatomy, because lymph nodes are found surrounding specific abdominal vessels. 6,13–16 Parietal Nodes The medial iliac lymph nodes are found caudal to the deep circumflex arteries along the lateral margins of the origins of the left and right external iliac arteries from the aorta (aortic trifurcation). These nodes are located immediately cranial to, at, or just caudal to the trifurcation of the caudal abdominal aorta. 16 They can be dorsolateral, lateral, or ventrolateral to the caudal abdominal vasculature. To identify medial iliac lymph nodes, therefore, it is necessary to sweep the transducer in a dorsoventral direction while imaging the vessels in long axis. Translation motion in the transverse plane while imaging the caudal aorta at the level of the trifurcation is also very useful to identify these nodes, often using a paralumbar acoustic window. In larger dogs, the medial iliac lymph nodes are typically 2 to 4 cm in length. They can be seen as fusiform to oval in shape and are isoechoic to slightly hypoechoic (relative to the surrounding fat) with a faint outer hyperechoic capsule. These nodes can be evaluated in long-axis (sagittal) or short- axis (transverse) view ( Figure 10 ) and are usually 3 to 5 mm in thickness in the adult dog. 2 The medial iliac lymph nodes receive afferent lymphatics that drain the caudal abdomen, pelvis, tail, and pelvic limbs. Features of malignancy that have been described include enlarged, round, hypoechoic to anechoic internal echogenicity with little echotexture. 17 In addition, focal effusion or hyperechoic fat may surround the abnormal lymph node in dogs. 17 FIGURE 8. Static ultrasound image from a dog with a pneumoperitoneum (arrows) with focal reverberation artifacts secondary to soft tissue-gas interface. This pneumoperitoneum was secondary to duodenal ulcer perforation from nonsteroidal anti-inflammatory drug administration. FIGURE 9. A focal hyperechoic nodule with distal acoustic shadowing from a focal area of nodular fat necrosis with resultant dystrophic mineralization (arrow). FIGURE 10. (A) Long-axis image of a normal mesenteric lymph node (arrow) from a dog with a normal abdominal ultrasound and no clinical signs. (B) Short-axis image of the medial iliac lymph node (arrow) in the same dog. The node is highlighted (+) to show the normal width. Ao, aorta, Cvc, caudal vena cava. A B