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66 IMAGING ESSENTIALS PEER REVIEWED If a small amount of peritoneal effusion is noted, focal collections of fluid may be seen first in the area of the liver (surrounding the hepatic lobes or gallbladder) or at the level of the cranial pole of the urinary bladder in the caudal abdomen ( Figure 4A ). These areas should be scrutinized selectively when a small amount of effusion is suspected. In cases of inflammatory or neoplastic effusion, the mesenteric fat can become hyperechoic and hyperattenuating, as is seen in small animals with pancreatitis or carcinomatosis ( Figure 4B and 4C ). Acute renal disorders can also result in a perinephric effusion and hyperechoic retroperitoneal fat 7 ( Figure 6 ). Neoplasia of the retroperitoneal space has been reported (hemangiosarcoma, undifferentiated carcinoma, extra-adrenal or adrenal pheochromocytoma). 3,4 Disseminated Abdominal Neoplasia Peritoneal effusion in cats and dogs may be secondary to carcinomatosis. In cats, the common sites of primary neoplasia resulting in peritoneal invasion and involvement are the pancreas, liver, and intestinal tract. 8 In dogs, sarcomatosis is most commonly secondary to ruptured splenic or hepatic hemangiosarcoma with peritoneal seeding 9 ( Figure 7 ). Pneumoperitoneum Common causes of pneumoperitoneum include rupture of a hollow viscus (typically gastrointestinal tract) or a penetrating wound. Small-volume pneumoperitoneum can be difficult to identify on ultrasound because the probe must be positioned on top of the free air for the air to be visualized. 10 Characteristic features of a pneumoperitoneum include gas and associated artifacts outside the gastrointestinal tract ( Figure 8 ). Nodular Fat Necrosis Nodular fat necrosis is an incidental finding in cats (primarily) and dogs that appears as an oval structure within the peritoneal space. Such structures are mineralized and therefore have a curvilinear hyperechoic boundary and distal acoustic shadowing. 11 They usually appear singly and can be found anywhere in the peritoneal (and, to a lesser extent, pleural) cavity ( Figure 9 ). ABDOMINAL LYMPH NODES (LYMPHOCENTERS) Abdominal lymph nodes can be divided into the parietal nodes (periaortic, renal, medial iliac, internal FIGURE 5. Effusion dorsal (far field, arrow) to the urinary bladder in a dog with ascites caused by right heart failure. FIGURE 6. Long-axis ultrasound image of the left kidney from a dog with acute renal failure secondary to leptospirosis. There is a perinephric effusion (arrow). FIGURE 7. Diffuse peritoneal nodules from 3 different animals. (A) A cat with carcinomatosis due to pancreatic adenocarcinoma. Multiple hypoechoic nodules are noted throughout the mesentery. (B) A dog with sarcomatosis secondary to peritoneal spread of hemangiosarcoma after rupture of a splenic hemangiosarcoma. Multiple hypoechoic nodules were noted adjacent to the jejunum within the mesentery (arrow). (C) A dog with lymphomatosis secondary to multicentric lymphoma. The heteroechoic to hypoechoic nodules (arrows) were within the mesentery and separate from the mesenteric lymph nodes that were also enlarged. A B C