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65 JULY/AUGUST 2017 ■ TVPJOURNAL.COM IMAGING ESSENTIALS space is a closed cavity with a serous mesothelial lining. 1 A scant amount of physiologic peritoneal effusion, typically not seen on ultrasonography, is normal; this effusion serves as a lubricant for the peritoneal organs. A small amount of anechoic peritoneal effusion can be seen in young puppies and kittens 2 ( Figure 2 ), but no effusion should be seen in healthy adult dogs and cats. 3,4 The fat within the peritoneum (located primarily in the mesentery and falciform ligament) has intermediate echogenicity and a coarse echotexture ( Figure 1 ). Effusions Peritoneal effusions can be detected by the presence of anechoic fluid separating and/or surrounding normal abdominal structures ( Figure 3 ). Peritoneal fluid accumulation in adult dogs and cats is considered abnormal. Peritoneal effusion should be evaluated with cytology as well as culture and antibiotic sensitivity testing if sepsis is suspected. Determining the type of effusion based on its echogenicity is problematic because ultrasound lacks specificity: anechoic effusions can be exudates and echogenic effusions can be transudates ( Figure 4 ). Nevertheless, a pure or modified transudate typically appears anechoic with no echogenic foci suspended in the fluid. 3–5 Intraperitoneal hemorrhage, inflammation, or neoplastic effusions typically appear echogenic with suspended echoes. 3–5 With moderate to severe peritoneal effusion, artifacts are more readily apparent. These include refraction artifacts along curved surfaces with urinary bladder wall dropout at the area of refraction, distal acoustic enhancement (through-transmission), or side lobe artifacts (see " Physical Principles of Artifacts & False Assumptions " [May/June 2015]). Focal effusions, compared with generalized effusion, can be seen in areas of focal organ pathology (eg, acute pancreatitis). Additionally, exudative effusions can incite an inflammatory response resulting in increased echogenicity of the surrounding peritoneal or retroperitoneal fat 6 ( Figure 5 ). In all cases of significant effusion, the echogenicity of the peritoneal and retroperitoneal fat is increased due to through- transmission or distal acoustic enhancement. FIGURE 3. Long-axis image of the cranial abdomen in an adult dog. Note the anechoic effusion separating the diaphragm and the spleen and liver. FIGURE 2. Anechoic focal effusion (arrow) from the abdomen of a normal 4-month-old dog. A transudate was identified on cytology. FIGURE 4. Protein-losing enteropathy and hypoalbuminemia causing a pure abdominal transudate in a 12-year-old dog. (A) Multiple curvilinear areas of anechoic peritoneal effusion can be seen (arrows). (B) Echogenic effusion (arrow) secondary to hemorrhage from a splenic mass rupture. (C) Hyperechoic, hyperattenuating mesenteric fat (MF) with distal acoustic dropout due to attenuation (arrow) surrounding an inflamed, enlarged, hypoechoic pancreas (PA). In addition a mild echogenic effusion is seen surrounding the pancreas. A B C